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+The Project Gutenberg eBook, Things a Boy Should Know About Electricity,
+by Thomas M. (Thomas Matthew) St. John
+
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+
+
+
+Title: Things a Boy Should Know About Electricity
+       Second Edition
+
+
+Author: Thomas M. (Thomas Matthew) St. John
+
+
+
+Release Date: January 14, 2014  [eBook #44665]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+
+***START OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT
+ELECTRICITY***
+
+
+E-text prepared by Chris Curnow, Emmy, and the Online Distributed
+Proofreading Team (http://www.pgdp.net) from page images generously made
+available by Internet Archive (https://archive.org)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 44665-h.htm or 44665-h.zip:
+      (http://www.gutenberg.org/files/44665/44665-h/44665-h.htm)
+      or
+      (http://www.gutenberg.org/files/44665/44665-h.zip)
+
+
+      Images of the original pages are available through
+      Internet Archive. See
+      https://archive.org/details/thingsboyshouldk00stjo
+
+
+Transcriber's note:
+
+      Text enclosed by underscores is in italics (_italics_).
+
+      Text enclosed by equal signs is in bold face (=bold=).
+
+      Characters enclosed by curly brackets after an underscore
+      are subscripts (example: CuSO_{4} [the chemical formula
+      of copper sulfate]).
+
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+
+[Illustration]
+
+
+       *       *       *       *       *
+
+_BY THE SAME AUTHOR._
+
+
+  =FUN WITH MAGNETISM.= A book and complete outfit of apparatus
+     for _Sixty-One Experiments_.
+
+  =FUN WITH ELECTRICITY.= A book and complete outfit of
+     apparatus for _Sixty Experiments_.
+
+  =FUN WITH PUZZLES.= A book, key and complete outfit for _Four
+     Hundred Puzzles_.
+
+  =FUN WITH SOAP-BUBBLES.= A book and complete outfit of
+     apparatus for _Fancy Bubbles and Films_.
+
+  =FUN WITH SHADOWS.= Including book of instructions with one
+     hundred illustrations and a complete outfit of apparatus
+     for _Shadow Pictures, Pantomimes, Entertainments, etc.,
+     etc._
+
+  =HUSTLE-BALL.= An American game. Played by means of magic
+     wands and polished balls of steel.
+
+  =JINGO.= The great war game, including JINGO JUNIOR.
+
+  =HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS.= A book
+     containing complete directions for making all kinds of
+     simple apparatus for the study of elementary electricity.
+
+  =THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY
+     EXPERIMENT.= This book is designed as a text-book for
+     amateurs, students, and others who wish to take up a
+     systematic course of simple experiments at home or in
+     school.
+
+  =THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.= This book
+     explains, in simple, straightforward language, many things
+     about electricity; things in which the American boy is
+     intensely interested; things he wants to know; things he
+     should know.
+
+  =ANS., OR ACCURACY, NEATNESS AND SPEED.= For teachers and
+     pupils. Containing study-charts, practice devices and
+     special methods for accurate, rapid work with figures.
+
+    _Ask Your Bookseller, Stationer, or Toy Dealer for our
+    Books, Games, Puzzles, Educational Amusements, Etc._
+
+
+    CATALOGUE UPON APPLICATION
+
+    Thomas M. St. John, 407 West 51st St., New York.
+
+       *       *       *       *       *
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+by
+
+THOMAS M. ST. JOHN, Met. E.
+
+Author of "Fun With Magnetism," "Fun With Electricity,"
+"How Two Boys Made Their Own Electrical Apparatus,"
+"The Study of Elementary Electricity
+and Magnetism by Experiment," etc.
+
+SECOND EDITION
+
+
+
+
+
+
+
+[Illustration]
+
+New York
+Thomas M. St. John
+407 West 51st Street
+1903
+
+Copyright, 1900.
+By Thomas M. St. John.
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+
+
+
+TABLE OF CONTENTS
+
+
+  CHAPTER                                                   PAGE
+      I. About Frictional Electricty                           7
+     II. About Magnets and Magnetism                          21
+    III. How Electricity is Generated by the Voltaic Cell,    32
+     IV. Various Voltaic Cells,                               36
+      V. About Push-Buttons, Switches and Binding-Posts,      43
+     VI. Units and Apparatus for Electrical Measurements,     48
+    VII. Chemical Effects of the Electric Current,            58
+   VIII. How Electroplating and Electrotyping are Done,       60
+     IX. The Storage Battery, and How it Works,               63
+      X. How Electricity is Generated by Heat,                68
+     XI. Magnetic Effects of the Electric Current,            71
+    XII. How Electricity is Generated by Induction,           77
+   XIII. How the Induction Coil Works,                        80
+    XIV. The Electric Telegraph, and How it Sends Messages,   84
+     XV. The Electric Bell and Some of its Uses,              91
+    XVI. The Telephone and How it Transmits Speech,           95
+   XVII. How Electricity is Generated by Dynamos,            101
+  XVIII. How the Electric Current is Transformed,            109
+    XIX. How Electric Currents are Distributed for Use,      114
+     XX. How Heat is Produced by the Electric Current,       124
+    XXI. How Light is Produced by the Incandescent Lamp,     129
+   XXII. How Light is Produced by the Arc Lamp,              135
+  XXIII. X-Rays, and How the Bones of the Human Body are
+             Photographed,                                   141
+   XXIV. The Electric Motor, and How it Does Work,           147
+    XXV. Electric Cars, Boats and Automobiles,               154
+   XXVI. A Word About Central Stations,                      162
+  XXVII. Miscellaneous Uses of Electricity,                  165
+
+
+
+
+TO THE READER
+
+
+For the benefit of those who wish to make their own electrical
+apparatus for experimental purposes, references have been made
+throughout this work to the "Apparatus Book;" by this is meant the
+author's "How Two Boys Made Their Own Electrical Apparatus."
+
+For those who wish to take up a course of elementary electrical
+experiments that can be performed with simple, home-made apparatus,
+references have been made to "Study;" by this is meant "The Study of
+Elementary Electricity and Magnetism by Experiment."
+
+                                                       THE AUTHOR.
+
+
+
+
+Things A Boy Should Know About Electricity
+
+
+
+
+CHAPTER I.
+
+ABOUT FRICTIONAL ELECTRICITY.
+
+
+=1. Some Simple Experiments.= Have you ever shuffled your feet along
+over the carpet on a winter's evening and then quickly touched your
+finger to the nose of an unsuspecting friend? Did he jump when a bright
+spark leaped from your finger and struck him fairly on the very tip of
+his sensitive nasal organ?
+
+[Illustration: Fig. 1.]
+
+Did you ever succeed in proving to the pussy-cat, Fig. 1, that
+something unusual occurs when you thoroughly rub his warm fur with your
+hand? Did you notice the bright sparks that passed to your hand when it
+was held just above the cat's back? You should be able to see, hear,
+and feel these sparks, especially when the air is dry and you are in a
+dark room.
+
+Did you ever heat a piece of paper before the fire until it was real
+hot, then lay it upon the table and rub it from end to end with your
+hand, and finally see it cling to the wall?
+
+Were you ever in a factory where there were large belts running rapidly
+over pulleys or wheels, and where large sparks would jump to your hands
+when held near the belts?
+
+If you have never performed any of the four experiments mentioned, you
+should try them the first time a chance occurs. There are dozens of
+simple, fascinating experiments that may be performed with this kind of
+electricity.
+
+=2. Name.= As this variety of electricity is made, or generated, by
+the friction of substances upon each other, it is called _frictional_
+electricity. It is also called _static_ electricity, because it
+generally stands still upon the surface of bodies and does not "flow in
+currents" as easily as some of the other varieties. Static electricity
+may be produced by induction as well as by friction.
+
+[Illustration: Fig. 2.]
+
+=3. History.= It has been known for over 2,000 years that certain
+substances act queerly when rubbed. Amber was the first substance upon
+which electricity was produced by friction, and as the Greek name for
+amber is _elektron_, bodies so affected were said to be _electrified_.
+When a body, like ebonite, is rubbed with a flannel cloth, we say that
+it becomes _charged with electricity_. Just what happens to the ebonite
+is not clearly understood. We know, however, that it will attract
+light bodies, and then quickly repel them if they be conductors. Fig.
+2 shows a piece of tissue-paper jumping toward a sheet of ebonite that
+has been electrified with a flannel cloth.
+
+=4. Conductors and Non-Conductors.= Electricity can be produced upon
+glass and ebonite because they do not carry or conduct it away. If a
+piece of iron be rubbed, the electricity passes from the iron into the
+earth as fast as it is generated, because the iron is a _conductor_ of
+electricity. Glass is an _insulator_ or _non-conductor_. Frictional
+electricity resides upon the outside, only, of conductors. A hollow
+tin box will hold as great a charge as a solid piece of metal having
+the same outside size and shape. When frictional electricity passes
+from one place to another, sparks are produced. Lightning is caused
+by the passage of static electricity from a cloud to the earth, or
+from one cloud to another. In this case air forms the conductor. (For
+experiments, see "Study," Chapter VII.)
+
+[Illustration: Fig. 3.]
+
+=5. Electroscopes.= A piece of carbon, pith, or even a small piece of
+damp tissue-paper will serve as an electroscope to test the presence of
+static electricity. The pith is usually tied to a piece of silk thread
+which is a non-conductor. Fig. 3 shows the ordinary form of _pith-ball
+electroscope_.
+
+The _leaf electroscope_ is a very delicate apparatus. Gold-leaf is
+generally used, but aluminum-leaf will stand handling and will do for
+all ordinary purposes. Fig. 4 shows a common form, the glass being
+used to keep currents of air from the leaves and at the same time to
+insulate them from the earth.
+
+Electroscopes are used to show the presence, relative amount, or kind
+of static electricity on a body. (See "Study," Chapter XI.)
+
+[Illustration: Fig. 4.]
+
+=6. Two Kinds of Electrification.= It can be shown that the
+electrification produced on all bodies by friction is not the same;
+for example, that generated with glass and silk is not the same as
+that made with ebonite and flannel. It has been agreed to call that
+produced by glass and silk _positive_, and that by ebonite and flannel
+_negative_. The signs + and - are used for positive and negative.
+
+=7. Laws of Electrification.= (1) Charges of the same kind repel each
+other; (2) charges of unlike kinds attract each other; (3) either kind
+of a charge attracts and is attracted by a neutral body.
+
+=8. Static Electric Machines.= In order to produce static electricity
+in quantities for experiments, some device is necessary.
+
+The _electrophorus_ (e-lec-troph´-o-rus) is about the simplest form
+of machine. Fig. 5 shows a simple electrophorus in which are two
+insulators and one conductor. The ebonite sheet E S is used with a
+flannel cloth to generate the electricity. The metal cover E C is
+lifted by the insulating handle E R. The cover E C is placed upon the
+thoroughly charged sheet E S, and then it is touched for an instant
+with the finger, before lifting it by E R. The charge upon E C can then
+be removed by bringing the hand near it. The bright spark that passes
+from E C to the hand indicates that E C has discharged itself into the
+earth. The action of the electrophorus depends upon induction. (For
+experiments, details of action, induced electrification, etc., see "The
+Study of Elementary Electricity and Magnetism by Experiment," Chapters
+VIII. and IX.)
+
+[Illustration: Fig. 5.]
+
+_The first electric machine_ consisted of a ball of sulphur fastened to
+a spindle which could be turned by a crank. By holding the hands or a
+pad of silk upon the revolving ball, electricity was produced.
+
+[Illustration: Fig. 6.]
+
+[Illustration: Fig. 7.]
+
+=9. The Cylinder Electric Machine= consists, as shown in Fig. 6, of a
+glass cylinder so mounted that it can be turned by a crank. Friction
+is produced by a pad of leather C, which presses against the cylinder
+as it turns. Electric sparks can be taken from the large "conductors"
+which are insulated from the earth. The opposite electricities unite
+with sparks across D and E. If use is to be made of the electricity,
+either the rubber or the prime conductor must be connected with the
+ground. In the former case positive electricity is obtained; in the
+latter, negative.
+
+=10. The Plate Electrical Machine.= Fig. 7 also shows an old form of
+machine. Such machines are made of circular plates of glass or ebonite,
+two rubbing pads being usually employed, one on each side of the plate.
+One operator is seen on an insulated stool (Fig. 7), the electricity
+passing through him before entering the earth by way of the body of the
+man at the right.
+
+[Illustration: Fig. 8.]
+
+=11. The Toepler-Holtz Machine=, in one form, is shown in Fig. 8. The
+electricity is produced by the principle of induction, and not by mere
+friction. This machine, used in connection with condensers, produces
+large sparks.
+
+=12. The Wimshurst Machine= is of recent date, and not being easily
+affected by atmospheric changes, is very useful for ordinary laboratory
+work. Fig. 9 shows one form of this machine.
+
+=13. Influence Machines for Medical Purposes= are made in a large
+variety of forms. A Wimshurst machine is generally used as an exciter
+to charge the plates of the large machine when they lose their charge
+on account of excessive moisture in the atmosphere. Fig. 10 shows a
+large machine.
+
+[Illustration: Fig. 9.]
+
+=14. Uses of Electrical Machines.= Static electricity has been used for
+many years in the laboratory for experimental purposes, for charging
+condensers, for medical purposes, etc. It is now being used for X-ray
+work, and considerable advancement has been made within a few years in
+the construction and efficiency of the machines.
+
+[Illustration: Fig. 10.]
+
+With the modern machines large sparks are produced by merely turning
+a crank, enough electricity being produced to imitate a small
+thunderstorm. The sparks of home-made lightning will jump several
+inches.
+
+Do not think that electricity is generated in a commercial way by
+static electric machines. The practical uses of static electricity are
+very few when compared with those of current electricity from batteries
+and dynamos.
+
+=15. Condensation of Static Electricity.= By means of apparatus called
+_condensers_, a terrific charge of static electricity may be stored.
+Fig. 11 shows the most common form of condenser, known as the _Leyden
+jar_. It consists of a glass jar with an inside and outside coating of
+tin-foil.
+
+[Illustration: Fig. 11.]
+
+[Illustration: Fig. 12.]
+
+_To charge_ the jar it is held in the hand so that the outside coating
+shall be connected with the earth, the sparks from an electric machine
+being passed to the knob at the top, which is connected by a chain to
+the inside coating.
+
+_To discharge_ the jar, Fig. 12, a conductor with an insulating handle
+is placed against the outside coat; when the other end of the conductor
+is swung over towards the knob, a bright spark passes between them.
+This device is called a discharger. Fig. 13 shows a discharge through
+ether which the spark ignites.
+
+[Illustration: Fig. 13.]
+
+=16. The Leyden Battery=, Fig. 14, consists of several jars connected
+in such a way that the area of the inner and outer coatings is greatly
+increased. The battery has a larger capacity than one of its jars. (For
+Experiments in Condensation, see "Study," Chapter X.)
+
+[Illustration: Fig. 14.]
+
+=17. Electromotive Force of Static Electricity.= Although the sparks
+of static electricity are large, the _quantity_ of electricity is very
+small. It would take thousands of galvanic cells to produce a spark
+an inch long. While the quantity of static electricity is small, its
+potential, or electromotive force (E. M. F.), is very high. We say that
+an ordinary gravity cell has an E. M. F. of a little over one volt.
+Five such cells joined in the proper way would have an E. M. F. of a
+little over five volts. You will understand, then, what is meant when
+we say that the E. M. F. of a lightning flash is millions of volts.
+
+=18. Atmospheric Electricity.= The air is usually electrified, even
+in clear weather, although its cause is not thoroughly understood. In
+1752 it was proved by Benjamin Franklin (Fig. 15), with his famous
+kite experiment, that atmospheric and frictional electricities are
+of the same nature. By means of a kite, the string being wet by the
+rain, he succeeded, during a thunderstorm, in drawing sparks, charging
+condensers, etc.
+
+[Illustration: Fig. 15.]
+
+[Illustration: Fig. 16.]
+
+=19. Lightning= may be produced by the passage of electricity between
+clouds, or between a cloud and the earth (Fig. 16), which, with the
+intervening air, have the effect of a condenser. When the attraction
+between the two electrifications gets great enough, a spark passes.
+When the spark has a zigzag motion it is called _chain lightning_.
+In hot weather flashes are often seen which light whole clouds, no
+thunder being heard. This is called _heat lightning_, and is generally
+considered to be due to distant discharges, the light of which is
+reflected by the clouds. The lightning flash represents billions of
+volts.
+
+[Illustration: Fig. 17.]
+
+=20. Thunder= is caused by the violent disturbances produced in the
+air by lightning. Clouds, hills, etc., produce echoes, which, with the
+original sound, make the rolling effect.
+
+=21. Lightning-Rods=, when well constructed, often prevent violent
+discharges. Their pointed prongs at the top allow the negative
+electricity of the earth to pass quietly into the air to neutralize
+the positive in the cloud above. In case of a discharge, or stroke of
+lightning, the rods aid in conducting the electricity to the earth. The
+ends of the rods are placed deep in the earth, Fig. 17.
+
+=22. St. Elmo's Fire.= Electrification from the earth is often drawn up
+from the earth through the masts of ships, Fig. 18, to neutralize that
+in the clouds, and, as it escapes from the points of the masts, light
+is produced.
+
+[Illustration: Fig. 18.]
+
+=23. Aurora Borealis=, also called Northern Lights, are luminous
+effects, Fig. 19, often seen in the north. They often occur at the
+same time with magnetic storms, when telegraph and telephone work may
+be disturbed. The exact cause of this light is not known, but it is
+thought by many to be due to disturbances in the earth's magnetism
+caused by the action of the sun.
+
+[Illustration: Fig. 19.]
+
+
+
+
+CHAPTER II.
+
+
+ABOUT MAGNETS AND MAGNETISM.
+
+=24. Natural Magnets.= Hundreds of years ago it was discovered that
+a certain ore of iron, called lodestone, had the power of picking up
+small pieces of iron. It was used to indicate the north and south
+line, and it was discovered later that small pieces of steel could be
+permanently magnetized by rubbing them upon the lodestone.
+
+=25. Artificial Magnets.= Pieces of steel, when magnetized, are called
+artificial magnets. They are made in many forms. The electromagnet is
+also an artificial magnet; this will be treated separately.
+
+[Illustration: Fig. 20]
+
+=26. The Horseshoe Magnet=, Fig. 20, is, however, the one with which we
+are the most familiar. They are always painted red, but the red paint
+has nothing to do with the magnetism.
+
+The little end-piece is called the keeper, or armature; it should
+always be kept in place when the magnet is not in use. The magnet
+itself is made of steel, while the armature is made of soft iron. Steel
+retains magnetism for a long time, while soft iron loses it almost
+instantly. The ends of the magnet are called its _poles_, and nearly
+all the strength of the magnet seems to reside at the poles, the curved
+part having no attraction for outside bodies. One of the poles of the
+magnet is marked with a line, or with the letter N. This is called the
+north pole of the magnet, the other being its south pole.
+
+[Illustration: Fig. 21.]
+
+=27. Bar Magnets= are straight magnets. Fig. 21 shows a round bar
+magnet. The screw in the end is for use in the telephone, described
+later.
+
+=28. Compound Magnets.= When several thin steel magnets are riveted
+together, a compound magnet is formed. These can be made with
+considerable strength. Fig. 22 shows a compound horseshoe magnet. Fig.
+23 shows a form of compound bar magnet used in telephones. The use of
+the coil of wire will be explained later. A thick piece of steel can
+not be magnetized through and through. In the compound magnet we have
+the effect of a thick magnet practically magnetized through and through.
+
+[Illustration: Fig. 22.]
+
+[Illustration: Fig. 23.]
+
+=29. Magnetic and Diamagnetic Bodies.= Iron, and substances containing
+iron, are the ones most readily attracted by a magnet. Iron is said to
+be _magnetic_. Some substances, like nickel, for example, are visibly
+attracted by very strong magnets only. Strange as it may seem, some
+substances are actually repelled by strong magnets; these are called
+_diamagnetic_ bodies. Brass, copper, zinc, etc., are not visibly
+affected by a magnet. Magnetism will act through paper, glass, copper,
+lead, etc.
+
+[Illustration: Fig. 24.]
+
+=30. Making Magnets.= One of the strangest properties that a magnet
+has is its power to give magnetism to another piece of steel. If
+a sewing-needle be properly rubbed upon one of the poles of a
+magnet, it will become strongly magnetized and will retain its
+magnetism for years. Strong permanent magnets are made with the aid
+of electromagnets. Any number of little magnets may be made from a
+horseshoe magnet without injuring it.
+
+[Illustration: Fig. 25.]
+
+31. Magnetic Needles and Compasses. If a bar magnet be suspended
+by a string, or floated upon a cork, which can easily be done with
+the magnet made from a sewing-needle, Fig. 24, it will swing around
+until its poles point north and south. Such an arrangement is called
+a _magnetic needle_. In the regular _compass_, a magnetic needle is
+supported upon a pivot. Compasses have been used for many centuries
+by mariners and others. Fig. 25 shows an ordinary pocket compass, and
+Fig. 26 a form of mariner's compass, in which the small bar magnets are
+fastened to a card which floats, the whole being so mounted that it
+keeps a horizontal position, even though the vessel rocks.
+
+[Illustration: Fig. 26.]
+
+32. Action of Magnets Upon Each Other. By making two small
+sewing-needle magnets, you can easily study the laws of attraction and
+repulsion. By bringing the two north poles, or the two south poles,
+near each other, a repulsion will be noticed. Unlike poles attract each
+other. The attraction between a magnet and iron is mutual; that is,
+each attracts the other. Either pole of a magnet attracts soft iron.
+
+In magnetizing a needle, either end may be made a north pole at will;
+in fact, the poles of a weak magnet can easily be reversed by properly
+rubbing it upon a stronger magnet.
+
+=33. Theory of Magnetism.= Each little particle of a piece of steel or
+iron is supposed to be a magnet, even before it touches a magnet. When
+these little magnets are thoroughly mixed up in the steel, they pull in
+all sorts of directions upon each other and tend to keep the steel from
+attracting outside bodies. When a magnet is properly rubbed upon a bar
+of steel, the north poles of the little molecular magnets of the steel
+are all made to point in the same direction. As the north poles help
+each other, the whole bar can attract outside bodies.
+
+By jarring a magnet its molecules are thoroughly shaken up; in fact,
+most of the magnetism can be knocked out of a weak magnet by hammering
+it.
+
+=34. Retentivity.= The power that a piece of steel has to hold
+magnetism is called _retentivity_. Different kinds of steel have
+different retentivities. A sewing-needle of good steel will retain
+magnetism for years, and it is almost impossible to knock the magnetism
+out by hammering it. Soft steel has very little retentivity, because
+it does not contain much carbon. Soft iron, which contains less
+carbon than steel, holds magnetism very poorly; so it is not used for
+permanent magnets. A little magnetism, however, will remain in the
+soft iron after it is removed from a magnet. This is called _residual
+magnetism_.
+
+=35. Heat and Magnetism.= Steel will completely lose its magnetism
+when heated to redness, and a magnet will not attract red-hot iron.
+The molecules of a piece of red-hot iron are in such a state of rapid
+vibration that they refuse to be brought into line by the magnet.
+
+=36. Induced Magnetism.= A piece of soft iron may be induced to become
+a magnet by holding it near a magnet, absolute contact not being
+necessary. When the soft iron is removed, again, from the influence of
+the magnet, its magnetism nearly all disappears. It is said to have
+_temporary_ magnetism; it had _induced_ magnetism. If a piece of soft
+iron be held near the north pole of a magnet, as in Fig. 27, poles will
+be produced in the soft iron, the one nearest the magnet being the
+south pole, and the other the north pole.
+
+[Illustration: Fig. 27.]
+
+[Illustration: Fig. 28.]
+
+=37. Magnetic Field.= If a bar magnet be laid upon the table, and a
+compass be moved about it, the compass-needle will be attracted by the
+magnet, and it will point in a different direction for every position
+given to the compass. This strange power, called magnetism, reaches out
+on all sides of a magnet. The magnet may be said to act by induction
+upon the compass-needle. The space around the magnet, in which this
+inductive action takes place, is called the _magnetic field_. Fig. 28
+shows some of the positions taken by a compass-needle when moved about
+on one side of a bar magnet.
+
+[Illustration: Fig. 29.]
+
+[Illustration: Fig. 30.]
+
+=38. Magnetic Figures= can be made by sprinkling iron filings upon a
+sheet of paper under which is placed a magnet. Fig. 29 shows a magnetic
+figure made with an ordinary bar magnet. The magnet was placed upon the
+table and over this was laid a piece of smooth paper. Fine iron filings
+were sifted upon the paper, which was gently tapped so that the filings
+could arrange themselves. As each particle of iron became a little
+magnet, by induction, its poles were attracted and repelled by the
+magnet; and when the paper was tapped they swung around to their final
+positions. Notice that the filings have arranged themselves in lines.
+These lines show the positions of some of the _lines of magnetic force_
+which surrounded the magnet.
+
+These lines of force pass from the north pole of a magnet through the
+air on all sides to its south pole.
+
+[Illustration: Fig. 31.]
+
+Fig. 30 shows a magnetic figure made from two bar magnets placed side
+by side, their unlike poles being next to each other. Fig. 31 shows
+the magnetic figure of a horseshoe magnet with round poles, the poles
+being uppermost.
+
+=39. The Use of Armatures.= A magnet attracts iron most strongly at its
+poles, because it is at the poles that the greatest number of lines
+of force pass into the air. Lines of force pass easily through soft
+iron, which is said to be a good conductor of them. Air is not a good
+conductor of the lines of force; in order, then, for the lines of force
+to pass from the north pole of a magnet to its south pole, they must
+overcome this resistance of the air, unless the armature is in place. A
+magnet will gradually grow weaker when its armature is left off.
+
+=40. Terrestrial Magnetism.= As the compass-needle points to the north
+and south, the earth must act like a magnet. There is a place very far
+north, about a thousand miles from the north pole of the earth, which
+is called the earth's north magnetic pole. Compass-needles point to
+this place, and not to the earth's real north pole. You can see, then,
+that if a compass be taken north of this magnetic pole, its north pole
+will point south. Lines of force pass from the earth's north magnetic
+pole through the air on all sides of the earth and enter the earth's
+south magnetic pole. The compass-needle, in pointing toward the north
+magnetic pole, merely takes the direction of the earth's lines of
+force, just as the particles of iron filings arrange themselves in the
+magnetic figures.
+
+=41. Declination.= As the magnetic needle does not point exactly to the
+north, an angle is formed between the true north and south line and the
+line of the needle. In Fig. 32 the line marked N S is the true north
+and south line. The _angle of variation_, or the declination, is the
+angle A between the line N S and the compass-needle.
+
+[Illustration: Fig. 32.]
+
+[Illustration: Fig. 33.]
+
+=42. Dip or Inclination.= If a piece of steel be carefully balanced
+upon a support, and then magnetized, it will be found that it will no
+longer balance. The north pole will _dip_ or point downward. Fig. 33
+shows what happens to a needle when it is held in different positions
+over a bar magnet. It simply takes the directions of the lines of
+force as they pass from the north to the south pole of the magnet.
+As the earth's lines of force pass in curves from the north to the
+south magnetic pole, you can see why the magnetic needle dips, unless
+its south pole is made heavier than its north. Magnetic needles are
+balanced after they are magnetized.
+
+[Illustration: Fig. 34.]
+
+Fig. 34 shows a simple form of dipping needle. These are often used
+by geologists and miners. In the hands of the prospector, the
+miner's compass, or dipping needle, proves a serviceable guide to the
+discovery and location of magnetic iron ore. In this instrument the
+magnetic needle is carefully balanced upon a horizontal axis within a
+graduated circle, and in which the needle will be found to assume a
+position inclined to the horizon. This angle of deviation is called the
+_inclination_ or _dip_, and varies in different latitudes, and even at
+different times in the same place.
+
+=43. The Earth's Inductive Influence.= The earth's magnetism acts
+inductively upon pieces of steel or iron upon its surface. If a piece
+of steel or iron, like a stove poker, for example, be held in a north
+and south line with its north end dipping considerably, it will be
+in the best position for the magnetism of the earth to act upon it;
+that is, it will lie in the direction taken by the earth's lines of
+force. If the poker be struck two or three times with a hammer to
+shake up its molecules, we shall find, upon testing it, that it has
+become magnetized. By this method we can pound magnetism right out of
+the air with a hammer. If the magnetized poker be held level, in an
+east and west direction, it will no longer be acted upon to advantage
+by the inductive influence of the earth, and we can easily hammer the
+magnetism out of it again. (For experiments on magnets and magnetism
+see "Study," Part I.)
+
+
+
+
+CHAPTER III.
+
+HOW ELECTRICITY IS GENERATED BY THE VOLTAIC CELL.
+
+
+=44. Early Experiments.= In 1786 Galvani, an Italian physician, made
+experiments to study the effect of static electricity upon the nervous
+excitability of animals, and especially upon the frog. He found that
+electric machines were not necessary to produce muscular contractions
+or kicks of the frog's legs, and that they could be produced when two
+different metals, Fig. 35, like iron and copper, for example, were
+placed in proper contact with a nerve and a muscle and then made to
+touch each other. Galvani first thought that the frog generated the
+electricity instead of the metals.
+
+[Illustration: Fig. 35.]
+
+Volta proved that the electricity was caused by the contact of the
+metals. He used the condensing electroscope as one means of proving
+that two dissimilar metals become charged differently when in contact.
+Volta also carried out his belief by constructing what is called a
+_Voltaic Pile_. He thought that by making several pairs of metals so
+arranged that all the little currents would help each other, a strong
+current could be generated. Fig. 36 shows a _pile_, it being made by
+placing a pair of zinc and copper discs in contact with one another,
+then laying on the copper disc a piece of flannel soaked in brine, then
+on top of this another pair, etc., etc. By connecting the first zinc
+and the last copper, quite a little current was produced. This was a
+start from which has been built our present knowledge of electricity.
+Strictly speaking, electricity is not generated by combinations of
+metals or by cells; they really keep up a difference of potential, as
+will be seen.
+
+[Illustration: Fig. 36.]
+
+[Illustration: Fig. 37.]
+
+[Illustration: Fig. 38.]
+
+=45. The Simple Cell.= It has been stated that two different kinds of
+electrifications may be produced by friction; one positive, the other
+negative. Either can be produced, at will, by using proper materials.
+Fig. 37 shows a section of a _simple cell_; Fig. 38 shows another view.
+Cu is a piece of copper, and Zn a piece of zinc. When they are placed
+in dilute sulphuric acid, it can be shown by delicate apparatus that
+they become charged differently, because the acid acts differently
+upon the plates. They become charged by chemical action, and not by
+friction. The zinc is gradually dissolved, and it is this chemical
+burning of the zinc that furnishes energy for the electric current in
+the simple cell. The electrification, or charge, on the plates tends to
+flow from the place of higher to the place of lower potential, just as
+water tends to flow down hill. If a wire be joined to the two metals, a
+constant current of electricity will flow through it, because the acid
+continues to act upon the plates. The simple cell is a _single-fluid_
+cell, as but one liquid is used in its construction.
+
+=45a. Plates and Poles.= The metal strips used in voltaic cells are
+called _plates_ or _elements_. The one most acted upon by the acid
+is called the positive (+) plate. In the simple cell the zinc is the
++ plate, and the copper the negative (-) plate. The end of a wire
+attached to the - plate is called the + pole, or electrode. Fig. 37
+shows the negative (-) electrode as the end of the wire attached to the
++ plate.
+
+=46. Direction of Current.= In the cell the current passes from the
+zinc to the copper; that is, from the positive to the negative plate,
+where bubbles of hydrogen gas are deposited. In the wire connecting the
+plates, the current passes from the copper to the zinc plate. In most
+cells, carbon takes the place of copper. (See "Study," § 268.)
+
+=47. Local Currents; Amalgamation.= Ordinary zinc contains impurities
+such as carbon, iron, etc., and when the acid comes in contact with
+these, they form with the zinc a small cell. This tends to eat away the
+zinc without producing useful currents. The little currents in the cell
+from this cause are called _local currents_. (See "Study," Exp. 111, §
+273.) This is largely overcome by coating the zinc with mercury. This
+process is called _amalgamation_. It makes the zinc act like pure zinc,
+which is not acted upon by dilute sulphuric acid when the current does
+not pass. (See "Study," § 257, 274.)
+
+=48. Polarization of Cells.= Bubbles of hydrogen gas are formed when
+zinc is dissolved by an acid. In the ordinary simple cell these bubbles
+collect on the copper plate, and not on the zinc plate, as might be
+expected. The hydrogen is not a conductor of electricity, so this film
+of gas holds the current back. The hydrogen acts like a metal and sets
+up a current that opposes the zinc to the copper current. Several
+methods are employed to get rid of the hydrogen. (See "Study," § 278,
+279, 280.)
+
+
+
+
+CHAPTER IV.
+
+VARIOUS VOLTAIC CELLS.
+
+
+=49. Single-Fluid and Two-Fluid Cells.= The simple cell (§ 45) is a
+single-fluid cell. The liquid is called the _electrolyte_, and this
+must act upon one of the plates; that is, chemical action must take
+place in order to produce a current. The simple cell polarizes rapidly,
+so something must be used with the dilute sulphuric acid to destroy the
+hydrogen bubbles. This is done in the _bichromate of potash cell_.
+
+In order to get complete depolarization--that is, to keep the carbon
+plate almost perfectly free from hydrogen, it is necessary to use
+_two-fluid cells_, or those to which some solid depolarizer is added to
+the one fluid.
+
+=50. Open and Closed Circuit Cells.= If we consider a voltaic cell, the
+wires attached to it, and perhaps some instrument through which the
+current passes, we have an _electric circuit_. When the current passes,
+the circuit is _closed_, but when the wire is cut, or in any way
+disconnected so that the current can not pass, the circuit is _open_ or
+_broken_. (See "Study," § 266.)
+
+_Open Circuit Cells_ are those which can give momentary currents at
+intervals, such as are needed for bells, telephones, etc. These must
+have plenty of time to rest, as they polarize when the circuit is
+closed for a long time. The _Leclanché_ and _dry_ cells are the most
+common open circuit cells.
+
+_Closed Circuit Cells._ For telegraph lines, motors, etc., where a
+current is needed for some time, the cell must be of such a nature
+that it will not polarize quickly; it must give a strong and constant
+current. The _bichromate_ and _gravity cells_ are examples of this
+variety. (See "Study," § 286.)
+
+=51. Bichromate of Potash Cells= are very useful for general laboratory
+work. They are especially useful for operating induction coils, small
+motors, small incandescent lamps, for heating platinum wires, etc.
+These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is
+used as the exciting fluid, and in this is dissolved the bichromate of
+potash which keeps the hydrogen bubbles from the carbon plate. (See
+"Apparatus Book," § 26.) Zinc and carbon are used for the plates, the +
+pole being the wire attached to the carbon.
+
+[Illustration: Fig. 39.]
+
+Fig. 39 shows one form of bichromate cell. It furnishes a large
+quantity of current, and as the zinc can be raised from the fluid, it
+may be kept charged ready for use for many months, and can be set in
+action any time when required by lowering the zinc into the liquid. Two
+of these cells will burn a one candle-power miniature incandescent lamp
+several hours. The carbon is indestructible.
+
+    =Note.= For various forms of home-made cells, see "Apparatus
+    Book," Chapter I., and for battery fluids see Chapter II.
+
+=52. The Grenet Cell.= Fig. 40 is another form of bichromate cell. The
+carbon plates are left in the fluid constantly. The zinc plate should
+be raised when the cell is not in use, to keep it from being uselessly
+dissolved.
+
+[Illustration: Fig. 40.]
+
+[Illustration: Fig. 41.]
+
+=53. Plunge Batteries.= Two or more cells are often arranged so that
+their elements can be quickly lowered into the acid solution. Such a
+combination, Fig. 41, is called a _plunge battery_. The binding-posts
+are so arranged that currents of different strengths can be taken from
+the combination. The two binding-posts on the right of the battery
+will give the current of one cell; the two binding-posts on the left
+of the battery will give the current of two cells, and the two end
+binding-posts will give the current of all three cells. When not in
+use the elements must always be hung on the hooks and kept out of the
+solution.
+
+=54. Large Plunge Batteries=. Fig. 42, are arranged with a winch and
+a bar above the cells; these afford a ready and convenient means of
+lifting or lowering the elements and avoiding waste. In the battery
+shown, Fig. 42, the zincs are 4×6 inches; the carbons have the same
+dimensions, but there are two carbon plates to each zinc, thus giving
+double the carbon surface.
+
+[Illustration: Fig. 42.]
+
+=55. The Fuller Cell=, Fig. 43, is another type of bichromate cell,
+used largely for long-distance telephone service, for telephone
+exchange and switch service, for running small motors, etc. It consists
+of a glass jar, a carbon plate, with proper connections, a clay porous
+cup, containing the zinc, which is made in the form of a cone. A little
+mercury is placed in the porous cup to keep the zinc well amalgamated.
+Either bichromate of potash or bichromate of soda can be used as a
+depolarizer.
+
+[Illustration: Fig. 43.]
+
+[Illustration: Fig. 44.]
+
+=56. The Gravity Cell=, sometimes called the _bluestone_ or _crowfoot_
+cell, is used largely for telegraph, police, and fire-alarm signal
+service, laboratory and experimental work, or whenever a closed circuit
+cell is required. The E.M.F. is about one volt. This is a modified form
+of the Daniell cell. Fig. 44 shows a home-made gravity cell.
+
+A copper plate is placed at the bottom of the glass jar, and upon
+this rests a solution of copper sulphate (bluestone). The zinc plate
+is supported about four inches above the copper, and is surrounded
+by a solution of zinc sulphate which floats upon the top of the blue
+solution. An insulated wire reaches from the copper to the top of the
+cell and forms the positive pole. (See "Apparatus Book," § 11 to 15,
+for home-made gravity cell, its regulation, etc. For experiments with
+two-fluid Daniell cell, see "Study," Exp. 113, § 281 to 286.)
+
+[Illustration: Fig. 45.]
+
+=56a. Bunsen Cells,= Fig. 45, are used for motors, small incandescent
+lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of
+which is a cylinder of zinc that stands in dilute sulphuric acid, the
+carbon being in nitric acid.
+
+=57. The Leclanché Cell= is an open circuit cell. Sal ammoniac is used
+as the exciting fluid, carbon and zinc being used for plates. Manganese
+dioxide is used as the depolarizer; this surrounds the carbon plate,
+the two being either packed together in a porous cup or held together
+in the form of cakes. The porous cup, or pressed cake, stands in the
+exciting fluid. The E. M. F. is about 1.5 volts.
+
+[Illustration: Fig. 46.]
+
+[Illustration: Fig. 47.]
+
+[Illustration: Fig. 48.]
+
+[Illustration: Fig. 49.]
+
+Fig. 46 shows a form with porous cup. The binding-post at the top of
+the carbon plate forms the + electrode, the current leaving the cell at
+this point.
+
+_The Gonda Prism Cell_ (Fig. 47), is a form of Leclanché in which the
+depolarizer is in the form of a cake.
+
+=58. Dry Cells= are open circuit cells, and can be carried about,
+although they are moist inside. The + pole is the end of the carbon
+plate. Zinc is used as the outside case and + plate. Fig. 48 shows the
+ordinary forms.
+
+Fig. 49 shows a number of dry cells arranged in a box with switch in
+front, so that the current can be regulated at will.
+
+[Illustration: Fig. 50.]
+
+=59. The Edison-Lelande Cells=, Fig. 50, are made in several sizes and
+types. Zinc and copper oxide, which is pressed into plates, form the
+elements. The exciting fluid consists of a 25 per cent. solution of
+caustic potash in water. They are designed for both open and closed
+circuit work.
+
+
+
+
+CHAPTER V.
+
+ABOUT PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.
+
+
+=60. Electrical Connections.= In experimental work, as well as in
+the everyday work of the electrician, electrical connections must
+constantly be made. One wire must be joined to another, just for a
+moment, perhaps, or one piece of apparatus must be put in an electric
+circuit with other apparatus, or the current must be turned on or off
+from motors, lamps, etc. In order to conveniently and quickly make such
+connections, apparatus called push-buttons, switches and binding-posts
+are used.
+
+[Illustration: Fig. 51.]
+
+[Illustration: Fig. 52.]
+
+=61. Push-Buttons.= The simple act of pressing your finger upon a
+movable button, or knob, may ring a bell a mile away, or do some other
+equally wonderful thing. Fig. 51 shows a simple push-button, somewhat
+like a simple key in construction. If we cut a wire, through which a
+current is passing, then join one of the free ends to the screw A and
+the other end to screw C, we shall be able to let the current pass at
+any instant by pressing the spring B firmly upon A.
+
+Push-buttons are made in all sorts of shapes and sizes. Fig. 52 gives
+an idea of the general internal construction. The current enters A by
+one wire, and leaves by another wire as soon as the button is pushed
+and B is forced down to A. The bottom of the little button rests upon
+the top of B.
+
+Fig. 53 shows a _Table Clamp-Push_ for use on dining-tables,
+card-tables, chairs, desks, and other movable furniture. Fig. 54 shows
+a combination of push-button, speaking-tube, and letter-box used in
+city apartment houses. Fig. 55 shows an _Indicating Push_. The buzzer
+indicates, by the sound, whether the call has been heard; that is, the
+person called answers back.
+
+[Illustration: Fig. 53.]
+
+[Illustration: Fig. 54.]
+
+_Modifications_ of ordinary push-buttons are used for floor
+push-buttons, on doors, windows, etc., for burglar-alarms, for turning
+off or on lights, etc., etc. (See "Apparatus Book," Chapter III., for
+home-made push-buttons.)
+
+[Illustration: Fig. 55.]
+
+=62. Switches= have a movable bar or plug of metal, moving on a pivot,
+to make or break a circuit, or transfer a current from one conductor to
+another.
+
+Fig. 56 shows a _single point switch_. The current entering the pivoted
+arm can go no farther when the switch is open, as shown. To close
+the circuit, the arm is pushed over until it presses down upon the
+contact-point. For neatness, both wires are joined to the under side of
+the switch or to binding-posts.
+
+[Illustration: Fig. 56.]
+
+Fig. 57 shows a _knife switch_. Copper blades are pressed down between
+copper spring clips to close the circuit. The handle is made of
+insulating material.
+
+_Pole-changing switches_, Fig. 58, are used for changing or reversing
+the poles of batteries, etc.
+
+Fig. 59 shows a home-made switch, useful in connection with resistance
+coils. By joining the ends of the coils A, B, C, D, with the
+contact-points 1, 2, 3, etc., more or less resistance can be easily
+thrown in by simply swinging the lever E around to the left or right.
+If E be turned to 1, the current will be obliged to pass through all
+the coils A, B, etc., before it can pass out at Y. If E be moved to
+3, coils A and B will be cut out of the circuit, thus decreasing the
+resistance to the current on its way from X to Y. Current regulators
+are made upon this principle. (See "Apparatus Book," Chapter IV., for
+home-made switches.)
+
+[Illustration: Fig. 57.]
+
+[Illustration: Fig. 58.]
+
+[Illustration: Fig. 59.]
+
+_Switchboards_ are made containing from two or three to hundreds of
+switches, and are used in telegraph and telephone work, in electric
+light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60
+shows a switch used for incandescent lighting currents.
+
+[Illustration: Fig. 60.]
+
+[Illustration: Fig. 61.]
+
+=63. Binding-Posts= are used to make connections between two pieces of
+apparatus, between two or more wires, between a wire and any apparatus,
+etc., etc. They allow the wires to be quickly fastened or unfastened
+to the apparatus. A large part of the apparatus shown in this book has
+binding-posts attached. Fig. 61 shows a few of the common forms used.
+(See "Apparatus Book," Chapter V., for home-made binding-posts.)
+
+
+
+
+CHAPTER VI.
+
+UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.
+
+
+=64. Electrical Units.= In order to measure electricity for
+experimental or commercial purposes, standards or units are just as
+necessary as the inch or foot for measuring distances.
+
+=65. Potential; Electromotive Force.= If water in a tall tank be
+allowed to squirt from two holes, one near the bottom, the other near
+the top, it is evident that the force of the water that comes from the
+hole at the bottom will be the greater. The pressure at the bottom is
+greater than that near the top, because the "head" is greater.
+
+When a spark of static electricity jumps a long distance, we say that
+the charge has a high _potential_; that is, it has a high electrical
+pressure. Potential, for electricity, means the same as pressure, for
+water. The greater the potential, or _electromotive force_ (E.M.F.) of
+a cell, the greater its power to push a current through wires. (See
+"Study," § 296 to 305, with experiments.)
+
+=66. Unit of E.M.F.; the Volt.=--In speaking of water, we say that its
+pressure is so many pounds to the square inch, or that it has a fall,
+or head, of so many feet. We speak of a current as having so many
+volts; for example, we say that a wire is carrying a 110-volt current.
+The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F.
+of about one volt. This name was given in honor of Volta.
+
+=67. Measurement of Electromotive Force.= There are several ways by
+which the E.M.F. of a cell, for example, can be measured. It is usually
+measured _relatively_, by comparison with the E. M. F. of some standard
+cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by
+comparison with the two-fluid cell.)
+
+[Illustration: Fig. 62.]
+
+_Voltmeters_ are instruments by means of which E. M. F. can be read on
+a printed scale. They are a variety of galvanometer, and are made with
+coils of such high resistance, compared with the resistance of a cell
+or dynamo, that the E. M. F. can be read direct. The reason for this
+will be seen by referring to Ohm's law ("Study," § 356); the resistance
+is so great that the strength of the current depends entirely upon the
+E. M. F.
+
+[Illustration: Fig. 63.]
+
+Voltmeters measure electrical pressure just as steam gauges measure
+the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63
+shows a voltmeter with illuminated dial. An electrical bulb behind the
+instrument furnishes light so that the readings can be easily taken.
+
+=68. Electrical Resistance.= Did you ever ride down hill on a
+hand-sled? How easily the sled glides over the snow! What happens,
+though, when you strike a bare place, or a place where some evil-minded
+person has sprinkled ashes? Does the sled pass easily over bare ground
+or ashes? Snow offers very little _resistance_ to the sled, while ashes
+offer a great resistance.
+
+[Illustration: Fig. 64.]
+
+All substances do not allow the electric current to pass through
+them with the same ease. Even the liquid in a cell tends to hold the
+current back and offers _internal resistance_. The various wires and
+instruments connected to a cell offer _external resistance_. (See
+"Study," Chapter XVIII., for experiments, etc.)
+
+=69. Unit of Resistance.= =The Ohm= is the name given to the unit of
+resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in.
+of No. 24 copper wire, will make a fairly accurate ohm.
+
+_Resistance coils_, having carefully measured resistances, are made
+for standards. (See "Apparatus Book," Chapter XVII., for home-made
+resistance coils.) Fig. 64 shows a commercial form of a standard
+resistance coil. The coil is inclosed in a case and has large wires
+leading from its ends for connections. Fig. 65 gives an idea of
+the way in which coils are wound and used with plugs to build up
+_resistance boxes_, Fig. 66.
+
+=70. Laws of Resistance.= 1. The resistance of a wire is directly
+proportional to its length, provided its cross-section, material, etc.,
+are uniform.
+
+2. The resistance of a wire is inversely proportional to its area of
+cross-section; or, in other words, inversely proportional to the square
+of its diameter, other things being equal.
+
+[Illustration: Fig. 65.]
+
+3. The resistance of a wire depends upon its material, as well as upon
+its length, size, etc.
+
+4. The resistance of a wire increases as its temperature rises. (See
+"Study," Chapters XVIII. and XIX., for experiments on resistance, its
+measurement, etc.)
+
+[Illustration: Fig. 66.]
+
+=71. Current Strength.= The strength of a current at the end of a
+circuit depends not only upon the _electrical pressure_, or E. M. F.,
+which drives the current, but also upon the _resistance_ which has to
+be overcome. The greater the resistance the weaker the current at the
+end of its journey.
+
+=72. Unit of Current Strength; The Ampere.= A current having an E. M.
+F. of _one volt_, pushing its way through a resistance of _one ohm_,
+would have a unit of strength, called _one ampere_. This current, one
+ampere strong, would deposit, under proper conditions, .0003277 gramme
+of copper in _one second_ from a solution of copper sulphate.
+
+=73. Measurement of Current Strength.= A magnetic needle is deflected
+when a current passes around it, as in instruments like the
+galvanometer. The _galvanoscope_ merely indicates the presence of a
+current. _Galvanometers_ measure the strength of a current, and they
+are made in many forms, depending upon the nature and strength of the
+currents to be measured. Galvanometers are standardized, or calibrated,
+by special measurements, or by comparison with some standard
+instrument, so that when the deflection is a certain number of degrees,
+the current passing through it is known to be of a certain strength.
+
+[Illustration: Fig. 67.]
+
+Fig. 67 shows an _astatic galvanometer_. Fig. 68 shows a _tangent
+galvanometer_, in which the strength of the current is proportional
+to the tangent of the angle of deflection. Fig. 69 shows a _D'Arsonval
+galvanometer_, in which a coil of wire is suspended between the poles
+of a permanent horseshoe magnet. The lines of force are concentrated
+by the iron core of the coil. The two thin suspending wires convey the
+current to the coil. A ray of light is reflected from the small mirror
+and acts as a pointer as in other forms of reflecting galvanometers.
+
+[Illustration: Fig. 68.]
+
+=74. The Ammeter=, Fig. 70, is a form of galvanometer in which the
+strength of a current, in amperes, can be read. In these the strength
+of current is proportional to the angular deflections. The coils are
+made with a small resistance, so that the current will not be greatly
+reduced in strength in passing through them.
+
+[Illustration: Fig. 69.]
+
+=75. Voltameters= measure the strength of a current by chemical means,
+the quantity of metal deposited or gas generated being proportional
+to the time that the current flows and to its strength. In the _water
+voltameter_, Fig. 71, the hydrogen and oxygen produced in a given time
+are measured. (See "Study," Chapter XXI.)
+
+[Illustration: Fig. 70.]
+
+The _copper voltameter_ measures the amount of copper deposited in a
+given time by the current. Fig. 72 shows one form. The copper cathode
+is weighed before and after the current flows. The weight of copper
+deposited and the time taken are used to calculate the current strength.
+
+[Illustration: Fig. 71.]
+
+=76. Unit of Quantity=; =The Coulomb= is the quantity of electricity
+given, in _one second_, by a current having a strength of one ampere.
+Time is an important element in considering the work a current can do.
+
+[Illustration: Fig. 72.]
+
+=77. Electrical Horse-power=; =The Watt= is the unit of electrical
+power. A current having the strength of one ampere, and an E. M.
+F. of one volt has a unit of power. 746 watts make one electrical
+horse-power. Watts = amperes × volts. Fig. 73 shows a direct reading
+wattmeter based on the international volt and ampere. They save taking
+simultaneous ammeter and voltmeter readings, which are otherwise
+necessary to get the product of volts and amperes, and are also used on
+alternating current measurements.
+
+[Illustration: Fig. 73.]
+
+There are also forms of wattmeters, Fig. 74, in which the watts are
+read from dials like those on an ordinary gas-meter, the records being
+permanent.
+
+Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit
+that readings can be taken. D represents a dynamo. A is placed so that
+the whole current passes through it, while V is placed between the main
+wires to measure the difference in potential. The product of the two
+readings in volts and amperes gives the number of watts.
+
+[Illustration: Fig. 74.]
+
+=78. Chemical Meters= also measure the quantity of current that is
+used; for example, one may be placed in the cellar to measure the
+quantity of current used to light the house.
+
+[Illustration: Fig. 75.]
+
+Fig. 76 shows a chemical meter, a part of the current passing through
+a jar containing zinc plates and a solution of zinc sulphate. Metallic
+zinc is dissolved from one plate and deposited upon the other. The
+increase in weight shows the amount of chemical action which is
+proportional to the ampere hours. Knowing the relation between the
+quantity of current that can pass through the solution to that which
+can pass through the meter by another conductor, a calculation can be
+made which will give the current used. A lamp is so arranged that it
+automatically lights before the meter gets to the freezing-point; this
+warms it up to the proper temperature, at which point the light goes
+out again.
+
+[Illustration: Fig. 76.]
+
+
+
+
+CHAPTER VII.
+
+CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.
+
+
+=79. Electrolysis.= It has been seen that in the voltaic cell
+electricity is generated by chemical action. Sulphuric acid acts upon
+zinc and dissolves it in the cell, hydrogen is produced, etc. When
+this process is reversed, that is, when the electric current is passed
+through some solutions, they are decomposed, or broken up into their
+constituents. This process is called _electrolysis_, and the compound
+decomposed is the _electrolyte_. (See "Study," § 369, etc., with
+experiments.)
+
+[Illustration: Fig. 77.]
+
+Fig. 77 shows how water can be decomposed into its two constituents,
+hydrogen and oxygen, there being twice as much hydrogen formed as
+oxygen.
+
+Fig. 78 shows a glass jar in which are placed two metal strips, A and
+C, these being connected with two cells. In this jar may be placed
+various conducting solutions to be tested. If, for example, we use
+a solution of copper sulphate, its chemical formula being CuSO_{4},
+the current will break it up into Cu (copper) and SO_{4}. The Cu will
+be deposited upon C as the current passes from A to C through the
+solution. A is called the _anode_, and C the _cathode_.
+
+[Illustration: Fig. 78.]
+
+Fig. 79 shows another form of jar used to study the decomposition of
+solutions by the electric current.
+
+[Illustration: Fig 79.]
+
+=80. Ions.= When a solution is decomposed into parts by a current, the
+parts are called the _Ions_. When copper sulphate (Cu SO_{4}) is used,
+the ions are Cu, which is a metal, and SO_{4}, called an acid radical.
+When silver nitrate (Ag NO_{3}) is used, Ag and NO_{3} are the ions.
+The metal part of the compound goes to the cathode.
+
+
+
+
+CHAPTER VIII.
+
+HOW ELECTROPLATING AND ELECTROTYPING ARE DONE.
+
+
+=81. Electricity and Chemical Action.= We have just seen, Chapter VII.,
+that the electric current has the power to decompose certain compounds
+when they are in solution. By choosing the right solutions, then, we
+shall be able to get copper, silver, and other metals set free by
+electrolysis.
+
+=82. Electroplating= consists in coating substances with metal with
+the aid of the electric current. If we wish to electroplate a piece
+of metal with copper, for example, we can use the arrangement shown
+in Fig. 78, in which C is the cathode plate to be covered, and A is
+a copper plate. The two are in a solution of copper sulphate, and,
+as explained in § 79, the solution will be decomposed. Copper will
+be deposited upon C, and the SO_{4} part of the solution will go to
+the anode A, which it will attack and gradually dissolve. The SO_{4},
+acting upon the copper anode, makes CuSO_{4} again, and this keeps the
+solution at a uniform strength. The amount of copper dissolved from the
+copper anode equals, nearly, the amount deposited upon the cathode. The
+metal is carried in the direction of the current.
+
+If we wish to plate something with silver or gold, it will be necessary
+to use a solution of silver or gold for the electrolyte, a plate of
+metallic silver or gold being used for the anode, as the case may be.
+
+Great care is used in cleaning substances to be plated, all dirt and
+grease being carefully removed.
+
+Fig. 80 shows a plating bath in which several articles can be plated
+at the same time by hanging them upon a metal bar which really forms a
+part of the cathode. If, for example, we wish to plate knives, spoons,
+etc., with silver, they would be hung from the bar shown, each being a
+part of the cathode. The vat would contain a solution of silver, and
+from the other bar would be hung a silver plate having a surface about
+equal to that of the combined knives, etc.
+
+[Illustration: Fig. 80.]
+
+Most metals are coated with copper before they are plated with silver
+or gold. When plating is done on a large scale, a current from a dynamo
+is used. For experimental purposes a Gravity cell will do very well.
+(See "Study," § 374 to 380 with experiments.)
+
+=83. Electrotyping.= It was observed by De La Rue in 1836 that in the
+Daniell cell an even coating of copper was deposited upon the copper
+plate. From this was developed the process of electrotyping, which
+consists in making a copy in metal of a wood-cut, page of type, etc.
+A mould or impression of the type or coin is first made in wax, or
+other suitable material. These moulds are, of course, the reverse
+of the original, and as they do not conduct electricity, have to be
+coated with graphite. This thin coating lines the mould with conducting
+material so that the current can get to every part of the mould.
+These are then hung upon the cathode in a bath of copper sulphate
+as described in § 82. The electric current which passes through the
+vat deposits a thin layer of metallic copper next to the graphite.
+When this copper gets thick enough, the wax is melted away from it,
+leaving a thin shell of copper, the side next to the graphite being
+exactly alike in shape to the type, but made of copper. These thin
+copper sheets are too thin to stand the pressure necessary on printing
+presses, so they are strengthened by backing them with soft metal which
+fills every crevice, making solid plates about ¼ in. thick. These
+plates or _electrotypes_ are used to print from, the original type
+being used to set up another page.
+
+
+
+
+CHAPTER IX.
+
+THE STORAGE BATTERY, AND HOW IT WORKS.
+
+
+=84. Polarization.= It has been stated that a simple cell polarizes
+rapidly on account of hydrogen bubbles that form upon the copper plate.
+They tend to send a current in the opposite direction to that of the
+main current, which is thereby weakened.
+
+[Illustration: Fig. 81.]
+
+=85. Electromotive Force of Polarization.= It has been shown, Fig. 71,
+that water can be decomposed by the electric current. Hydrogen and
+oxygen have a strong attraction or chemical affinity for each other, or
+they would not unite to form water. This attraction has to be overcome
+before the water can be decomposed. As soon as the decomposing current
+ceases to flow, the gases formed try to rush together again; in fact,
+if the water voltameter be disconnected from the cells and connected
+with a galvanoscope, the presence of a current will be shown. This
+voltameter will give a current with an E. M. F. of nearly 1.5 volts; so
+it is evident that we must have a current with a higher voltage than
+this to decompose water. This E. M. F., due to polarization, is called
+the E. M. F. of polarization.
+
+=86. Secondary or Storage Batteries=, also called _accumulators_, do
+not really store electricity. They must be charged by a current before
+they can give out any electricity. Chemical changes are produced in the
+storage cells by the charging current just as they are in voltameters,
+electroplating solutions, etc.; so it is potential chemical energy
+that is really stored. When the new products are allowed to go back to
+their original state, by joining the electrodes of the charged cell, a
+current is produced.
+
+Fig. 81 shows two lead plates, A and B, immersed in dilute sulphuric
+acid, and connected with two ordinary cells. A strong current will pass
+through the liquid between A and B at first, but it will quickly become
+weaker, as chemical changes take place in the liquid. This may be shown
+by a galvanometer put in the circuit before beginning the experiment.
+By disconnecting the wires from the cells and joining them to the
+galvanometer, it will be shown that a current comes from the lead
+plates. This arrangement may be called a simple storage cell. Regular
+storage cells are charged with the current from a dynamo. (See "Study,"
+Exp. 151.)
+
+[Illustration: Fig. 82.]
+
+The first storage cells were made of plain lead plates, rolled up in
+such a way that they were close to each other, but did not touch. These
+were placed in dilute sulphuric acid. They were charged in alternate
+directions several times, until the lead became properly acted upon, at
+which time the cell would furnish a current.
+
+A great improvement was made in 1881, by Faure, who coated the plates
+with red lead.
+
+[Illustration: Fig. 83.]
+
+The method now generally practiced is to cast a frame of lead, with
+raised right-angled ribs on each side, thus forming little depressed
+squares, or to punch a lead plate full of holes, which squares or holes
+are then filled with a pasty mixture of red oxide of lead in positive
+plates, and with litharge in negatives. In a form called the chloride
+battery, instead of cementing lead oxide paste into or against a lead
+framing in order to obtain the necessary active material, the latter is
+obtained by a strictly chemical process.
+
+Fig. 82 shows a storage cell with plates, etc., contained in a glass
+jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood
+tank. Fig. 84 shows three cells joined in series. Many storage cells
+are used in central electric light stations to help the dynamos during
+the "rush" hours at night. They are charged during the day when the
+load on the dynamos is not heavy.
+
+Fig. 85 shows another form of storage cell containing a number of
+plates.
+
+[Illustration: Fig. 84.]
+
+=87. The Uses of Storage Batteries= are almost numberless. The current
+can be used for nearly everything for which a constant current is
+adapted, the following being some of its applications: Carriage
+propulsion; electric launch propulsion; train lighting; yacht lighting;
+carriage lighting; bicycle lighting; miners' lamps; dental, medical,
+surgical, and laboratory work; phonographs; kinetoscopes; automaton
+pianos; sewing-machine motors; fan motors; telegraph; telephone;
+electric bell; electric fire-alarm; heat regulating; railroad switch
+and signal apparatus.
+
+By the installing of a storage plant many natural but small sources
+of power may be utilized in furnishing light and power; sources which
+otherwise are not available, because not large enough to supply maximum
+demands. The force of the tides, of small water powers from irrigating
+ditches, and even of the wind, come under this heading.
+
+[Illustration: Fig. 85.]
+
+As a regulator of pressure, in case of fluctuations in the load, the
+value of a storage plant is inestimable. These fluctuations of load are
+particularly noticeable in electric railway plants, where the demand is
+constantly rising and falling, sometimes jumping from almost nothing to
+the maximum, and _vice versa_, in a few seconds. If for no other reason
+than the prevention of severe strain on the engines and generators,
+caused by these fluctuations of demand, a storage plant will be
+valuable.
+
+
+
+
+CHAPTER X.
+
+HOW ELECTRICITY IS GENERATED BY HEAT.
+
+
+=88. Thermoelectricity= is the name given to electricity that is
+generated by heat. If a strip of iron, I, be connected between two
+strips of copper, C C, these being joined by a copper wire, C W, we
+shall have an arrangement that will generate a current when heated at
+either of the junctions between C and I. When it is heated at A the
+current will flow as shown by arrows, from C to I. If we heat at B,
+the current will flow in the opposite direction through the metals,
+although it will still go from C to I as before. Such currents are
+called _thermoelectric currents_.
+
+[Illustration: Fig. 86.]
+
+Different pairs of metals produce different results. Antimony and
+bismuth are generally used, because the greatest effect is produced
+by them. If the end of a strip of bismuth be soldered to the end of
+a similar strip of antimony, and the free ends be connected to a
+galvanometer of low resistance, the presence of a current will be shown
+when the point of contact becomes hotter than the rest of the circuit.
+The current will flow from bismuth to antimony across the joint. By
+cooling the juncture below the temperature of the rest of the circuit,
+a current will be produced in the opposite direction to the above. The
+energy of the current is kept up by the heat absorbed, just as it is
+kept up by chemical action in the voltaic cell.
+
+=89. Peltier Effect.= If an electric current be passed through pairs of
+metals, the parts at the junction become slightly warmer or cooler than
+before, depending upon the direction of the current. This action is
+really the reverse of that in which currents are produced by heat.
+
+[Illustration: Fig. 87.]
+
+=90. Thermopiles.= As the E.M.F. of the current produced by a single
+pair of metals is very small, several pairs are usually joined in
+series, so that the different currents will help each other by flowing
+in the same direction. Such combinations are called thermoelectric
+piles, or simply _thermopiles_.
+
+Fig. 87 shows such an arrangement, in which a large number of elements
+are placed in a small space. The junctures are so arranged that the
+alternate ones come together at one side.
+
+Fig. 88 shows a thermopile connected with a galvanometer. The heat of
+a match, or the cold of a piece of ice, will produce a current, even if
+held at some distance from the thermopile. The galvanometer should be
+a short-coil astatic one. (See "Study," Chapter XXIV., for experiments
+and home-made thermopile.)
+
+[Illustration: Fig. 88.]
+
+
+
+
+CHAPTER XI.
+
+MAGNETIC EFFECTS OF THE ELECTRIC CURRENT.
+
+
+=91. Electromagnetism= is the name given to magnetism that is developed
+by electricity. We have seen that if a magnetic needle be placed in the
+field of a magnet, its N pole will point in the direction taken by the
+lines of force as they pass from the N to the S pole of the magnet.
+
+[Illustration: Fig. 89.]
+
+=92. Lines of Force about a Wire.= When a current passes through a
+wire, the magnetic needle placed over or under it tends to take a
+position at right angles to the wire. Fig. 89 shows such a wire and
+needle, and how the needle is deflected; it twists right around from
+its N and S position as soon as the current begins to flow. This shows
+that the lines of force pass _around_ the wire and not in the direction
+of its length. The needle does not swing entirely perpendicular to the
+wire, that is, to the E and W line, because the earth is at the same
+time pulling its N pole toward the N.
+
+Fig. 90 shows a bent wire through which a current passes from C to Z.
+If you look along the wire from C toward the points A and B, you will
+see that _under_ the wire the lines of force pass to the left. Looking
+along the wire from Z toward D you will see that the lines of force
+pass opposite to the above, as the current comes _toward_ you. This is
+learned by experiment. (See "Study," Exp. 152, § 385, etc.)
+
+[Illustration: Fig. 90.]
+
+[Illustration: Fig. 91.]
+
+_Rule._ Hold the right hand with the thumb extended (Fig. 89) and with
+the fingers pointing in the direction of the current, the palm being
+toward the needle and on the opposite side of the wire from the needle.
+The north-seeking pole will then be deflected in the direction in which
+the thumb points.
+
+=93. Current Detectors.= As there is a magnetic field about a wire when
+a current passes through it, and as the magnetic needle is affected, we
+have a means of detecting the presence of a current. When the current
+is strong it is simply necessary to let it pass once over or under a
+needle; when it is weak, the wire must pass several times above and
+below the needle, Fig. 91, to give the needle motion. (See "Apparatus
+Book," Chapter XIII., for home-made detectors.)
+
+[Illustration: Fig. 92.]
+
+=94. Astatic Needles and Detectors.= By arranging two magnetized
+needles with their poles opposite each other, Fig. 92, an _astatic
+needle_ is formed. The pointing-power is almost nothing, although
+their magnetic fields are retained. This combination is used to detect
+feeble currents. In the ordinary detector, the tendency of the needle
+to point to the N and S has to be overcome by the magnetic field about
+the coil before the needle can be moved; but in the _astatic detector_
+and _galvanoscope_ this pointing-power is done away with. Fig. 93 shows
+a simple _astatic galvanoscope_. Fig. 67 shows an astatic galvanometer
+for measuring weak currents.
+
+[Illustration: Fig. 93.]
+
+=95. Polarity of Coils.= When a current of electricity passes through
+a coil of wire, the coil acts very much like a magnet, although no
+iron enters into its construction. The coil becomes magnetized by the
+electric current, lines of force pass from it into the air, etc. Fig.
+94 shows a coil connected to copper and zinc plates, so arranged with
+cork that the whole can float in a dish of dilute sulphuric acid. The
+current passes as shown by the arrows, and when the N pole of a magnet
+is brought near the right-hand end, there is a repulsion, showing that
+that end of the coil has a N pole.
+
+_Rule._ When you face the right-hand end of the coil, the current is
+seen to pass around it in an anti-clockwise direction; this produces a
+N pole. When the current passes in a clockwise direction a S pole is
+produced.
+
+[Illustration: Fig. 94.]
+
+=96. Electromagnets.= A coil of wire has a stronger field than a
+straight wire carrying the same current, because each turn adds its
+field to the fields of the other turns. By having the central part of
+the coil made of iron, or by having the coil of insulated wire wound
+upon an iron _core_, the strength of the magnetic field of the coil is
+greatly increased.
+
+Lines of force do not pass as readily through air as through iron;
+in fact, lines of force will go out of their way to go through iron.
+With a coil of wire the lines of force pass from its N pole through
+the air on all sides of the coil to its S pole; they then pass through
+the inside of the coil and through the air back to the N pole. When
+the resistance to their passage through the coil is decreased by the
+core, the magnetic field is greatly strengthened, and we have an
+_electromagnet_.
+
+The coil of wire temporarily magnetizes the iron core; it can
+permanently magnetize a piece of steel used as a core. (See "Study,"
+Chapter XXII., for experiments.)
+
+[Illustration: Fig. 95.]
+
+=97. Forms of Electromagnets.= Fig. 95 shows a _straight, or
+bar electromagnet_. Fig. 96 shows a simple form of _horseshoe
+electromagnet_. As this form is not easily wound, the coils are
+generally wound on two separate cores which are then joined by a
+_yoke_. The yoke merely takes the place of the curved part shown
+in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe
+electromagnet used for all sorts of electrical instruments. (See
+"Apparatus Book," Chapter IX., for home-made electromagnets.)
+
+=98. Yokes and Armatures.= In the horseshoe magnet there are two poles
+to attract and two to induce. The lines of force pass through the yoke
+on their way from one core to the other, instead of going through
+the air. This reduces the resistance to them. If we had no yoke we
+should simply have two straight electromagnets, and the resistance to
+the lines of force would be so great that the total strength would
+be much reduced. Yokes are made of soft iron, as well as the cores
+and armature. The _armature_, as with permanent horseshoe magnets, is
+strongly drawn toward the poles. As soon as the current ceases to flow,
+the attraction also ceases.
+
+[Illustration: Fig. 96.]
+
+[Illustration: Fig. 97.]
+
+[Illustration: Fig. 98.]
+
+Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98
+shows that the coils must be joined so that the current can pass around
+the cores in opposite directions to make unlike poles. (See "Study,"
+Exp. 164 to 173.)
+
+
+
+
+CHAPTER XII.
+
+HOW ELECTRICITY IS GENERATED BY INDUCTION.
+
+
+=99. Electromagnetic Induction.= We have seen that a magnet has the
+power to act through space and induce another piece of iron or steel
+to become a magnet. A charge of static electricity can induce a
+charge upon another conductor. We have now to see how a _current_ of
+electricity in one conductor can induce a current in another conductor,
+not in any way connected with the first, and how a magnet and a coil
+can generate a current.
+
+[Illustration: Fig. 99.]
+
+[Illustration: Fig. 100.]
+
+=100. Current from Magnet and Coil.= If a bar magnet, Fig. 99, be
+suddenly thrust into a hollow coil of wire, a momentary current of
+electricity will be generated in the coil. No current passes when the
+magnet and coil are still; at least one of them must be in motion. Such
+a current is said to be _induced_, and is an _inverse_ one when the
+magnet is inserted, and a _direct_ one when the magnet is withdrawn
+from the coil.
+
+=101. Induced Currents and Lines of Force.= Permanent magnets are
+constantly sending out thousands of lines of force. Fig. 100 shows
+a bar magnet entering a coil of wire; the number of lines of force
+is increasing, and the induced current passes in an anti-clockwise
+direction when looking down into the coil along the lines of force.
+This produces an indirect current. If an iron core be used in the coil,
+the induced current will be greatly strengthened.
+
+[Illustration: Fig. 101.]
+
+It takes force to move a magnet through the center of a coil, and it
+is this work that is the source of the induced current. We have, in
+this simple experiment, the key to the action of the dynamo and other
+electrical machines.
+
+=102. Current from two Coils.= Fig. 101 shows two coils of wire, the
+smaller being connected to a cell, the larger to a galvanometer.
+By moving the small coil up and down inside of the large one,
+induced currents are generated, first in one direction and then in
+the opposite. We have here two entirely separate circuits, in no
+way connected. The _primary_ current comes from the cell, while the
+_secondary_ current is an induced one. By placing a core in the small
+coil of Fig. 101, the induced current will be greatly strengthened.
+
+It is not necessary to have the two coils so that one or both of them
+can move. They may be wound on the same core, or otherwise arranged as
+in the induction coil. (See "Study," Chapter XXV., for experiments on
+induced currents.)
+
+
+
+
+CHAPTER XIII.
+
+HOW THE INDUCTION COIL WORKS.
+
+
+=103. The Coils.= We saw, § 102, that an induced current was generated
+when a current-carrying coil, Fig. 101, was thrust into another coil
+connected with a galvanometer. The galvanometer was used merely to show
+the presence of the current. The _primary coil_ is the one connected
+with the cell; the other one is called the _secondary coil_.
+
+[Illustration: Fig. 102.]
+
+When a current suddenly begins to flow through a coil, the effect upon
+a neighboring coil is the same as that produced by suddenly bringing
+a magnet near it; and when the current stops, the opposite effect is
+produced. It is evident, then, that we can keep the small coil of
+Fig. 101 with its core inside of the large coil, and generate induced
+currents by merely making and breaking the primary circuit.
+
+We may consider that when the primary circuit is closed, the lines of
+force shoot out through the turns of the secondary coil just as they
+do when a magnet or a current-carrying coil is thrust into it. Upon
+opening the circuit, the lines of force cease to exist; that is, we may
+imagine them drawn in again.
+
+=104. Construction.= Fig. 102 shows one form of home-made induction
+coil, given here merely to explain the action and connections. Nearly
+all induction coils have some form of automatic current interrupter,
+placed in the primary circuit, to rapidly turn the current off and on.
+
+_Details of Figs. 102 and 103._ Wires 5 and 6 are the ends of the
+primary coil, while wires 7 and 8 are the terminals of the secondary
+coil. The primary coil is wound on a bolt which serves as the core, and
+on this coil is wound the secondary which consists of many turns of
+fine wire. The wires from a battery should be joined to binding-posts W
+and X, and the handles, from which the shock is felt, to Y and Z. Fig.
+103 shows the details of the interrupter.
+
+[Illustration: Fig. 103.]
+
+If the current from a cell enters at W, it will pass through the
+primary coil and out at X, after going through 5, R, F, S I, B, E and
+C. The instant the current passes, the bolt becomes magnetized; this
+attracts A, which pulls B away from the end of S I, thus automatically
+opening the circuit. B at once springs back to its former position
+against SI, as A is no longer attracted; the circuit being closed, the
+operation is rapidly repeated.
+
+A _condenser_ is usually connected to commercial forms. It is placed
+under the wood-work and decreases sparking at the interrupter. (See
+"Apparatus Book," Chapter XI., for home-made induction coils.)
+
+[Illustration: Fig. 104.]
+
+Fig. 104 shows one form of coil. The battery wires are joined to the
+binding-posts at the left. The secondary coil ends in two rods, and the
+spark jumps from one to the other. The interrupter and a switch are
+shown at the left.
+
+Fig. 105 shows a small coil for medical purposes. A dry cell is placed
+under the coil and all is included in a neat box. The handles form the
+terminals of the secondary coil.
+
+=105. The Currents.= It should be noted that the current from the
+cell does not get into the secondary coil. The coils are thoroughly
+insulated from each other. The secondary current is an induced one,
+its voltage depending upon the relative number of turns of wire there
+are in the two coils. (See Transformers.) The secondary current is
+an alternating one; that is, it flows in one direction for an instant
+and then immediately reverses its direction. The rapidity of the
+alternations depends upon the speed of the interrupter. Coils are made
+that give a secondary current with an enormous voltage; so high, in
+fact, that the spark will pass many inches, and otherwise act like
+those produced by static electric machines.
+
+[Illustration: Fig. 105.]
+
+=106. Uses of Induction Coils.= Gas-jets can be lighted at a distance
+with the spark from a coil, by extending wires from the secondary
+coil to the jet. Powder can be fired at a distance, and other things
+performed, when a high voltage current is needed. Its use in medicine
+has been noted. It is largely used in telephone work. Of late, great
+use has been made of the secondary current in experiments with
+vacuum-tubes, X-ray work, etc.
+
+
+
+
+CHAPTER XIV.
+
+THE ELECTRIC TELEGRAPH, AND HOW IT SENDS MESSAGES.
+
+
+=107. The Complete Telegraph Line= consists of several instruments,
+switches, etc., etc., but its essential parts are: The _Line_, or wire,
+which connects the different stations; the _Transmitter_ or _Key_; the
+_Receiver_ or _Sounder_, and the _Battery_ or _Dynamo_.
+
+=108. The Line= is made of strong copper, iron, or soft steel wire. To
+keep the current in the line it is insulated, generally upon poles, by
+glass insulators. For very short lines two wires can be used, the line
+wire and the return; but for long lines the earth is used as a return,
+a wire from each end being joined to large metal plates sunk in the
+earth.
+
+[Illustration: Fig. 106.]
+
+=109. Telegraph Keys= are merely instruments by which the circuit
+can be conveniently and rapidly opened or closed at the will of the
+operator. An ordinary push-button may be used to turn the current off
+and on, but it is not so convenient as a key.
+
+Fig. 106 shows a side view of a simple key which can be put anywhere
+in the circuit, one end of the cut wire being attached to X and the
+other to Y. By moving the lever C up and down according to a previously
+arranged set of signals, a current will be allowed to pass to a
+distant station. As X and Y are insulated from each other, the current
+can pass only when C presses against Y.
+
+Fig. 107 shows a regular key, with switch, which is used to allow the
+current to pass through the instrument when receiving a message.
+
+[Illustration: Fig. 107.]
+
+=110. Telegraph Sounders= receive the current from some distant
+station, and with its electromagnet produce sounds that can be
+translated into messages.
+
+[Illustration: Fig. 108.]
+
+Fig. 108 shows simply an electromagnet H, the coil being connected in
+series with a key K and a cell D C. The key and D C are shown by a top
+view. The lever of K does not touch the other metal strap until it is
+pressed down. A little above the core of H is held a strip of iron, on
+armature I. As soon as the circuit is closed at K, the current rushes
+through the circuit, and the core attracts I making a distinct _click_.
+As soon as K is raised, I springs away from the core, if it has been
+properly held. In regular instruments a click is also made when the
+armature springs back again.
+
+The time between the two clicks can be short or long, to represent
+_dots_ or _dashes_, which, together with _spaces_, represent letters.
+(For Telegraph Alphabet and complete directions for home-made keys,
+sounders, etc., see "Apparatus Book," Chapter XIV.)
+
+[Illustration: Fig. 109.]
+
+[Illustration: Fig. 110.]
+
+Fig. 109 shows a form of home-made sounder. Fig. 110 shows one form of
+telegraph sounder. Over the poles of the horseshoe electromagnet is an
+armature fixed to a metal bar that can rock up and down. The instant
+the current passes through the coils the armature comes down until a
+stop-screw strikes firmly upon the metal frame, making the down click.
+As soon as the distant key is raised, the armature is firmly pulled
+back and another click is made. The two clicks differ in sound, and can
+be readily recognized by the operator.
+
+=111. Connections for Simple Line.= Fig. 111 shows complete connections
+for a home-made telegraph line. The capital letters are used for the
+right side, R, and small letters for the left side, L. Gravity cells,
+B and b, are used. The _sounders_, S and s, and the _keys_, K and k,
+are shown by a top view. The broad black lines of S and s represent the
+armatures which are directly over the electromagnets. The keys have
+switches, E and e.
+
+The two stations, R and L, may be in the same room, or in different
+houses. The _return wire_, R W, passes from the copper of b to the zinc
+of B. This is important, as the cells must help each other; that is,
+they are in series. The _line wire_, L W, passes from one station to
+the other, and the return may be through the wire, R W, or through the
+earth; but for short lines a wire is best.
+
+[Illustration: Fig. 111.]
+
+=112. Operation of Simple Line.= Suppose two boys, R (right) and L
+(left) have a line. Fig. 111 shows that R's switch, E, is open, while
+e is closed. The entire circuit, then, is broken at but one point. As
+soon as R presses his key, the circuit is closed, and the current from
+both cells rushes around from B, through K, S, L W, s, k, b, R W, and
+back to B. This makes the armatures of S and s come down with a click
+at the same time. As soon as the key is raised, the armatures lift and
+make the up-click. As soon as R has finished, he closes his switch E.
+As the armatures are then held down, L knows that R has finished, so
+he opens his switch e, and answers R. Both E and e are closed when the
+line is not in use, so that either can open his switch at any time and
+call up the other. Closed circuit cells must be used for such lines. On
+very large lines dynamos are used to furnish the current.
+
+=113. The Relay.= Owing to the large resistance of long telegraph
+lines, the current is weak when it reaches a distant station, and not
+strong enough to work an ordinary sounder. To get around this, relays
+are used; these are very delicate instruments that replace the sounder
+in the line wire circuit. Their coils are usually wound with many turns
+of fine wire, so that a feeble current will move its nicely adjusted
+armature. The relay armature merely acts as an automatic key to open
+and close a local circuit which includes a battery and sounder. The
+line current does not enter the sounder; it passes back from the relay
+to the sending station through the earth.
+
+[Illustration: Fig. 112.]
+
+Fig. 112 gives an idea of simple relay connections. The key K, and
+cell D C, represent a distant sending station. E is the electromagnet
+of the relay, and R A is its armature. L W and R W represent the line
+and return wires. R A will vibrate toward E every time K is pressed,
+and close the local circuit, which includes a local battery, L B, and
+a sounder. It is evident that as soon as K is pressed the sounder will
+work with a good strong click, as the local battery can be made as
+strong as desired.
+
+Fig. 113 shows a regular instrument which opens and closes the local
+circuit at the top of the armature.
+
+[Illustration: Fig. 113.]
+
+=114. Ink Writing Registers= are frequently used instead of sounders.
+Fig. 114 shows a writing register that starts itself promptly at the
+opening of the circuit, and stops automatically as soon as the circuit
+returns to its normal condition. A strip of narrow paper is slowly
+pulled from the reel by the machine, a mark being made upon it every
+time the armature of an inclosed electromagnet is attracted. When the
+circuit is simply closed for an instant, a short line, representing a
+_dot_, is made.
+
+Registers are built both single pen and double pen. In the latter case,
+as the record of one wire is made with a fine pen, and the other with
+a coarse pen, they can always be identified. The record being blocked
+out upon white tape in solid black color, in a series of clean-cut dots
+and dashes, it can be read at a glance, and as it is indelible, it may
+be read years afterward. Registers are made for local circuits, for
+use in connection with relays, or for direct use on main lines, as is
+usually desirable in fire-alarm circuits.
+
+[Illustration: Fig. 114.]
+
+
+
+
+CHAPTER XV.
+
+THE ELECTRIC BELL AND SOME OF ITS USES.
+
+
+[Illustration: Fig. 115.]
+
+[Illustration: Fig. 116.]
+
+=115. Automatic Current Interrupters= are used on most common bells,
+as well as on induction coils, etc. (See § 104.) Fig. 115 shows a
+simple form of interrupter. The wire 1, from a cell D C, is joined to
+an iron strip I a short distance from its end. The other wire from D C
+passes to one end of the electromagnet coil H. The remaining end of H
+is placed in contact with I as shown, completing the circuit. As soon
+as the current passes, I is pulled down and away from the upper wire
+2, breaking the circuit. I, being held by its left-hand end firmly in
+the hand, immediately springs back to its former position, closing the
+circuit again. This action is repeated, the rapidity of the vibrations
+depending somewhat upon the position of the wires on I. In regular
+instruments a platinum point is used where the circuit is broken; this
+stands the sparking when the armature vibrates.
+
+=116. Electric Bells= may be illustrated by referring to Fig. 116,
+which shows a circuit similar to that described in § 115, but which
+also contains a key K, in the circuit. This allows the circuit to
+be opened and closed at a distance from the vibrating armature. The
+circuit must not be broken at two places at the same time, so wires
+should touch at the end of I before pressing K. Upon pressing K the
+armature I will vibrate rapidly. By placing a small bell near the end
+of the vibrating armature, so that it will be struck by I at each
+vibration, we should have a simple electric bell. This form of electric
+bell is called a _trembling_ bell, on account of its vibrating armature.
+
+[Illustration: Fig. 117.]
+
+[Illustration: Fig. 118.]
+
+Fig. 117 shows a form of trembling bell with cover removed. Fig. 118
+shows a _single-stroke_ bell, used for fire-alarms and other signal
+work. In this the armature is attracted but once each time the current
+passes. As many taps of the bell can be given as desired by pressing
+the push-button. Fig. 119 shows a gong for railway crossings, signals,
+etc. Fig. 120 shows a circuit including cell, push-button, and bell,
+with extra wire for lengthening the line.
+
+[Illustration: Fig. 119.]
+
+_Electro-Mechanical Gongs_ are used to give loud signals for special
+purposes. The mechanical device is started by the electric current when
+the armature of the electromagnet is attracted. Springs, weights, etc.,
+are used as the power. Fig. 121 shows a small bell of this kind.
+
+[Illustration: Fig. 120.]
+
+=117. Magneto Testing Bells=, Fig. 122, are really small hand-power
+dynamos. The armature is made to revolve between the poles of strong
+permanent magnets, and it is so wound that it gives a current with a
+large E. M. F., so that it can ring through the large resistance of a
+long line to test it.
+
+_Magneto Signal Bells_, Fig. 123, are used as generator and bell in
+connection with telephones. The generator, used to ring a bell at a
+distant station, stands at the bottom of the box. The bell is fastened
+to the lid, and receives current from a distant bell.
+
+[Illustration: Fig. 121.]
+
+[Illustration: Fig. 122.]
+
+[Illustration: Fig. 123.]
+
+[Illustration: Fig. 124.]
+
+=118. Electric Buzzers= have the same general construction as electric
+bells; in fact, you will have a buzzer by removing the bell from an
+ordinary electric bell. Buzzers are used in places where the loud sound
+of a bell would be objectionable. Fig. 124 shows the usual form of
+buzzers, the cover being removed.
+
+
+
+
+CHAPTER XVI.
+
+THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.
+
+
+=119. The Telephone= is an instrument for reproducing sounds at a
+distance, and electricity is the agent by which this is generally
+accomplished. The part spoken to is called the _transmitter_, and
+the part which gives sound out again is called the _receiver_. Sound
+itself does not pass over the line. While the same apparatus can be
+used for both transmitter and receiver, they are generally different in
+construction to get the best results.
+
+[Illustration: Fig. 125.]
+
+[Illustration: Fig. 126.]
+
+[Illustration: Fig. 127.]
+
+=120. The Bell or Magneto-transmitter= generates its own current, and
+is, strictly speaking, a dynamo that is run by the voice. It depends
+upon induction for its action.
+
+[Illustration: Fig. 128.]
+
+Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the
+wires being connected to a delicate galvanoscope. If one pole of the
+magnet H M be suddenly moved up and down near the core, an alternating
+current will be generated in the coil, the circuit being completed
+through the galvanoscope. As H M approaches the core the current will
+flow in one direction, and as H M is withdrawn it will pass in the
+opposite direction. The combination makes a miniature alternating
+dynamo.
+
+[Illustration: Fig. 129.]
+
+If we imagine the soft iron core of H, Fig. 125, taken out, and one
+pole of H M, or preferably that of a bar magnet stuck through the coil,
+a feeble current will also be produced by moving the soft iron back and
+forth near the magnet's pole. This is really what is done in the Bell
+transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being
+made to vibrate by the voice immediately in front of a coil having
+a permanent magnet for a core. The disc, or _diaphragm_, as it is
+called, is fixed near, but it does not touch, the magnet. It is under
+a constant strain, being attracted by the magnet, so its slightest
+movement changes the strength of the magnetic field, causing more or
+less lines of force to shoot through the turns of the coil and induce a
+current. The coil consists of many turns of fine, insulated wire. The
+current generated is an alternating one, and although exceedingly small
+can force its way through a long length of wire.
+
+[Illustration: Fig. 130.]
+
+Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form
+of compound magnet frequently used in the transmitter. Fig. 129 shows a
+transmitter with cords which contain flexible wires.
+
+[Illustration: Fig. 131.]
+
+=121. The Receiver=, for short lines, may have the same construction as
+the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers,
+either being used as the transmitter and the other as the receiver.
+As the alternating current goes to the distant receiver, it flies
+through the coil first in one direction and then in the other. This
+alternately strengthens and weakens the magnetic field near the
+diaphragm, causing it to vibrate back and forth as the magnet pulls
+more or less. The receiver diaphragm repeats the vibrations in the
+transmitter. Nothing but the induced electric current passes over the
+wires.
+
+[Illustration: Fig. 132.]
+
+=122. The Microphone.= If a current of electricity be allowed to
+pass through a circuit like that shown in Fig. 131, which includes a
+battery, a Bell receiver, and a microphone, any slight sound near the
+microphone will be greatly magnified in the receiver. The microphone
+consists of pieces of carbon so fixed that they form loose contacts.
+Any slight movement of the carbon causes the resistance to the current
+to be greatly changed. The rapidly varying resistance allows more or
+less current to pass, the result being that this pulsating current
+causes the diaphragm to vibrate. The diaphragm has a constantly varying
+pull upon it when the carbons are in any way disturbed by the voice, or
+by the ticking of a watch, etc. This principle has been made use of in
+carbon transmitters, which are made in a large variety of forms.
+
+[Illustration: Fig. 133.]
+
+=123. The Carbon Transmitter= does not, in itself, generate a
+current like the magneto-transmitter; it merely produces changes in
+the strength of a current that flows through it and that comes from
+some outside source. In Fig. 132, X and Y are two carbon buttons, X
+being attached to the diaphragm D. Button Y presses gently against X,
+allowing a little current to pass through the circuit which includes
+a battery, D C, and a receiver, R. When D is caused to vibrate by the
+voice, X is made to press more or less against Y, and this allows more
+or less current to pass through the circuit. This direct undulating
+current changes the pull upon the diaphragm of R, causing it to vibrate
+and reproduce the original sounds spoken into the transmitter. In
+regular lines, of course, a receiver and transmitter are connected at
+each end, together with bells, etc., for signaling.
+
+[Illustration: Fig. 134.]
+
+=124. Induction Coils in Telephone Work.= As the resistance of long
+telephone lines is great, a high electrical pressure, or E.M.F. is
+desired. While the current from one or two cells is sufficient to work
+the transmitter properly, and cause undulating currents in the short
+line, it does not have power enough to force its way over a long line.
+
+To get around this difficulty, an induction coil, Fig. 133, is used
+to transform the battery current, that flows through the carbon
+transmitter and primary coil, into a current with a high E. M. F. The
+battery current in the primary coil is undulating, but always passes in
+the same direction, making the magnetic field around the core weaker
+and stronger. This causes an alternating current in the secondary coil
+and main line. In Fig. 133 P and S represent the primary and secondary
+coils. P is joined in series with a cell and carbon transmitter; S
+is joined to the distant receiver. One end of S can be grounded, the
+current completing the circuit through the earth and into the receiver
+through another wire entering the earth.
+
+[Illustration: Fig. 135.]
+
+=125. Various forms= of telephones are shown in Figs. 134, 135, 136.
+Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form
+of wall telephone; Fig. 136 shows head-telephones for switchboard
+operators.
+
+[Illustration: Fig. 136.]
+
+
+
+
+CHAPTER XVII.
+
+HOW ELECTRICITY IS GENERATED BY DYNAMOS.
+
+
+=126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a
+machine for converting mechanical energy into an electric current,
+through electromagnetic induction. The dynamo is a machine that will
+convert steam power, for example, into an electric current. Strictly
+speaking, a dynamo creates electrical pressure, or electromotive force,
+and not electricity, just as a force-pump creates water-pressure, and
+not water. They are generally run by steam or water power.
+
+[Illustration: Fig. 137.]
+
+=127. Induced Currents.= We have already spoken about currents being
+induced by moving a coil of wire in a magnetic field. We shall now
+see how this principle is used in the dynamo which is a generator of
+induced currents.
+
+[Illustration: Fig. 138.]
+
+Fig. 137 shows how a current can be generated by a bar magnet and
+a coil of wire. Fig. 138 shows how a current can be generated by a
+horseshoe magnet and a coil of wire having an iron core. The ends of
+the coil are to be connected to an astatic galvanoscope; this forms a
+closed circuit. The coil may be moved past the magnet, or the magnet
+past the coil.
+
+[Illustration: Fig. 139.]
+
+[Illustration: Fig. 140.]
+
+[Illustration: Fig. 141.]
+
+[Illustration: Fig. 142.]
+
+Fig. 139 shows how a current can be generated by two coils, H being
+connected to an astatic galvanoscope and E to a battery. By suddenly
+bringing E toward H or the core of E past that of H, a current is
+produced. We have in this arrangement the main features of a dynamo.
+We can reverse the operation, holding E in one position and moving H
+rapidly toward it. In this case H would represent the armature and E
+the field-magnet. When H is moved toward E, the induced current in H
+flows in one direction, and when H is suddenly withdrawn from E the
+current is reversed in H. (See "Study," Chapter XXV., for experiments.)
+
+[Illustration: Fig. 143.]
+
+=128. Induced Currents by Rotary Motion.= The motions of the coils in
+straight lines are not suitable for producing currents strong enough
+for commercial purposes. In order to generate currents of considerable
+strength and pressure, the coils of wire have to be pushed past
+magnets, or electromagnets, with great speed. In the dynamo the coils
+are so wound that they can be given a rapid rotary motion as they fly
+past strong electromagnets. In this way the coil can keep on passing
+the same magnets, in the same direction, as long as force is applied to
+the shaft that carries them.
+
+[Illustration: Fig. 144.]
+
+=129. Field-Magnets; Armature; Commutator.= What we need then, to
+produce an induced current by a rotary motion, is a strong magnetic
+field, a rotating coil of wire properly placed in the field, and some
+means of leading the current from the machine.
+
+[Illustration: Fig. 145.]
+
+[Illustration: Fig. 146.]
+
+If a loop of wire, Fig. 140, be so arranged on bearings at its ends
+that it can be made to revolve, a current will flow through it in
+one direction during one-half of the revolution, and in the opposite
+direction during the other half, it being insulated from all external
+conductors. This agrees with the experiments suggested in § 127, when
+the current generated in a coil passed in one direction during its
+motion _toward_ the strongest part of the field, and in the opposite
+direction when the coil passed _out_ of it. A coil must be cut by
+lines of force to generate a current. A current inside of the machine,
+as in Fig. 140, would be of no value; it must be led out to external
+conductors where it can do work. Some sort of sliding contact is
+necessary to connect a revolving conductor with outside stationary
+ones. The magnet, called the _field-magnet_, is merely to furnish lines
+of magnetic force. The one turn of wire represents the simplest form of
+_armature_.
+
+Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated
+from each other, and rotating with the coil. The two stationary pieces
+of carbon, A, B, called _brushes_, press against the rings, and to
+these are joined wires, which complete the circuit, and which lead out
+where the current can do work. The arrows show the direction of the
+current during one-half of a revolution. The rings form a _collector_,
+and this arrangement gives an _alternating current_.
+
+[Illustration: Fig. 147.]
+
+In Fig. 142 the ends of the coil are joined to the two halves of a
+cylinder. These halves, X and Y, are insulated from each other, and
+from the axis. The current flows from X onto the brush A, through some
+external circuit, to do the work, and thence back through brush B onto
+Y. By the time that Y gets around to A, the direction of the current in
+the loop has reversed, so that it passes toward Y, but it still enters
+the outside circuit through A, because Y is then in contact with A.
+This device is called a _commutator_, and it allows a constant or
+_direct current_ to leave the machine.
+
+[Illustration: Fig. 148.]
+
+In regular machines, the field-magnets are electromagnets, the whole
+or a part of the current from the dynamo passing around them on its
+way out, to excite them and make a powerful field between the poles.
+To lessen the resistance to the lines of force on their way from the
+N to the S pole of the field-magnets, the armature coils are wound on
+an iron core; this greatly increases the strength of the field, as
+the lines of force have to jump across but two small air-gaps. There
+are many loops of wire on regular armatures, and many segments to the
+commutator, carefully insulated from each other, each getting its
+current from the coil attached to it.
+
+=130. Types of Dynamos.= While there is an almost endless number of
+different makes and shapes of dynamos, they may be divided into two
+great types; the _continuous_ or _direct current_, and the _alternating
+current_ dynamo. Direct current machines give out a current which
+constantly flows in one direction, and this is because a commutator is
+used. Alternating currents come from collectors or rings, as shown in
+Fig. 141; and as an alternating current cannot be used to excite the
+fields, an outside current from a small direct current machine must be
+used. These are called exciters.
+
+[Illustration: Fig. 149.]
+
+In direct current machines enough residual magnetism is left in the
+field to induce a slight current in the armature when the machine is
+started. This immediately adds strength to the field-magnets, which, in
+turn, induce a stronger current in the armature.
+
+=131. Winding of Dynamos.= There are several ways of winding dynamos,
+depending upon the special uses to be made of the current.
+
+The _series wound_ dynamo, Fig. 143, is so arranged that the entire
+current passes around the field-magnet cores on its way from the
+machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the
+current from the machine is carried around the field-magnet cores
+through many turns of fine wire. The _compound wound_ dynamo is really
+a combination of the two methods just given. In _separately-excited_
+dynamos, the current from a separate machine is used to excite the
+field-magnets.
+
+=132. Various Machines.= Fig. 145 shows a hand power dynamo
+which produces a current for experimental work. Fig. 146 shows a
+magneto-electrical generator which produces a current for medical use.
+Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps
+are connected in series to dynamos.
+
+[Illustration]
+
+
+
+
+CHAPTER XVIII.
+
+HOW THE ELECTRIC CURRENT IS TRANSFORMED.
+
+
+=133. Electric Current and Work.= The amount of work a current can do
+depends upon two factors; the strength (amperes), and the pressure,
+or E. M. F. (volts). A current of 10 amperes with a pressure of 1,000
+volts = 10 × 1,000 = 10,000 watts. This furnishes the same amount of
+energy as a current of 50 amperes at 200 volts; 50 × 200 = 10,000 watts.
+
+=134. Transmission of Currents.= It is often necessary to carry a
+current a long distance before it is used. A current of 50 amperes
+would need a copper conductor 25 times as large (sectional area) as one
+to carry the 10 ampere current mentioned in § 133. As copper conductors
+are very expensive, electric light companies, etc., generally try to
+carry the current on as small a wire as possible. To do this, the
+voltage is kept high, and the amperage low. Thus, as seen in § 133,
+the current of 1,000 volts and 10 amperes could be carried on a much
+smaller wire than the other current of equal energy. A current of
+1,000 volts, however, is not adapted for lights, etc., so it has to be
+changed to lower voltage by some form of transformer before it can be
+used.
+
+=135. Transformers=, like induction coils, are instruments for changing
+the E. M. F. and strength of currents. There is very little loss of
+energy in well-made transformers. They consist of two coils of wire on
+one core; in fact, an induction coil may be considered a transformer,
+but in this a direct current has to be interrupted. If the secondary
+coil has 100 times as many turns of wire as the primary, a current of
+100 volts can be taken from the secondary coil when the primary current
+is but 1 volt; but the _strength_ (amperes) of this new current will be
+but one-hundredth that of the primary current.
+
+By using the coil of fine wire as the primary, we can lower the voltage
+and increase the strength in the same proportion.
+
+[Illustration: Fig. 150.]
+
+[Illustration: Fig. 151.]
+
+Fig. 150 shows about the simplest form of transformer with a solid iron
+core, on which are wound two coils, the one, P, being the primary, and
+the other, S, the secondary. Fig. 151 shows the general appearance of
+one make of transformer. The operation of this apparatus, as already
+mentioned, is to reduce the high pressure alternating current sent out
+over the conductors from the dynamo, to a potential at which it can
+be employed with convenience and safety, for illumination and other
+purposes. They consist of two or more coils of wire most carefully
+insulated from one another. A core or magnetic circuit of soft iron,
+composed of very thin punchings, is then formed around these coils,
+the purpose of the iron core being to reduce the magnetic resistance
+and increase the inductive effect. One set of these coils is connected
+with the primary or high-pressure wires, while the other set, which are
+called the secondary coils, is connected to the house or low-pressure
+wires, or wherever the current is required for use. The rapidly
+alternating current impulses in the primary or high-pressure wires
+induce secondary currents similar in form but opposite in direction
+in the secondary coils. These current impulses are of a much lower
+pressure, depending upon the ratio of the number of turns of wire
+in the respective coils, it being customary to wind transformers in
+such a manner as to reduce from 1,000 or 2,000-volt primaries to 50
+or 100-volt secondaries, at which voltage the secondary current is
+perfectly harmless.
+
+[Illustration: Fig. 152.]
+
+=136. Motor-Dynamos.= Fig. 152. These consist essentially of two
+belt-type machines on a common base, direct coupled together, one
+machine acting as a motor to receive current at a certain voltage,
+and the other acting as a dynamo to give out the current usually
+at a different voltage. As they transform current from one voltage
+to another, motor-dynamos are sometimes called Double Field Direct
+Current Transformers. The larger sizes have three bearings, one bearing
+being between the two machines, while the smaller sizes have but two
+bearings, the two armatures being fastened to a common spider.
+
+[Illustration: Fig. 153.]
+
+_Applications._ The uses to which motor-dynamos are put are very
+various. They are extensively used in the larger sizes as "Boosters,"
+for giving the necessary extra force on long electric supply circuits
+to carry the current to the end with the same pressure as that which
+reaches the ends of the shorter circuits from the station.
+
+Motor-dynamos have the advantage over dynamotors, described later, of
+having the secondary voltage easily and economically varied over wide
+ranges by means of a regulator in the dynamo field.
+
+=137. Dynamotors.= Fig. 153. In Dynamotors the motor and dynamo
+armatures are combined in one, thus requiring a single field only.
+The primary armature winding, which operates as a motor to drive the
+machine, and the secondary or dynamo winding, which operates as a
+generator to produce a new current, are upon the same armature core,
+so that the armature reaction of one winding neutralizes that of the
+other. They therefore have no tendency to spark, and do not require
+shifting of the brushes with varying load. Having but one field and two
+bearings, they are also more efficient than motor-dynamos.
+
+_Applications._ They have largely displaced batteries for telegraph
+work. The size shown, occupying a space of about 8-inch cube, and
+having an output of 40 watts, will displace about 800 gravity cells,
+occupying a space of about 10 feet cube. The cost of maintenance of
+such a battery per year, exclusive of rent, is about $800, whereas the
+1-6 dynamotor can be operated at an annual expense of $150.
+
+Dynamotors are largely used by telephone companies for charging storage
+batteries, and for transforming from direct to alternating current, for
+ringing telephone bells. Electro-cautery, electroplating, and electric
+heating also give use to dynamotors.
+
+
+
+
+CHAPTER XIX.
+
+HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.
+
+
+[Illustration: Fig. 154.]
+
+[Illustration: Fig. 155.]
+
+[Illustration: Fig. 156.]
+
+=138. Conductors and Insulators.= To carry the powerful current from
+the generating station to distant places where it is to give heat,
+power, or light, or even to carry the small current of a single cell
+from one room to another, _conductors_ must be used. To keep the
+current from passing into the earth before it reaches its destination
+_insulators_ must be used. The form of conductors and insulators used
+will depend upon the current and many other conditions. It should be
+remembered that the current has to be carried to the lamp or motor,
+through which it passes, and then back again to the dynamo, to form a
+complete circuit. A break anywhere in the circuit stops the current.
+Insulators are as important as conductors.
+
+[Illustration: Fig. 157.]
+
+[Illustration: Fig. 158.]
+
+=139. Mains, Service Wires, etc.= From the switchboard the current
+flows out through the streets in large conductors, or _mains_, the
+supply being kept up by the dynamos, just as water-pressure is kept up
+by the constant working of pumps. Branches, called _service wires_, are
+led off from the mains to supply houses or factories, one wire leading
+the current into the house from one main, and a similar one leading it
+out of the house again to the other main.
+
+[Illustration: Fig. 159.]
+
+[Illustration: Fig. 160.]
+
+In large buildings, pairs of wires, called _risers_, branch out from
+the service wires and carry the current up through the building. These
+have still other branches--_floor mains_, _etc._, that pass through
+halls, etc., smaller branches finally reaching the lamps. The sizes of
+all of these wires depend upon how much current has to pass through
+them. The mains in large cities are usually placed underground. In some
+places they are carried on poles.
+
+[Illustration: Fig. 161.]
+
+=140. Electric Conduits= are underground passages for electric wires,
+cables, etc. There are several ways of insulating the conductors.
+Sometimes they are placed in earthenware or iron tubes, or in wood that
+has been treated to make it water-proof. At short distances are placed
+man-holes, where the different lengths are joined, and where branches
+are attached.
+
+[Illustration: Fig. 162.]
+
+Fig. 154 shows creosoted wooden pipes; Fig. 155 shows another form of
+wooden pipe. Fig. 156 shows a coupling-box used to join Edison tubes.
+The three wires, used in the three-wire system, are insulated from each
+other, the whole being surrounded by an iron pipe of convenient length
+for handling. Fig. 157 shows sections of man-holes and various devices
+used in conduit work.
+
+[Illustration: Fig. 163.]
+
+=141. Miscellaneous Appliances.= When the current enters a house for
+incandescent lighting purposes, for example, quite a number of things
+are necessary. To measure the current a meter is usually placed in the
+cellar. In new houses the insulated conductors are usually run through
+some sort of tube which acts as a double protection, all being hidden
+from view. Fig. 158 shows a short length of iron tube with a lining of
+insulating material. Wires are often run through tubes made of rubber
+and various other insulating materials.
+
+Where the current is to be put into houses after the plastering has
+been done, the wires are usually run through _mouldings_ or supported
+by _cleats_. Fig. 159 shows a cross-section of moulding. The insulated
+wires are placed in the slots, which are then covered.
+
+[Illustration: Fig. 164.]
+
+[Illustration: Fig. 165.]
+
+[Illustration: Fig. 166.]
+
+[Illustration: Fig. 167.]
+
+Fig. 160 shows a form of porcelain cleat. These are fastened to
+ceilings or walls, and firmly hold the insulated wires in place. Fig.
+161 shows a wood cleat. Fig. 162 shows small porcelain _insulators_.
+These may be screwed to walls, etc., the wire being then fastened to
+them. Fig. 163 shows how telegraph wires are supported and insulated.
+Fig. 164 shows how wires may be carried by tree and insulated from them.
+
+[Illustration: Fig. 168.]
+
+[Illustration: Fig. 169.]
+
+[Illustration: Fig. 170.]
+
+=142. Safety Devices.= We have seen that when too large a current
+passes through a wire, the wire becomes heated and may even be melted.
+Buildings are wired to use certain currents, and if from any cause much
+more current than the regular amount should suddenly pass through the
+service wires into the house, the various smaller wires would become
+overheated, and perhaps melt or start a fire. An accidental short
+circuit, for example, would so reduce resistance that too much current
+would suddenly rush through the wires. There are several devices by
+which the over-heating of wires is obviated.
+
+[Illustration: Figs. 171 to 175.]
+
+Fig. 165 shows a _safety fuse_, or _safety cut-out_, which consists of
+a short length of easily fusible wire, called _fuse wire_, placed in
+the circuit and supported by a porcelain block. These wires are tested,
+different sizes being used for different currents. As soon as there
+is any tendency toward over-heating, the fuse _blows_; that is, it
+promptly melts and opens the circuit before any damage can be done to
+the regular conductors. Fig. 166 shows a cross-section of a _fuse plug_
+that can be screwed into an ordinary socket. The fuse wire is shown
+black.
+
+Fig. 167 shows a _fuse link_. These are also of fusible material, and
+so made that they can be firmly held under screw-heads. For heavy
+currents _fuse ribbons_ are used, or several wires or links may be
+used side by side. Fig. 168 shows a _fusible rosette_. Fig. 169 shows
+two fuse wires fixed between screw-heads, the current passing through
+them in opposite directions, both sides of the circuit being included.
+Fig. 170 shows various forms of cut-outs.
+
+[Illustration: Fig. 176.]
+
+=143. Wires and Cables= are made in many sizes. Figs. 171 to 175 show
+various ways of making small conductors. They are made very flexible,
+for some purposes, by twisting many small copper wires together, the
+whole being then covered with insulating material.
+
+[Illustration: Fig. 177.]
+
+Figs. 176, 177, show sections of submarine cables. Such cables consist
+of copper conductors insulated with pure gutta-percha. These are then
+surrounded by hempen yarn or other elastic material, and around the
+whole are placed galvanized iron armor wires for protection. Each core,
+or conductor, contains a conductor consisting of a single copper wire
+or a strand of three or more twisted copper wires.
+
+=144. Lamp Circuits.= As has been noted before, in order to have the
+electric current do its work, we must have a complete circuit. The
+current must be brought back to the dynamo, much of it, of course,
+having been used to produce light, heat, power, etc. For lighting
+purposes this is accomplished in two principal ways.
+
+[Illustration: Fig. 178.]
+
+Fig. 178 shows a number of lamps so arranged, "in series," that the
+same current passes through them all, one after the other. The total
+resistance of the circuit is large, as all of the lamp resistances are
+added together.
+
+[Illustration: Fig. 179.]
+
+Fig. 179 shows lamps arranged side by side, or "in parallel," between
+the two main wires. The current divides, a part going through each lamp
+that operates. The total resistance of the circuit is not as large
+as in the series arrangement, as the current has many small paths in
+going from one main wire to the other. Fig. 179 also shows the ordinary
+_two-wire system_ for incandescent lighting, the two main wires having
+usually a difference of potential equal to 50 or 110 volts. These
+comparatively small pressures require fairly large conductors.
+
+_The Three-Wire System_, Fig. 180, uses the current from two dynamos,
+arranged with three main wires. While the total voltage is 220, one of
+the wires being neutral, 110 volts can be had for ordinary lamps. This
+voltage saves in the cost of conductors.
+
+[Illustration: Fig. 180.]
+
+[Illustration: Fig. 181.]
+
+_The Alternating System_, Fig. 181, uses transformers. The high
+potential of the current allows small main wires, from which branches
+can be run to the primary coil of the transformer. The secondary coil
+sends out an induced current of 50 or 110 volts, while that in the
+primary may be 1,000 to 10,000 volts.
+
+
+
+
+CHAPTER XX.
+
+HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.
+
+
+=145. Resistance and Heat.= We have seen that all wires and conductors
+offer resistance to the electric current. The smaller the wire the
+greater its resistance. Whenever resistance is offered to the current,
+heat is produced. By proper appliances, the heat of resistance can be
+used to advantage for many commercial enterprises. Dynamos are used to
+generate the current for heating and lighting purposes.
+
+[Illustration: Fig. 182.]
+
+Fig. 182 shows how the current from two strong cells can be used to
+heat a short length of very fine platinum or German-silver wire.
+The copper conductors attached to the cells do not offer very much
+resistance.
+
+It will be seen from the above that in all electrical work the sizes
+of the wires used have to be such that they do not overheat. The coils
+of dynamos, motors, transformers, ampere-meters, etc., etc., become
+somewhat heated by the currents passing through them, great care being
+taken that they are properly designed and ventilated so that they will
+not burn out.
+
+[Illustration: Fig. 183.]
+
+[Illustration: Fig. 184.]
+
+=146. Electric Welding.= Fig. 183 shows one form of electric welding
+machine. The principle involved in the art of electric welding is
+that of causing currents of electricity to pass through the abutting
+ends of the pieces of metal which are to be welded, thereby generating
+heat at the point of contact, which also becomes the point of greatest
+resistance, while at the same time mechanical pressure is applied
+to force the parts together. As the current heats the metal at the
+junction to the welding temperature, the pressure follows up the
+softening surface until a complete union or weld is effected; and, as
+the heat is first developed in the interior of the parts to be welded,
+the interior of the joint is as efficiently united as the visible
+exterior. With such a method and apparatus, it is found possible to
+accomplish not only the common kinds of welding of iron and steel, but
+also of metals which have heretofore resisted attempts at welding, and
+have had to be brazed or soldered.
+
+[Illustration: Figs. 185 to 189.]
+
+The introduction of the electric transformer enables enormous currents
+to be so applied to the weld as to spend their energy just at the point
+where heating is required. They need, therefore, only to be applied
+for a few seconds, and the operation is completed before the heat
+generated at the weld has had time to escape by conduction to any other
+part.
+
+Although the quantity of the current so employed in the pieces to be
+welded is enormous, the potential at which it is applied is extremely
+low, not much exceeding that of the batteries of cells used for ringing
+electric bells in houses.
+
+[Illustration: Fig. 190.]
+
+=147. Miscellaneous Applications.= Magneto Blasting Machines are now
+in very common use for blasting rocks, etc. Fig. 184 shows one, it
+being really a small hand dynamo, occupying less than one-half a cubic
+foot of space. The armature is made to revolve rapidly between the
+poles of the field-magnet by means of a handle that works up and down.
+The current is carried by wires from the binding-posts to fuses. The
+heat generated by resistance in the fuse ignites the powder or other
+explosive.
+
+_Electric soldering irons_, _flat-irons_, _teakettles_, _griddles_,
+_broilers_, _glue pots_, _chafing-dishes_, _stoves_, etc., etc., are
+now made. Figs. 185 to 189 show some of these applications. The coils
+for producing the resistance are inclosed in the apparatus.
+
+[Illustration: Fig. 191.]
+
+Fig. 190 shows a complete electric kitchen. Any kettle or part of the
+outfit can be made hot by simply turning a switch. Fig. 191 shows an
+electric heater placed under a car seat. Many large industries that
+make use of the heating effects of the current are now being carried
+on.
+
+
+
+
+CHAPTER XXI.
+
+HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.
+
+
+[Illustration: Fig. 192.]
+
+[Illustration: Fig. 193.]
+
+=148. Incandescence.= We have just seen that the electric current
+produces heat when it flows through a conductor that offers
+considerable resistance to it. As soon as this was discovered men
+began to experiment to find whether a practical light could also be
+produced. It was found that a wire could be kept hot by constantly
+passing a current through it, and that the light given out from it
+became whiter and whiter as the wire became hotter. The wire was said
+to be _incandescent_, or glowing with heat. As metal wires are good
+conductors of electricity, they had to be made extremely fine to offer
+enough resistance; too fine, in fact, to be properly handled.
+
+=149. The Incandescent Lamp.= Many substances were experimented upon
+to find a proper material out of which could be made a _filament_
+that would give the proper resistance and at the same time be strong
+and lasting. It was found that hair-like pieces of carbon offered the
+proper resistance to the current. When heated in the air, however,
+carbon burns; so it became necessary to place the carbon filaments in a
+globe from which all the air had been pumped before passing the current
+through them. This proved to be a success.
+
+[Illustration: Fig. 194.]
+
+[Illustration: Fig. 195.]
+
+[Illustration: Fig. 196.]
+
+Fig. 192 shows the ordinary form of lamp. The _carbon filament_ is
+attached, by carbon paste, to short platinum wires that are sealed in
+the glass, their lower ends being connected to short copper wires that
+are joined to the terminals of the lamp. When the lamp is screwed
+into its socket, the current can pass up one side of the filament
+and down the other. The filaments used have been made of every form
+of carbonized vegetable matter. Bamboo has been largely used, fine
+strips being cut by dies and then heated in air-tight boxes containing
+fine carbon until they were thoroughly carbonized. This baking of the
+bamboo produces a tough fiber of carbon. Various forms of thread have
+been carbonized and used. Filaments are now made by pressing finely
+pulverized carbon, with a binding material, through small dies. The
+filaments are made of such sizes and lengths that will adapt them to
+the particular current with which they are to be used. The longer the
+filament, the greater its resistance, and the greater the voltage
+necessary to push the current through it.
+
+[Illustration: Fig. 197.]
+
+[Illustration: Fig. 198.]
+
+After the filaments are properly attached, the air is pumped from the
+bulb or globe. This is done with some form of mercury pump, and the air
+is so thoroughly removed from the bulb that about one-millionth only of
+the original air remains. Before sealing off the lamp, a current is
+passed through the filament to drive out absorbed air and gases, and
+these are carried away by the pump. By proper treatment the filaments
+have a uniform resistance throughout, and glow uniformly when the
+current passes.
+
+[Illustration: Fig. 199.]
+
+[Illustration: Fig. 200.]
+
+=150. Candle-Power.= A lamp is said to have 4, 8, 16 or more
+candle-power. A 16-candle-power lamp, for example, means one that will
+give as much light as sixteen standard candles. A standard sperm candle
+burns two grains a minute. The candle-power of a lamp can be increased
+by forcing a strong current through it, but this shortens its life.
+
+_The Current_ used for incandescent lamps has to be strong enough to
+force its way through the filament and produce a heat sufficient to
+give a good light. The usual current has 50 or 110 volts, although
+small lamps are made that can be run by two or three cells. If the
+voltage of the current is less than that for which the lamp was made,
+the light will be dim. The filament can be instantly burned out by
+passing a current of too high pressure through it.
+
+Even with the proper current, lamps soon begin to deteriorate, as small
+particles of carbon leave the filament and cling to the glass. This is
+due to the evaporation, and it makes the filament smaller, and a higher
+pressure is then needed to force the current through the increased
+resistance; besides this, the darkened bulb does not properly let the
+light out. The current may be direct or alternating.
+
+[Illustration: Fig. 201.]
+
+[Illustration: Fig. 202.]
+
+=151. The Uses= to which incandescent lamps are put are almost
+numberless. Fig. 193 shows a decorative lamp. Fancy lamps are made in
+all colors. Fig. 194 shows a conic candle lamp, to imitate a candle.
+What corresponds to the body of the candle (see figure B to C) is a
+delicately tinted opal glass tube surmounted (see figure A to B) by a
+finely proportioned conic lamp with frosted globe. C to D in the figure
+represents the regular base, and thus the relative proportions of the
+parts are shown. Fig. 195 shows another form of candelabra lamp. Fig.
+196 shows small dental lamps. Fig. 197 shows a small lamp with mirror
+for use in the throat. Fig. 198 shows lamp with half shade attached,
+used for library tables. Fig. 199 shows an electric pendant for several
+lamps, with shade. Fig. 200 shows a lamp guard. Fig. 201 shows a lamp
+socket, into which the lamp is screwed. Fig. 202 shows incandescent
+bulbs joined in parallel to the + and - mains. Fig. 203 shows how the
+lamp cord can be adjusted to desired length. Fig. 204 shows a lamp
+with reflector placed on a desk. Fig. 205 shows a form of shade and
+reflector.
+
+[Illustration: Fig. 203.]
+
+[Illustration: Fig. 204.]
+
+[Illustration: Fig. 205.]
+
+
+
+
+CHAPTER XXII.
+
+HOW LIGHT IS PRODUCED BY THE ARC LAMP.
+
+
+=152. The Electric Arc.= When a strong current passes from one carbon
+rod to another across an air-space, an _electric arc_ is produced.
+When the ends of two carbon rods touch, a current can pass from one to
+the other, but the imperfect contact causes resistance enough to heat
+the ends red-hot. If the rods be separated slightly, the current will
+continue to flow, as the intensely heated air and flying particles of
+carbon reduce the resistance of the air-space.
+
+Fig. 206 shows two carbon rods which are joined to the two terminals
+of a dynamo. The upper, or positive, carbon gradually wears away and
+becomes slightly hollow. The heated _crater_, as it is called, is the
+hottest part. The negative carbon becomes pointed. The arc will pass in
+a vacuum, and even under water.
+
+[Illustration: Fig. 206.]
+
+As the electric arc is extremely hot, metals are easily vaporized in
+it; in fact, even the carbon rods themselves slowly melt and vaporize.
+This extreme heat is used for many industrial purposes.
+
+[Illustration: Fig. 207.]
+
+[Illustration: Fig. 208.]
+
+"The phenomenon of the electric arc was first noticed by Humphrey
+Davy in 1800, and its explanation appears to be the following: Before
+contact the difference of potential between the points is insufficient
+to permit a spark to leap across even 1/10000 of an inch of air-space,
+but when the carbons are made to touch, a current is established.
+On separating the carbons, the momentary extra current due to
+self-induction of the circuit, which possesses a high electromotive
+force, can leap the short distance, and in doing so volatilizes a small
+quantity of carbon between the points. Carbon vapor, being a partial
+conductor, allows the current to continue to flow across the gap,
+provided it be not too wide; but as the carbon vapor has a very high
+resistance it becomes intensely heated by the passage of the current,
+and the carbon points also grow hot. Since, however, solid matter is a
+better radiator than gaseous matter, the carbon points emit far more
+light than the arc itself, though they are not so hot. It is observed,
+also, that particles of carbon are torn away from the + electrode,
+which becomes hollowed out to a cup-shape, and some of these are
+deposited on the - electrode."
+
+[Illustration: Fig. 209.]
+
+=153. Arc Lamps.= As the carbons gradually wear away, some device is
+necessary to keep their ends the right distance apart. If they are too
+near, the arc is very small; and if too far apart, the current can not
+pass and the light goes out. The positive carbon gives the more intense
+light and wears away about twice as fast as the - carbon, so it is
+placed above the - carbon, to throw the light downwards.
+
+[Illustration: Fig. 210.]
+
+[Illustration: Fig. 211.]
+
+Arc lamps contain some device by which the proper distance between
+the carbons can be kept. Most of them grip the upper carbon and pull
+it far enough above the lower one to establish the arc. As soon as
+the distance between them gets too great again, the grip on the upper
+carbon is loosened, allowing the carbon to drop until it comes in
+contact with the lower one, thus starting the current again. These
+motions are accomplished by electromagnets. Fig. 207 shows a form of
+arc lamp with _single carbons_ that will burn from 7 to 9 hours.
+
+[Illustration: Fig. 212.]
+
+[Illustration: Fig. 213.]
+
+[Illustration: Fig. 214.]
+
+Fig. 208 shows the mechanism by which the carbons are regulated. Fig.
+209 shows a form of _double carbon_, or _all-night_ lamp, one set of
+carbons being first used, the other set being automatically switched in
+at the proper time.
+
+[Illustration: Fig. 215.]
+
+Figs. 210, 211 show forms of _short arc lamps_, for use under low
+ceilings, so common in basements, etc.
+
+Fig. 212 shows a _hand-feed focussing_ type of _arc lamp_. In regular
+street lamps, the upper carbon only is fed by mechanism, as it burns
+away about twice as fast as the lower one, thus bringing the arc lower
+and lower. When it is desired to keep the arc at the focus of a
+reflector, both carbons must be fed.
+
+Fig. 213 shows a _theatre arc lamp_, used to throw a strong beam of
+light from the balcony to the stage.
+
+Fig. 214 shows the arc lamp used as a search-light. The reflector
+throws a powerful beam of light that can be seen for miles; in
+fact, the light is used for signalling at night. Fig. 215 shows how
+search-lights are used at night on war-vessels.
+
+
+
+
+CHAPTER XXIII.
+
+X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.
+
+
+[Illustration: Fig. 216.]
+
+[Illustration: Fig. 217.]
+
+=154. Disruptive Discharges.= We have seen, in the study of induction
+coils, that a spark can jump several inches between the terminals
+of the secondary coil. The attraction between the two oppositely
+charged terminals gets so great that it overcomes the resistance of
+the air-space between them, a brilliant spark passes, and they are
+discharged. This sudden discharge is said to be _disruptive_, and it
+is accompanied by a flash of light and a loud report. The _path_ of
+the discharge may be nearly straight, or crooked, depending upon the
+nature of the material in the gap between the terminals.
+
+[Illustration: Fig. 218.]
+
+[Illustration: Fig. 219.]
+
+=155. Effect of Air Pressure on Spark.= The disruptive spark takes
+place in air at ordinary pressures. The nature of the spark is greatly
+changed when the pressure of the air decreases. Fig. 216 shows an
+air-tight glass tube so arranged that the air can be slowly removed
+with an air-pump. The upper rod shown can be raised or lowered to
+increase the distance between it and the lower rod, these acting as the
+terminals of an induction coil. Before exhausting any air, the spark
+will jump a small distance between the rods and act as in open air. As
+soon as a small amount of air is removed, a change takes place. The
+spark is not so intense and has no definite path, there being a general
+glow throughout the tube. As the air pressure becomes still less, the
+glow becomes brighter, until the entire tube is full of purple light
+that is able to pass the entire length of it; that is, the discharge
+takes place better in rarefied air than it does in ordinary air.
+
+=156. Vacuum-Tubes.= As electricity passes through rarefied gases much
+easier than through ordinary air, regular tubes, called _vacuum-tubes_,
+are made for such study. Fig. 217 shows a plain tube of this kind,
+platinum terminals being fused in the glass for connections. These
+tubes are often made in complicated forms, Fig. 218, with colored
+glass, and are called _Geissler tubes_. They are often made in such a
+way that the electrodes are in the shape of discs, etc., and are called
+_Crookes tubes_, Fig. 219. A slight amount of gas is left in the tubes.
+
+[Illustration: Fig. 220.]
+
+[Illustration: Fig. 220-A.]
+
+=157. Cathode Rays.= The _cathode_ is the electrode of a vacuum-tube
+by which the current leaves the tube, and it has been known for some
+time that some kind of influence passes in straight lines from this
+point. Shadows, Fig. 219, are cast by such rays, a screen being placed
+in their path.
+
+=158. X-Rays.= Professor Roentgen of Würzburg discovered that when the
+cathode rays are allowed to fall upon a solid body, the solid body
+gives out still other rays which differ somewhat from the original
+cathode rays. They can penetrate, more or less, through many bodies
+that are usually considered opaque. The hand, for example, may be used
+as a negative for producing a photograph of the bones, as the rays do
+not pass equally well through flesh and bone.
+
+[Illustration: Fig. 221.]
+
+Fig. 220 shows a Crookes tube fitted with a metal plate, so that
+the cathode rays coming from C will strike it. The X-rays are given
+out from P. These rays are invisible and are even given out where
+the cathode rays strike the glass. Some chemical compounds are made
+luminous by these rays; so screens are made and coated with them in
+order that the shadows produced by the X-rays can be seen by the
+eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a
+_fluoroscope_ that contains a screen covered with proper chemicals.
+
+[Illustration: Fig. 222.]
+
+[Illustration: Fig. 223.]
+
+=159. X-Ray Photographs.= Bone does not allow the X-rays to pass
+through it as readily as flesh, so if the hand be placed over a
+sensitized photographic plate, Fig. 221, and proper connections be
+made with the induction coil, etc., the hand acts as a photographic
+negative. Upon developing the plate, as in ordinary photography,
+a picture or shadow of the bones will be seen. Fig. 222 shows the
+arrangement of battery, induction coil, focus tube, etc., for examining
+the bones of the human body.
+
+Fig. 223 shows the bones of a fish. Such photographs have been very
+valuable in discovering the location of bullets, needles, etc., that
+have become imbedded in the flesh, as well as in locating breaks in the
+bones.
+
+
+
+
+CHAPTER XXIV.
+
+THE ELECTRIC MOTOR, AND HOW IT DOES WORK.
+
+
+=160. Currents and Motion.= We have seen, Chapter XII., that when coils
+of wire are rapidly moved across a strong magnetic field, a current
+of electricity is generated. We have now to deal with the opposite of
+this; that is, we are to study how _motion_ can be produced by allowing
+a current of electricity to pass through the armature of a machine.
+
+[Illustration: Fig. 224.]
+
+[Illustration: Fig. 225.]
+
+Fig. 224 shows, by diagram, a coil H, suspended so that it can move
+easily, its ends being joined to a current reverser, and this, in turn,
+to a dry cell D C. A magnet, H M, will attract the core of H when
+no current passes. When the current is allowed to pass first in one
+direction and then in the opposite direction, by using the reverser,
+the core of H will jump back and forth from one pole of H M to the
+other. There are many ways by which motion can be produced by the
+current, but to have it practical, the motion must be a rotary one.
+(See "Study," Chapter XXVI., for numerous experiments.)
+
+[Illustration: Fig. 226.]
+
+=161. The Electric Motor= is a machine for transforming electric
+energy into mechanical power. The construction of motors is very
+similar to that of dynamos. They have field-magnets, armature coils,
+commutator, etc.; in fact, the armature of an ordinary direct current
+dynamo will revolve if a current be passed through it, entering by one
+brush and leaving by the other. There are many little differences of
+construction, for mechanical and electrical reasons, but we may say
+that the general construction of dynamos and motors is the same.
+
+Fig. 225 shows a coil of wire, the ends of which are connected to
+copper and zinc plates. These plates are floated in dilute sulphuric
+acid, and form a simple cell which sends a current through the wire, as
+shown by the arrows.
+
+[Illustration: Fig. 227.]
+
+We have seen that a current-carrying wire has a magnetic field and
+acts like a magnet; so it will be easily seen that if a magnet be held
+near the wire it will be either attracted or repelled, the motion
+depending upon the poles that come near each other. As shown in the
+figure, the N pole of the magnet repels the field of the wire, causing
+it to revolve. We see that this action is just the reverse to that in
+galvanometers, where the coil is fixed, and the magnet, or magnetic
+needle, is allowed to move. As soon as the part of the wire, marked A
+in Fig. 225, gets a little distance from the pole, the opposite side
+of the wire, B, begins to be attracted by it, the attraction getting
+stronger and stronger, until it gets opposite the N pole. If the N pole
+were still held in place, B would vibrate back and forth a few times,
+and finally come to rest near the pole. If, however, as soon as B gets
+opposite N the S pole of the magnet be quickly turned toward B, the
+coil will be repelled and the rotary motion will continue.
+
+[Illustration: Fig. 228.]
+
+[Illustration: Figs. 229 to 231.]
+
+[Illustration: Fig. 232.]
+
+[Illustration: Fig. 233.]
+
+Let us now see how this helps to explain electric motors. We may
+consider the wire of Fig. 225 as one coil of an armature, and the
+plates, C and Z, as the halves of a commutator. In this arrangement, it
+must be noted, the current always flows through the armature coil in
+the same direction, the rotation being kept up by reversing the poles
+of the field-magnet. In ordinary simple motors the current is reversed
+in the armature coils, the field-magnets remaining in one position
+without changing the poles. This produces the same effect as the above.
+The current is reversed automatically as the brushes allow the current
+to enter first one commutator bar and then the opposite one as the
+armature revolves. The regular armatures have many coils and many
+commutator bars, as will be seen by examining the illustrations shown.
+
+The ordinary galvanometer may be considered a form of motor. By
+properly opening and closing the circuit, the rotary motion of the
+needle can be kept up as long as current is supplied. Even an electric
+bell or telegraph sounder may be considered a motor, giving motion
+straight forward and back.
+
+=162. The Uses of Motors= are many. It would be impossible to mention
+all the things that are done with the power from motors. A few
+illustrations will give an idea of the way motors are attached to
+machines.
+
+Fig. 226 shows one form of motor, the parts being shown in Fig. 227.
+
+[Illustration: Fig. 234.]
+
+Fig. 228 shows a fan motor run by a battery. They are generally run
+by the current from the street. Figs. 229-231 show other forms of fan
+motors. Fig. 232 shows an electric hat polisher. A church organ bellows
+is shown in Fig. 233, so arranged that it can be pumped by an electric
+motor. Fig. 234 shows a motor direct connected to a drill press.
+
+=163. Starting Boxes.= If too much current were suddenly allowed to
+pass into the armature of a motor, the coils would be over-heated,
+and perhaps destroyed, before it attained its full speed. A rapidly
+revolving armature will take more current, without being overheated,
+than one not in motion. A motor at full speed acts like a dynamo, and
+generates a current which tends to flow from the machine in a direction
+opposite to that which produces the motion. It is evident, then, that
+when the armature is at rest, all the current turned on passes through
+it without meeting with this opposing current.
+
+[Illustration: Fig. 235.]
+
+[Illustration: Fig. 236.]
+
+Fig. 235 shows a starting, stopping, and regulating box, inside of
+which are a number of German-silver resistance coils properly connected
+to contact-points at the top. By turning the knob, the field of the
+motor is immediately charged first through resistance, then direct, and
+then the current is put on the armature gradually through a series of
+coils, the amount of current depending upon the distance the switch is
+turned. Fig. 236 shows a cross section of the same.
+
+
+
+
+CHAPTER XXV.
+
+ELECTRIC CARS, BOATS, AND AUTOMOBILES.
+
+
+=164. Electric Cars=, as well as boats, automobiles, etc., etc., are
+moved by the power that comes from electric motors, these receiving
+current from the dynamos placed at some "central station." We have
+already seen how the motor can do many kinds of work. By properly
+gearing it to the car wheels, motion can be given to them which will
+move the car.
+
+[Illustration: Fig. 237.]
+
+Fig. 237 shows two dynamos which will be supposed to be at a power
+house and which send out a current to propel cars. From the figure
+it will be seen that the wires over the cars, called trolley-wires,
+are connected to the positive (+) terminals of the dynamos, and that
+the negative (-) terminals are connected to the tracks. In case a
+wire were allowed to join the trolley-wire and track, we should have
+a short circuit, and current would not only rush back to the dynamo
+without doing useful work, but it would probably injure the machines.
+When some of the current is allowed to pass through a car, motion is
+produced in the motors, as has been explained. As the number of cars
+increases, more current passes back to the dynamos, which must do more
+work to furnish such current.
+
+_Trolley-poles_, fastened to the top of the cars and which end in
+grooved wheels, called _trolley-wheels_, are pressed by springs against
+the trolley-wires. The current passes down these through switches to
+_controllers_ at each end of the car, one set being used at a time.
+
+[Illustration: Fig. 238.]
+
+[Illustration: Fig. 239.]
+
+=165. The Controllers=, as the name suggests, control the speed of the
+car by allowing more or less current to pass through the motors. The
+motors, resistance coils and controllers are so connected with each
+other that the amount of current used can be regulated.
+
+[Illustration: Fig. 240.]
+
+[Illustration: Fig. 241.]
+
+When the motorman turns the handle of the controller to the first
+notch, the current passes through all of the resistance wires placed
+under the car, then through one motor after the other. The motors being
+joined in series by the proper connections at the controller, the
+greatest resistance is offered to the current and the car runs at the
+slowest speed at this first notch. As more resistance is cut out by
+turning the handle to other notches, the car increases its speed; but
+as the resistance wires become heated and the heat passes into the air,
+there is a loss of energy. It is not economical to run a car at such a
+speed that energy is wasted as heat. As soon as the resistance is all
+cut out, the current simply passes through the motors joined in series.
+This gives a fairly slow speed and one that is economical because all
+the current tends to produce motion.
+
+By allowing the current to pass through the motors joined in parallel,
+that is, by allowing each to take a part of the current, the resistance
+is greatly reduced, and a higher speed attained. This is not instantly
+done, however, as too much strain would be put upon the motors. As soon
+as the next notch is reached, the motors are joined in parallel and
+the resistance also thrown in again. By turning the handle still more,
+resistance is gradually cut out, and the highest speed produced when
+the current passes only through the motors in parallel.
+
+[Illustration: Fig. 242.]
+
+[Illustration: Fig. 243.]
+
+Fig. 238 represents a controller, by diagram, showing the relative
+positions of the controller cylinder, reversing and cut-out cylinders,
+arrangements for blowing out the short electric arcs formed, etc. A
+ratchet and pawl is provided, which indicates positively the running
+notches, at the same time permitting the cylinder to move with ease.
+Fig. 239 shows a top view of the controller.
+
+[Illustration: Fig. 244.]
+
+=166. Overhead and Underground Systems.= When wires for furnishing
+current are placed over the tracks, as in Fig. 237, we have the
+overhead system. In cities the underground system is largely used.
+The location of the conducting wires beneath the surface of the
+street removes all danger to the public, and protects them from all
+interference, leaving the street free from poles and wires.
+
+Fig. 240 shows a cross-section of an underground conduit. The rails,
+R R, are supported by cast-iron yokes, A, placed five feet apart, and
+thoroughly imbedded in concrete. The conduit has sewer connections
+every 100 feet. Conducting bars, C C, are placed on each side of
+the conduit, and these are divided into sections of about 500 feet.
+Insulators, D D, are placed every 15 feet. They are attached to, and
+directly under, the slot-rails, the stem passing through the conductor
+bar.
+
+[Illustration: Fig. 245.]
+
+Figs. 240 and 241 show the plow E. The contact plates are carried on
+coiled springs to allow a free motion. Two guide-wheels, F F, are
+attached to the leg of the plow. The conducting wires are carried up
+through the leg of the plow.
+
+=167. Appliances.= A large number of articles are needed in the
+construction of electric railroads. A few, only, can be shown that are
+used for the overhead system. Fig. 242 shows a pole insulator. Fig. 243
+shows a feeder-wire insulator. Fig. 244 shows a line suspension. Fig.
+245 shows a form of right-angle cross which allows the trolley-wheels
+of crossing lines to pass. Fig. 246 shows a switch. In winter a part of
+the current is allowed to pass through electric heaters placed under
+the seats of electric cars.
+
+[Illustration: Fig. 246.]
+
+=168. Electric Boats= are run by the current from storage batteries
+which are usually placed under the seats. An electric motor large
+enough to run a small boat takes up very little room and is generally
+placed under the floor. This leaves the entire boat for the use of
+passengers. The motor is connected to the shaft that turns the screw.
+Fig. 247 shows one design.
+
+=169. Electric Automobiles= represent the highest type of electrical
+and mechanical construction. The _running-gear_ is usually made of the
+best cold-drawn seamless steel tubing, to get the greatest strength
+from a given weight of material. The wheels are made in a variety of
+styles, but nearly all have ball bearings and pneumatic tires. In the
+lightest styles the wheels have wire spokes.
+
+The _electric motors_, supported by the running-gear, are geared to
+the rear wheels. The motors are made as nearly dust-proof as possible.
+
+_Storage batteries_ are put in a convenient place, depending upon the
+design of the carriage, and from these the motors receive the current.
+These can be charged from the ordinary 110-volt lighting circuits or
+from private dynamos. The proper plugs and attachments are usually
+furnished by the various makers for connecting the batteries with the
+street current, which is shut off when the batteries are full by an
+automatic switch.
+
+[Illustration: Fig. 247.]
+
+_Controllers_ are used, as on electric cars, the lever for starting,
+stopping, etc., being usually placed on the left-hand side of the seat.
+The _steering_ is done by a lever that moves the front wheels. Strong
+brakes, and the ability to quickly reverse the motors, allow electric
+carriages to be stopped suddenly in case of accidents.
+
+Electric automobiles are largely used in cities, or where the current
+can be easily had. The batteries must be re-charged after they have
+run the motors for a certain time which depends upon the speed and
+road, as well as upon the construction. Where carriages are to be run
+almost constantly, as is the case with those used for general passenger
+service in cities, duplicate batteries are necessary, so that one or
+two sets can be charged while another is in use. Fig. 248 shows one
+form of electric vehicle, the storage batteries being placed under and
+back of the seat.
+
+[Illustration: Fig. 248.]
+
+
+
+
+CHAPTER XXVI.
+
+A WORD ABOUT CENTRAL STATIONS.
+
+
+=170. Central Stations=, as the word implies, are places where, for
+example, electricity is generated for the incandescent or arc lights
+used in a certain neighborhood; where telephone or telegraph messages
+are sent to be resent to some other station; where operators are kept
+to switch different lines together, so that those on one line can
+talk to those on another, etc., etc. There are many kinds of central
+stations, each requiring a large amount of special apparatus to carry
+on the work. Fig. 249 gives a hint in regard to the way car lines
+get their power from a central power station. As a large part of the
+apparatus required in ordinary central stations has already been
+described, it is not necessary to go into the details of such stations.
+
+[Illustration: Fig. 249.]
+
+In lighting stations, for example, we have three principal kinds of
+apparatus. Boilers produce the steam that runs the steam engines, and
+these run the dynamos that give the current. Besides these there are
+many other things needed. The electrical energy that goes over the
+wires to furnish light, heat, and power, really comes indirectly from
+the coal that is used to boil water and convert it into steam. The
+various parts of the central station merely aid in this transformation
+of energy.
+
+[Illustration: Fig. 250.]
+
+[Illustration: Fig. 251.]
+
+The dynamos are connected to the engines by belts, or they are direct
+connected. Figs. 250, 251, show dynamos connected to engines without
+belts.
+
+The current from the dynamos is led to large switchboards which contain
+switches, voltmeters, ammeters, lightning arresters, and various other
+apparatus for the proper control and measurement of the current. From
+the switchboard it is allowed to pass through the various street mains,
+from which it is finally led to lamps, motors, etc.
+
+Water-power is frequently used to drive the dynamos instead of steam
+engines. The water turns some form of water-wheel which is connected
+to the dynamos. At Niagara Falls, for example, immense quantities of
+current are generated for light, heat, power, and industrial purposes.
+
+[Illustration]
+
+
+
+
+CHAPTER XXVII.
+
+MISCELLANEOUS USES OF ELECTRICITY.
+
+
+=171. The Many Uses= to which the electric current is put are almost
+numberless. New uses are being found for it every day. Some of the
+common applications are given below.
+
+=172. Automatic Electric Program Clocks=, Fig. 252, are largely used
+in all sorts of establishments, schools, etc., for ringing bells at
+certain stated periods. The lower dial shown has many contact-points
+that can be inserted to correspond to given times. As this revolves,
+the circuits are closed, one after the other, and it may be so set that
+bells will be rung in different parts of the house every five minutes,
+if desired.
+
+[Illustration: Fig. 252.]
+
+[Illustration: Fig. 253.]
+
+=173. Call Boxes= are used to send in calls of various kinds to
+central stations. Fig. 253 shows one form. The number of different
+calls provided includes messenger, carrier, coupé, express wagon,
+doctor, laborer, police, fire, together with three more, which may be
+made special to suit the convenience of the individual customer. The
+instruments are provided with apparatus for receiving a return signal,
+the object of which is to notify the subscriber that his call has been
+received and is having attention.
+
+[Illustration: Fig. 254.]
+
+[Illustration: Fig. 255.]
+
+Fig. 254 shows another form of call box, the handle being moved around
+to the call desired. As it springs back to the original position, an
+interrupted current passes through the box to the central station,
+causing a bell to tap a certain number of times, giving the call and
+location of the box.
+
+=174. Electric Gas-Lighters.= Fig. 255 shows a _ratchet burner_. The
+first pull of the chain turns on the gas through a four-way gas-cock,
+governed by a ratchet-wheel and pawl. The issuing gas is lighted by a
+wipe-spark at the tip of the burner. Alternate pulls shut off the gas.
+As the lever brings the attached wire A, in contact with the wire B,
+a bright spark passes, which ignites the gas, the burner being joined
+with a battery and induction or spark coil.
+
+_Automatic burners_ are used when it is desired to light gas at
+a distance from the push-button. Fig. 256 shows one form. Two
+electromagnets are shown, one being generally joined to a white
+push-button for turning on the gas and lighting it, the other being
+joined to a black button which turns off the gas when it is pressed.
+The armatures of the magnets work the gas-valve. Sparks ignite the gas,
+as explained above.
+
+[Illustration: Fig. 256.]
+
+[Illustration: Fig. 257.]
+
+=175. Door Openers.= Fig. 257 shows one form. They contain
+electromagnets so arranged that when the armature is attracted by the
+pushing of a button anywhere in the building, the door can be pushed
+open.
+
+=176. Dental Outfits.= Fig. 258 shows a motor arranged to run dental
+apparatus. The motor can be connected to an ordinary incandescent light
+socket. In case the current gives out, the drills, etc., can be run by
+foot power.
+
+[Illustration: Fig. 258.]
+
+=177. Annunciators= of various kinds are used in hotels, factories,
+etc., to indicate a certain room when a bell rings at the office.
+The bell indicates that some one has called, and the annunciator
+shows the location of the call by displaying the number of the room
+or its location. Fig. 259 shows a small annunciator. They contain
+electromagnets which are connected to push-buttons located in the
+building, and which bring the numbers into place as soon as the current
+passes through them.
+
+[Illustration: Fig. 259.]
+
+
+
+
+INDEX.
+
+
+Numbers refer to paragraphs. See Table of Contents for the titles of
+the various chapters.
+
+    Action of magnets upon each other, 32.
+
+    Adjuster, for lamp cords, 151.
+
+    Air pressure, effect of spark upon, 155.
+
+    Aluminum-leaf, for electroscopes, 5.
+
+    Alternating current, 129, 130;
+      system of wiring for, 144.
+
+    Amalgamation of zincs, 47.
+
+    Amber, electrification upon, 3.
+
+    Ammeter, the, 74;
+      how placed in circuit, 77.
+
+    Ampere, the, 72.
+
+    Annunciators, 177.
+
+    Anode, 79, 82.
+
+    Apparatus for electrical measurements, Chap. VI.
+
+    Appliances, for distribution of currents, 141;
+      for electric railways, 167;
+      for heating by electricity, 147.
+
+    Arc, the electric, 152.
+
+    Arc lamp, the, 153;
+      how light is produced by, Chap. XXII.;
+      double carbon, 153;
+      hand-feed focussing, 153;
+      for search-lights, 153;
+      short, for basements, 153;
+      single carbon, 153;
+      for theater use, 153.
+
+    Armature, of dynamo, 127, 129;
+      of electromagnets, 98;
+      of horseshoe magnet, 26;
+      of motors, 161;
+      uses of, 39.
+
+    Artificial magnets, 25.
+
+    Astatic, detectors, 94;
+      galvanometer, 73;
+      needles, 94.
+
+    Aurora borealis, 23.
+
+    Automatic, current interrupters, 104, 115;
+      gas lighters, 174;
+      program clocks, 172.
+
+    Automobiles, 169;
+      controllers for, 169;
+      motors for, 169;
+      steering of, 169;
+      storage batteries for, 169.
+
+
+    Bamboo filaments, 149.
+
+    Bar magnets, 27;
+      magnetic figures of, 38.
+
+    Batteries, large plunge, 54;
+      plunge, 53;
+      secondary, 86;
+      storage, and how they work, Chap. IX.
+
+    Bell, the electric, and some of its uses, Chap. XV.;
+      electric, 116;
+      magneto testing, 117;
+      trembling, etc., 116.
+
+    Bell transmitter, 120.
+
+    Belts, electricity generated by friction upon, 1.
+
+    Benjamin Franklin, 18.
+
+    Bichromate of potash cells, 51, etc.
+
+    Binding-posts, Chap. V.;
+      common forms of, 63.
+
+    Blasting, by electricity, 147;
+      electric machines for, 147.
+
+    Bluestone cell, 56.
+
+    Boats, electric, 168.
+
+    Boilers, use of in central stations, 170.
+
+    Bones, photographed by x-rays, Chap. XXIII.
+
+    Boosters, 136.
+
+    Brushes, 129.
+
+    Bunsen cells, 56_a_.
+
+    Burner, automatic, 174;
+      for gas-lights, 174;
+      ratchet, 174.
+
+    Buzzers, electric, 118.
+
+
+    Cables and wires, 143.
+
+    Call boxes, electric, 173.
+
+    Carbon, in arc lamps, 152, 153;
+      filament, 149;
+      transmitter, 123.
+
+    Carpet, electricity generated upon, 1.
+
+    Cars, electric, 164;
+      controllers for, 165;
+      heating by electricity, 167;
+      overhead system for, 166;
+      underground system for, 166.
+
+    Cat, electricity generated upon, 1.
+
+    Cathode, definition of, 79;
+      rays, 157.
+
+    Cells, Bunsen, 56_a_;
+      bichromate of potash, 51;
+      closed circuit, 50;
+      dry, 58;
+      Edison-Lelande, 59;
+      electricity generated by, Chap. III.;
+      Fuller, 55;
+      Gonda, 57;
+      gravity, 56;
+      Grenet, 52;
+      Leclanché, 57;
+      open circuit, 50;
+      plates and poles of, 45_a_;
+      polarization of, 48;
+      simple, 45, 49;
+      single-fluid, 49;
+      two-fluid, 49;
+      various voltaic, Chap. IV.
+
+    Central stations, 170;
+      a word about, Chap. XXVI.
+
+    Chain lightning, 19.
+
+    Chafing-dishes, electrical, 147.
+
+    Charging condensers, 15.
+
+    Chemical action, and electricity, 81.
+
+    Chemical effects of electric current, Chap. VII.
+
+    Chemical meters, 78.
+
+    Church organs, pumped by motors, 162.
+
+    Circuits, electric, 50;
+      for lamps, 144.
+
+    Cleats, porcelain, 141;
+      wooden, 141.
+
+    Clocks, automatic electric, 172.
+
+    Closed circuit cells, 50.
+
+    Coils, induction, and how they work, Chap. XIII.;
+      induction, construction of, 104;
+      method of joining, 98;
+      primary and secondary, 103;
+      resistance, 69;
+      rotation of, 95;
+      of transformers, 135.
+
+    Collectors on dynamos, 129.
+
+    Commutators, 129.
+
+    Compasses, magnetic, 31.
+
+    Compound, magnets, 28;
+      wound dynamo, 131.
+
+    Condensation of static electricity, 15.
+
+    Condensers, 15;
+      for induction coils, 104.
+
+    Conductors, and insulators, 4, 138.
+
+    Conduits, electric, 140.
+
+    Connections, electrical, 60;
+      for telegraph lines, 111.
+
+    Controllers, for automobiles, 169;
+      for electric cars, 165.
+
+    Copper sulphate, effects of current on, 82;
+      formula of, 79.
+
+    Copper voltameters, 75.
+
+    Cords, adjustable for lamps, 151.
+
+    Coulomb, the, 76.
+
+    Crater of hot carbons, 152.
+
+    Crookes tubes, 156, 158.
+
+    Current, detectors, 93;
+      direction of in cell, 46;
+      from magnet and coil, 100;
+      from two coils, 102;
+      induced, 127;
+      of induction coils, 105;
+      interrupters, automatic, 104, 115;
+      local, 47;
+      primary and secondary, 102;
+      transformation of, Chap. XVIII.;
+      transmission of, 134.
+
+    Currents, and motion, 160;
+      how distributed for use, Chap. XIX.
+
+    Current strength, 71;
+      measurement of, 73;
+      unit of, 72.
+
+    Cylinder electric machines, 9.
+
+
+    Daniell cell, 56.
+
+    D'Arsonval galvanometer, 73.
+
+    Declination, 41.
+
+    Decorative incandescent lamps, 151.
+
+    Dental, lamps, 151;
+      outfits, 176.
+
+    Detectors, astatic, 94;
+      current, 93.
+
+    Diamagnetic bodies, 29.
+
+    Diaphragm for telephones, 120.
+
+    Dip, of magnetic needle, 42.
+
+    Direct current, 129, 130.
+
+    Direction of current in cell, 46.
+
+    Discharging condensers, 15.
+
+    Disruptive discharges, 154.
+
+    Distribution of currents for use, Chap. XIX.
+
+    Door opener, electric, 175.
+
+    Dots and dashes, 110.
+
+    Drill press, run by motor, 162.
+
+    Dry cells, 58.
+
+    Dynamo, the, 126;
+      alternating current, 130;
+      commutator of, 129;
+      compound wound, 131;
+      direct current, 130;
+      lamps connected to, 132;
+      series wound, 131;
+      shunt wound, 131;
+      used as motor, 161;
+      use of in central stations, 170;
+      used with water power, 170.
+
+    Dynamos, electricity generated by, Chap. XVII.;
+      types of, 130;
+      various machines, 132;
+      winding of, 131.
+
+    Dynamotors, 137.
+
+
+    Earth, inductive influence of, 43;
+      lines of force about, 40, 42.
+
+    Ebonite, electricity by friction upon, 3, 4.
+
+    Edison-Lelande cells, 59.
+
+    Electric, automobiles, 169;
+      bell, and some of its uses, Chap. XV.;
+      boats, 168;
+      buzzers, 118;
+      cars, 164;
+      conduits, 140;
+      fans, 162;
+      flat-irons, 146;
+      gas lighters, 174;
+      griddles, 147;
+      kitchen, 147;
+      lights, arc, Chap. XXII.;
+      lights, incandescent, Chap. XXI.;
+      machines, static, 7 to 13;
+      machines, uses of, 14;
+      motor, the, 161;
+      motor, and how it does work, Chap. XXIV.;
+      soldering irons, 146;
+      telegraph, and how it sends messages, Chap. XIV.;
+      telephone, and how it transmits speech, Chap. XVI.;
+      welding, 146.
+
+    Electric current, and work, 133;
+      and chemical action, 81;
+      chemical effects of, Chap. VII.;
+      how distributed for use, Chap. XIX.;
+      magnetic effects of, Chap. XI.;
+      how transformed, Chap. XVIII.
+
+    Electrical, connections, 60;
+      horse-power, 77;
+      measurements, Chap. VI.;
+      resistance, 68;
+      resistance, unit of, 69;
+      units, Chap. VI.
+
+    Electricity, about frictional, Chap. I.;
+      and chemical action, 81;
+      atmospheric, 18;
+      heat produced by, Chap. XX.;
+      history of, 3;
+      how generated upon cat, 1;
+      how generated by dynamos, Chap. XVII.;
+      how generated by heat, Chap. X.;
+      how generated by induction, Chap. XII.;
+      how generated by voltaic cell, Chap. III.;
+      origin of name, 2.
+
+    Electrification, kinds of, 6;
+      laws of, 7.
+
+    Electrolysis, 79.
+
+    Electrolyte, 79.
+
+    Electromagnetic induction, 99.
+
+    Electromagnetism, 91.
+
+    Electromagnets, 96;
+      forms of, 97.
+
+    Electro-mechanical gong, 116.
+
+    Electromotive force, defined, 65, 71;
+      measurement of, 67;
+      of polarization, 85;
+      of static electricity, 17;
+      unit of, 66.
+
+    Electrophorus, the, 8.
+
+    Electroplating, 82.
+
+    Electroscopes, 5.
+
+    Electrotyping, 83.
+
+    Experiments, early, with currents, 44;
+      some simple, 1.
+
+    External resistance, 68.
+
+
+    Fan motors, 162.
+
+    Field, magnetic, 37.
+
+    Field-magnets, 129.
+
+    Figures, magnetic, 38.
+
+    Filaments, carbon, 149;
+      bamboo, etc., 149.
+
+    Fire, St. Elmo's, 22.
+
+    Flat-irons, electric, 147.
+
+    Floor mains, 139.
+
+    Fluoroscope, 158.
+
+    Force, and induced currents, 101;
+      lines of magnetic, 38;
+      lines of about a wire, 92, 96;
+      lines of about a magnet, 37, 38.
+
+    Frictional electricity, about, Chap, I.;
+      location of charge of, 4;
+      sparks from, 4.
+
+    Fuller cell, the, 55.
+
+    Fuse, link, 142;
+      plug, 142;
+      ribbons, 142;
+      wire, 142.
+
+    Fusible rosettes, 142.
+
+
+    Galvani, early experiments of, 44.
+
+    Galvanometers, 73;
+      astatic, 73;
+      considered as motor, 161;
+      D'Arsonval, 73;
+      tangent, 73.
+
+    Galvanoscope, 73;
+      astatic, 94.
+
+    Gas lighters, electric, 174.
+
+    Geissler tubes, 156.
+
+    Generators, electric, 126.
+
+    Glass, electricity generated upon, 4.
+
+    Glue pots, electric, 147.
+
+    Gold-leaf, for electroscopes, 5.
+
+    Gold plating, 82.
+
+    Gonda cell, 57.
+
+    Gong, electro-mechanical, 116.
+
+    Gravity cell, the, 56;
+      replaced by dynamotors, 137.
+
+    Grenet cell, 52.
+
+    Griddles, electric, 147.
+
+    Guard, for lamps, 151.
+
+
+    Heat, how generated by electricity, Chap. X.;
+      and magnetism, 35;
+      and resistance, 145.
+
+    Heat lightning, 19.
+
+    Heaters, for cars, 167.
+
+    History of electricity, 3.
+
+    Horse-power, electrical, 77.
+
+    Horseshoe, permanent magnets, 26;
+      electromagnets, 97, 98.
+
+    Human body, bones of, photographed by x-rays, Chap. XXIII.
+
+    Hydrogen, action of in cell, 48;
+      attraction of for oxygen, 85.
+
+    Incandescence, 148.
+
+    Incandescent lamp, 149;
+      candle-power of, 150;
+      current for, 150;
+      light produced by, Chap. XXI.;
+      construction of, 149;
+      uses of, 151.
+
+    Inclination of magnetic needle, 42.
+
+    Indicating push-button, 61.
+
+    Induced currents, 127;
+      and lines of force, 101;
+      by rotary motion, 128;
+      of induction coils, 105;
+      of transformers, 135.
+
+    Induced magnetism, 36.
+
+    Induction, electricity generated by, Chap. XII.;
+       electromagnetic, 99.
+
+    Induction coils, condensers for, 104;
+      construction of, 104;
+      currents of, 105;
+      how they work, Chap. XIII.;
+      in telephone work, 124;
+      uses of, 106.
+
+    Inductive influence of earth, 43.
+
+    Influence machines for medical purposes, 13.
+
+    Ink writing registers, 114.
+
+    Insulating tubing, 141.
+
+    Insulators, 141;
+      and conductors, 4, 138;
+      feeder-wire, 167;
+      for poles, 167;
+      porcelain, 141.
+
+    Internal resistance, 68.
+
+    Interrupters, automatic current, 104, 115.
+
+    Ions, 80.
+
+    Iron, electricity upon, by friction, 4.
+
+
+    Jar, Leyden, 15.
+
+    Jarring magnets, effects of, 33.
+
+
+    Keeper of magnets, 26.
+
+    Keys, telegraph, 109.
+
+    Kinds of electrification, 6.
+
+    Kitchen, electric, 147.
+
+    Knife switch, 62.
+
+
+    Lamp, incandescent, candle-power of, 150;
+      cord, adjustable, 151;
+      current for, 150;
+      dental, 151;
+      for desks, 151;
+      for throat, 151;
+      guard for, 151;
+      incandescent, 149;
+      socket, 151;
+      with half shade, 151.
+
+    Lamp, the arc, 153;
+      how light is produced by, Chap. XXII.;
+      double carbon, 153;
+      hand-feed focussing, 153;
+      for search-lights, 153;
+      single carbon, 153;
+      short, for basements, 153;
+      for theater use, 153.
+
+    Lamp circuits, alternating system, 144.
+
+    Lamps, in parallel, 144;
+      lamps in series, 144;
+      three-wire system, 144;
+      two-wire system, 144.
+
+    Laws, of electrification, 7;
+      of magnetic attraction, 32;
+      of resistance, 70.
+
+    Leaf electroscopes, 5.
+
+    Leclanché cell, 57.
+
+    Leyden, battery, 16;
+      jar, 15.
+
+    Light, how produced by arc lamp, Chap. XXII.;
+      how produced by incandescent lamp, Chap. XXI.
+
+    Lightning, 19;
+      rods, 21.
+
+    Line, telegraph, Chap. XIV.;
+      connections for, 111;
+      operation of, 112.
+
+    Line suspension, for trolley-wires, 167.
+
+    Line wire, 111.
+
+    Lines of force, conductors of, 39, 96;
+      about the earth, 40, 42;
+      and induced currents, 101;
+      about a magnet, 38;
+      about a wire, 92.
+
+    Local currents, 47.
+
+
+    Magnetic, bodies, 29;
+      declination, 41;
+      effects of electric current, Chap. XI.;
+      field, 37;
+      figure of one bar magnet, 38;
+      figure of two bar magnets, 38;
+      figure of horseshoe magnet, 38;
+      needle, dip of, 42;
+      needles and compasses, 31.
+
+    Magnetism, and heat, 35;
+      induced, 36;
+      laws of, 32;
+      residual, 34;
+      retentivity, 34;
+      temporary, 36;
+      terrestrial, 40;
+      theory of, 33.
+
+    Magneto, signal bells, 117;
+      testing bells, 117;
+      transmitter, 120.
+
+    Magnets, action upon each other, 32;
+      artificial, 25;
+      bar, 27;
+      compound, 28;
+      effects of jarring, 33;
+      electro, 96;
+      electro, forms of, 97;
+      horseshoe, 26;
+      and magnetism, about, Chap. II.;
+      making of, 30;
+      natural, 24.
+
+    Mains, electric, 139.
+
+    Man-holes, in conduits, 140.
+
+    Measurements, electric, Chap. VI.;
+      of current strength, 73;
+      of E.M.F., 67.
+
+    Meters, chemical, 78;
+      permanent record, 77.
+
+    Microphone, the, 122.
+
+    Motion and currents, 160.
+
+    Motor, acting like dynamo, 163;
+      armature of, 161;
+      controlling speed of, 165;
+      electric, 161;
+      electric, and how it does work, Chap. XXIV.;
+      fans, 162;
+      for automobiles, 169;
+      for boats, 168;
+      for pumping bellows, 162;
+      for running drill press, 162;
+      parts of, 162;
+      starting boxes for, 163;
+      uses of, 162.
+
+    Motor-dynamos, 136.
+
+    Mouldings, for wires, 141.
+
+
+    Name, electricity, origin of, 2.
+
+    Natural magnets, 24.
+
+    Needles, astatic, 94;
+      dipping, 42;
+      magnetic, 31.
+
+    Negative electrification, 5.
+
+    Non-conductors, 4.
+
+    North pole, magnetic of earth, 40;
+      of magnets, 26.
+
+    Northern lights, 23.
+
+
+    Ohm, the, 69.
+
+    Open circuit cells, 50.
+
+    Openers, for doors, 175.
+
+    Outfits, dental, 175.
+
+    Overhead trolley system, 166.
+
+    Oxygen, attraction for hydrogen, 85.
+
+
+    Parallel arrangement of lamps, 144.
+
+    Peltier effect, 89.
+
+    Pendant, electric, 151.
+
+    Pith-ball electroscope, 5.
+
+    Plate electrical machine, 10.
+
+    Plates of cells, 45_a_.
+
+    Plunge batteries, 53;
+      large, 54.
+
+    Polarity of coils, 95.
+
+    Polarization, 84;
+      electromotive force of, 85;
+      of cells, 48.
+
+    Pole-changing switch, 62.
+
+    Poles, of cells, 45_a_;
+      of horseshoe magnet, 26.
+
+    Positive electrification, 6.
+
+    Potential, defined, 65.
+
+    Push-buttons, Chap. V.;
+      indicating, 61;
+      modifications of, 61;
+      table clamp, 61.
+
+
+    Quantity of electricity, 76;
+      unit of, 76.
+
+    Rays, cathode, 157;
+      x-rays, 158.
+
+    Receiver, telephone, 121.
+
+    Reflectors, for lamps, 151.
+
+    Registers, ink writing, 114.
+
+    Relay, the, 113.
+
+    Residual magnetism, 34.
+
+    Resistance, coils and boxes, 69;
+      electrical, 68;
+      external, 68;
+      and heat, 145;
+      internal, 68;
+      laws of, 70;
+      unit of, 69.
+
+    Retentivity, 34.
+
+    Risers, in buildings, 139.
+
+    Rods, lightning, 21.
+
+    Roentgen, Prof., 158.
+
+    Rosette, fusible, 142.
+
+    Running-gear, of automobiles, 169.
+
+
+    Safety, devices, 142;
+      fuse, 142;
+      fuse link, 142;
+      fuse plug, 142;
+      fuse ribbon, 142;
+      fuse wire, 142.
+
+    Search-lights, 153;
+      signals sent by, 153.
+
+    Secondary batteries, 86;
+      uses of, 87.
+
+    Series arrangement of lamps, 144.
+
+    Series wound dynamo, 131.
+
+    Service wires, 139.
+
+    Shunt-wound dynamo, 131.
+
+    Signal bells, magneto, 117.
+
+    Simple cell, the, 45, 49.
+
+    Single-fluid cells, 49.
+
+    Single-point switch, 62.
+
+    Single-stroke bell, 116.
+
+    Socket, for incandescent lamps, 151.
+
+    Soldering irons, electric, 147.
+
+    Sounders, telegraph, 110;
+      home-made, 110.
+
+    Spark, effect of air pressure on, 155.
+
+    Sparks, from cells, 17;
+      from frictional electricity, 4.
+
+    St. Elmo's fire, 22.
+
+    Starting boxes, for motors, 163.
+
+    Static electric machines, 8.
+
+    Static electricity, condensation of, 15;
+      electromotive force of, 17;
+      to test presence of, 5;
+      uses of, 14.
+
+    Steam engines, in central stations, 170.
+
+    Steel, inductive influence of earth upon, 43;
+      retentivity of, 26.
+
+    Storage batteries, the, and how they work, Chap. IX.;
+      for automobiles, 169;
+      for boats, 168;
+      for natural sources of power, 87.
+
+    Stoves, electric, 147.
+
+    Strength of current, 71;
+      measurement of, 73;
+      unit of, 72.
+
+    Switchboards, 62.
+
+    Switches, Chap. V.;
+      knife, 62;
+      pole-changing, 62;
+      single point, 62;
+      for trolley lines, 167.
+
+    Table clamp-push, 61.
+
+    Tangent galvanometer, 73.
+
+    Teakettles, electric, 147.
+
+    Telegraph, electric, and how it sends messages, Chap. XIV.;
+      ink writing registers, 114;
+      keys, 109;
+      relay, 113;
+      sounders, 110.
+
+    Telegraph line, 107, 108;
+      operation of, 112;
+      simple connections of, 111.
+
+    Telephone, the, and how it transmits speech, Chap. XVI.;
+      receiver, 121;
+      transmitter, 120;
+      use of induction coil with, 124;
+      various forms of, 125.
+
+    Temporary magnetism, 36.
+
+    Terrestrial magnetism, 40.
+
+    Theory of magnetism, 33.
+
+    Thermoelectricity, 88.
+
+    Thermopiles, 90.
+
+    Three-wire system, 144.
+
+    Throat, lamp for, 151.
+
+    Thunder, 20.
+
+    Toepler-Holtz machines, 11.
+
+    Transformers, 135.
+
+    Transforming electric current, Chap. XVIII.;
+      for electric welding, 146.
+
+    Transmission of currents, 134.
+
+    Transmitter, Bell, 120;
+      carbon, 123.
+
+    Trembling bell, 116.
+
+    Trolley-wires, 164;
+      -poles, 164;
+      -wheels, 164.
+
+    Tubes, Crookes, 156, 158;
+      Geissler, 156;
+      vacuum, 156.
+
+    Two-fluid cells, 49.
+
+    Two-wire system, 144.
+
+
+    Underground trolley system 166;
+      conduits for, 166.
+
+    Unit, of current strength, 72;
+      of electromotive force, 66;
+      of quantity, 76;
+      of resistance, 69.
+
+    Units, electrical, Chap. VI.
+
+    Uses, of armatures, 39;
+      of electricity, miscellaneous, Chap. XXVII.;
+      of induction coils, 106;
+      of motors, 162;
+      of storage batteries, 87.
+
+
+    Vacuum-tubes, 156.
+
+    Variation, angle of, 41.
+
+    Volt, the, 66.
+
+    Volta, 66;
+      early experiments of, 44.
+
+    Voltaic cell, electricity generated by, Chap. III.
+
+    Voltaic pile, 44.
+
+    Voltameters, 75;
+      copper, 75;
+      water, 75.
+
+    Voltmeters, 67, 77.
+
+
+    Water, decomposition of, 79;
+      power, source of energy, 170;
+      voltameters, 73.
+
+    Watt, the, 77.
+
+    Wattmeters, 77.
+
+    Welding, electric, 146.
+
+    Wimshurst electric machine, 12.
+
+    Wires and cables, 143.
+
+    Wiring, for alternating system, 144;
+      three-wire system, 144;
+      two-wire system, 144.
+
+    Work, and electric current, 133.
+
+
+    X-ray photographs, 159.
+
+    X-rays, 156;
+      and how the bones of the human body are photographed, Chap. XXIII.
+
+
+    Yokes, 97, 98.
+
+
+    Zincs, amalgamation of, 47.
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.
+
+
+    By THOMAS M. ST. JOHN, Met. E.
+
+
+    The book contains 180 pages, and 260 illustrations; it measures
+    5 x 7½ in., and is bound in cloth.
+
+    PRICE, POST-PAID, $1.00.
+
+    =CONTENTS:= _Chapter_ I. About Frictional Electricity.--II.
+    About Magnets and Magnetism.--III. How Electricity
+    is Generated by the Voltaic Cell.--IV. Various
+    Voltaic Cells.--V. About Push-Buttons, Switches and
+    Binding-Posts.--VI. Units and Apparatus for Electrical
+    Measurements.--VII. Chemical Effects of the Electric
+    Current.--VIII. How Electroplating and Electrotyping are
+    Done.--IX. The Storage Battery and How it Works.--X. How
+    Electricity is Generated by Heat.--XI. Magnetic Effects of
+    the Electric Current.--XII. How Electricity is Generated
+    by Induction.--XIII. How the Induction Coil Works.--XIV.
+    The Electric Telegraph, and How it Sends Messages.--XV. The
+    Electric Bell and Some of its Uses.--XVI. The Telephone,
+    and How it Transmits Speech.--XVII. How Electricity
+    is Generated by Dynamos.--XVIII. How the Electric
+    Current is Transformed.--XIX. How Electric Currents are
+    Distributed for Use.--XX. How Heat is Produced by the
+    Electric Current.--XXI. How Light is Produced by the
+    Incandescent Lamp.--XXII. How Light is Produced by the Arc
+    Lamp.--XXIII. X-Rays, and How the Bones of the Human Body
+    are Photographed.--XXIV. The Electric Motor and How it Does
+    Work.--XXV. Electric Cars, Boats and Automobiles.--XXVI. A
+    Word About Central Stations.--XXVII. Miscellaneous Uses of
+    Electricity.
+
+This book explains, in simple, straightforward language, many things
+about electricity; things in which the American boy is intensely
+interested; things he wants to know; things he should know.
+
+It is free from technical language and rhetorical frills, but it tells
+how things work, and why they work.
+
+It is brimful of illustrations--the best that can be had--illustrations
+that are taken directly from apparatus and machinery, and that show
+what they are intended to show.
+
+This book does not contain experiments, or tell how to make apparatus;
+our other books do that. After explaining the simple principles of
+electricity, it shows how these principles are used and combined to
+make electricity do every-day work.
+
+    _Everyone Should Know About Electricity._
+
+    A VERY APPROPRIATE PRESENT
+
+
+
+
+THIRD EDITION
+
+How Two Boys Made Their Own Electrical Apparatus.
+
+
+    Containing complete directions for making all kinds of
+    simple electrical apparatus for the study of elementary
+    electricity. By PROFESSOR THOMAS M. ST. JOHN, New York City.
+
+    The book measures 5 × 7½ in., and is beautifully bound in
+    cloth. It contains 141 pages and 125 illustrations. Complete
+    directions are given for making 152 different pieces of
+    Apparatus for the practical use of students, teachers, and
+    others who wish to experiment.
+
+    PRICE, POST-PAID, $1.00.
+
+The shocking coils, telegraph instruments, batteries, electromagnets,
+motors, etc., etc., are so simple in construction that any boy of
+average ability can make them; in fact, the illustrations have been
+made directly from apparatus constructed by young boys.
+
+The author has been working along this line for several years, and he
+has been able, _with the help of boys_, to devise a complete line of
+simple electrical apparatus.
+
+
+  =_THE APPARATUS IS SIMPLE because the designs and methods
+      of construction have been worked out practically in the
+      school-room, absolutely no machine-work being required._=
+
+  =_THE APPARATUS IS PRACTICAL because it has been designed
+      for real use in the experimental study of elementary
+      electricity._=
+
+  =_THE APPARATUS IS CHEAP because most of the parts can be
+      made of old tin cans and cracker boxes, bolts, screws, wires
+      and wood._=
+
+
+    =Address, THOMAS M. ST. JOHN,=
+                =407 West 51st Street,=
+                                  =New York.=
+
+
+
+
+How Two Boys Made Their Own Electrical Apparatus.
+
+
+=CONTENTS:= _Chapter_ I. Cells and Batteries.--II. Battery Fluids
+and Solutions.--III. Miscellaneous Apparatus and Methods of
+Construction.--IV. Switches and Cut-Outs.--V. Binding-Posts and
+Connectors.--VI. Permanent Magnets,--VII. Magnetic Needles and
+Compasses.--VIII. Yokes and Armatures.--IX. Electro-Magnets.--X.
+Wire-Winding Apparatus.--XI. Induction Coils and Their
+Attachments.--XII. Contact Breakers and Current Interrupters.--XIII.
+Current Detectors and Galvanometers.--XIV. Telegraph Keys and
+Sounders.--XV. Electric Bells and Buzzers.--XVI. Commutators and
+Current Reversers.--XVII. Resistance Coils.--XVIII. Apparatus for
+Static Electricity.--XIX. Electric Motors.--XX. Odds and Ends.--XXI.
+Tools and Materials.
+
+"The author of this book is a teacher and wirier of great ingenuity,
+and we imagine that the effect of such a book as this falling into
+juvenile hands must be highly stimulating and beneficial. It is
+full of explicit details and instructions in regard to a great
+variety of apparatus, and the materials required are all within the
+compass of very modest pocket-money. Moreover, it is systematic and
+entirely without rhetorical frills, so that the student can go right
+along without being diverted from good helpful work that will lead
+him to build useful apparatus and make him understand what he is
+about. The drawings are plain and excellent. We heartily commend the
+book."--_Electrical Engineer._
+
+
+"Those who visited the electrical exhibition last May cannot have
+failed to notice on the south gallery a very interesting exhibit,
+consisting, as it did, of electrical apparatus made by boys. The
+various devices there shown, comprising electro-magnets, telegraph keys
+and sounders, resistance coils, etc., were turned out by boys following
+the instructions given in the book with the above title, which is
+unquestionably one of the most practical little works yet written that
+treat of similar subjects, for with but a limited amount of mechanical
+knowledge, and by closely following the instructions given, almost any
+electrical device may be made at very small expense. That such a book
+fills a long-felt want may be inferred from the number of inquiries
+we are constantly receiving from persons desiring to make their own
+induction coils and other apparatus."--_Electricity._
+
+
+"At the electrical show in New York last May one of the most
+interesting exhibits was that of simple electrical apparatus made by
+the boys in one of the private schools in the city. This apparatus,
+made by boys of thirteen to fifteen years of age, was from designs
+by the author of this clever little book, and it was remarkable to
+see what an ingenious use had been made of old tin tomato-cans,
+cracker-boxes, bolts, screws, wire, and wood. With these simple
+materials telegraph instruments, coils, buzzers, current detectors,
+motors, switches, armatures, and an almost endless variety of apparatus
+were made, In this book Mr. St. John has given directions in simple
+language for making and using these devices, and has illustrated
+these directions with admirable diagrams and cuts. The little volume
+is unique, and will prove exceedingly helpful to those of our young
+readers who are fortunate enough to possess themselves of a copy. For
+schools where a course of elementary science is taught, no better
+text-book in the first-steps in electricity is obtainable."--_The Great
+Round World._
+
+
+
+
+Exhibit of Experimental Electrical Apparatus
+
+AT THE ELECTRICAL SHOW, MADISON SQUARE GARDEN, NEW YORK.
+
+
+While only 40 pieces of simple apparatus were shown in this exhibit, it
+gave visitors something of an idea of what young boys can do if given
+proper designs.
+
+[Illustration: "HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS"
+
+Gives Proper Designs--Designs for over 150 Things.]
+
+
+
+
+Fun With Photography
+
+BOOK AND COMPLETE OUTFIT.
+
+
+[Illustration]
+
+=PHOTOGRAPHY= is now an educational amusement, and to many it is the
+most fascinating of all amusements. The magic of sunshine, the wonders
+of nature, and the beauties of art are tools in the hand of the amateur
+photographer.
+
+A great many things can be done with this outfit, and it will give an
+insight into this most popular pastime.
+
+
+    =THE OUTFIT= contains everything necessary for making
+    ordinary prints--together with other articles to be used
+    in various ways. The following things are included:
+    One Illustrated Book of Instructions, called "Fun With
+    Photography;" 1 Package of Sensitized Paper; 1 Printing
+    Frame, including Glass, Back, and Spring; 1 Set of Masks for
+    Printing Frame; 1 Set of Patterns for Fancy Shapes; 1 Book
+    of Negatives (Patent Pending) Ready for Use; 6 Sheets of
+    Blank Negative Paper; 1 Alphabet Sheet; 1 Package of Card
+    Mounts; 1 Package of Folding Mounts; 1 Package of "Fixo."
+
+    =CONTENTS OF BOOK:=--=Chapter I.
+    Introduction.=--Photography.--Magic Sunshine.--The
+    Outfit.--=II. General Instructions.=--The
+    Sensitized Paper.--How the Effects are
+    Produced.--Negatives.--Prints.--Printing Frames.--Our
+    Printing Frame.--Putting Negatives in Printing
+    Frame.--Printing.--Developing.--Fixing.--Drying.--Trimming.--Fancy
+    Shapes.--Mounting.--=III. Negatives and How to Make
+    Them.=--The Paper.--Making Transparent Paper.--Making
+    the Negatives.--Printed Negatives.--Perforated
+    Negatives.--Negatives Made from Magazine Pictures.--Ground
+    Glass Negatives.--=IV. Nature Photography.=--Aids
+    to Nature Study.--Ferns and Leaves.--Photographing
+    Leaves.--Perforating Leaves.--Drying Leaves, Ferns,
+    etc., for Negatives.--Flowers.--=V. Miscellaneous
+    Photographs.=--Magnetic Photographs.--Combination
+    Pictures.--Initial Pictures.--Name Plates.--Christmas,
+    Easter and Birthday Cards.
+
+    _The Book and Complete Outfit will be sent, by mail or
+    express, Charges Prepaid, upon receipt of 65 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+Fun With Magnetism.
+
+BOOK AND COMPLETE OUTFIT FOR SIXTY-ONE EXPERIMENTS IN MAGNETISM...
+
+
+[Illustration]
+
+Children like to do experiments; and in this way, better than in any
+other, _a practical knowledge of the elements of magnetism_ may be
+obtained.
+
+These experiments, although arranged to _amuse_ boys and girls, have
+been found to be very _useful in the class-room_ to supplement the
+ordinary exercises given in text-books of science.
+
+To secure the _best possible quality of apparatus_, the horseshoe
+magnets were made at Sheffield, England, especially for these sets.
+They are new and strong. Other parts of the apparatus have also been
+selected and made with great care, to adapt them particularly to these
+experiments.--_From the author's preface._
+
+
+    =CONTENTS.=--Experiments With Horseshoe Magnet.--Experiments
+    With Magnetized Needles.--Experiments With Needles,
+    Corks, Wires, Nails, etc.--Experiments With Bar
+    Magnets.--Experiments With Floating Magnets.--Miscellaneous
+    Experiments.--Miscellaneous Illustrations showing what very
+    small children can do with the Apparatus.--Diagrams showing
+    how Magnetized Needles may be used by little children to
+    make hundreds of pretty designs upon paper.
+
+
+    =AMUSING EXPERIMENTS.=--Something for Nervous People to
+    Try.--The Jersey Mosquito.--The Stampede.--The Runaway.--The
+    Dog-fight.--The Whirligig.--The Naval Battle.--A
+    String of Fish.--A Magnetic Gun.--A Top Upsidedown.--A
+    Magnetic Windmill.--A Compass Upsidedown.--The Magnetic
+    Acrobat.--The Busy Ant-hill.--The Magnetic Bridge.--The
+    Merry-go-Round.--The Tight-rope Walker.--A Magnetic Motor
+    Using Attractions and Repulsions.
+
+    _The Book and Complete Outfit will be sent, Post-paid,
+    upon receipt of 35 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+FUN WITH SHADOWS
+
+BOOK AND COMPLETE OUTFIT FOR SHADOW PICTURES, PANTOMIMES,
+ENTERTAINMENTS, Etc., Etc.
+
+
+[Illustration]
+
+=Shadow Making= has been a very popular amusement for several
+centuries. There is a great deal of _fun_ and instruction in it, and
+its long life is due to the fact that it has always been a source of
+keen delight to grown people as well as to children.
+
+In getting material together for this little book, the author has been
+greatly aided by English, French and American authors, some of whom are
+professional shadowists. It has been the author's special effort to get
+the subject and apparatus into a practical, cheap form for boys and
+girls.
+
+
+    =THE OUTFIT= contains everything necessary for all ordinary
+    shadow pictures, shadow entertainments, shadow plays, etc.
+    The following articles are included:
+
+    One book of Instructions called "Fun with Shadows"; 1 Shadow
+    Screen; 2 Sheets of Tracing Paper; 1 Coil of Wire for
+    Movable Figures; 1 Cardboard Frame for Circular Screen; 1
+    Cardboard House for Stage Scenery; 1 Jointed Wire Fish-pole
+    and Line; 2 Bent Wire Scenery Holders; 4 Clamps for Screen;
+    1 Wire Figure Support; 1 Wire for Oar; 2 Spring Wire Table
+    Clamps; 1 Wire Candlestick Holder; 5 Cardboard Plates
+    containing the following printed figures that should be cut
+    out with shears: 12 Character Hats; 1 Boat; 1 Oar-blade; 1
+    Fish; 1 Candlestick; 1 Cardboard Plate containing printed
+    parts for making movable figures.
+
+    =CONTENTS OF BOOK:= One Hundred Illustrations and Diagrams,
+    including Ten Full-page Book Plates, together with Six
+    Full-page Plates on Cardboard.
+
+    _Chapter_ I. Introduction.--II. General Instructions.--III.
+    Hand Shadows of Animals.--IV. Hand Shadows of Heads,
+    Character Faces, etc.--V. Moving Shadow Figures and How
+    to Make Them.--VI. Shadow Pantomimes.--VII. Miscellaneous
+    Shadows.
+
+    _The Book and Complete Outfit will be sent, =POST-PAID=,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+Fun With Electricity.
+
+BOOK AND COMPLETE OUTFIT FOR SIXTY EXPERIMENTS IN ELECTRICITY....
+
+
+[Illustration]
+
+Enough of the principles of electricity are brought out to make the
+book instructive as well as amusing. The experiments are systematically
+arranged, and make a fascinating science course. No chemicals, no
+danger.
+
+The book is conversational and not at all "schooly," Harry and Ned
+being two boys who perform the experiments and talk over the results as
+they go along.
+
+"The book reads like a story."--"An appropriate present for a
+boy or girl."--"Intelligent parents will appreciate 'Fun With
+Electricity.'"--"Very complete, because it contains both book and
+apparatus."--"There is no end to the fun which a boy or girl can have
+with this fascinating amusement."
+
+
+    =THERE IS FUN IN THESE EXPERIMENTS.=--Chain Lightning.--An
+    Electric Whirligig.--The Baby Thunderstorm.--A Race
+    with Electricity.--An Electric Frog Pond.--An Electric
+    Ding-Dong.--The Magic Finger.--Daddy Long-Legs.--Jumping
+    Sally.--An Electric Kite.--Very Shocking.--Condensed
+    Lightning.--An Electric Fly-Trap.--The Merry Pendulum.--An
+    Electric Ferry-Boat.--A Funny Piece of Paper.--A Joke on the
+    Family Cat.--Electricity Plays Leap-Frog.--Lightning Goes
+    Over a Bridge.--Electricity Carries a Lantern.--And _=40
+    Others=_.
+
+    The =_OUTFIT_= contains 20 different articles. The =_BOOK
+    OF INSTRUCTION=_ measures 5 x 7½ inches, and has 38
+    illustrations, 55 pages, good paper and clear type.
+
+    _The Book, and Complete Outfit will be sent, by mail or
+    express, Charges Prepaid, upon receipt of 65 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+Fun With Puzzles.
+
+BOOK, KEY, AND COMPLETE OUTFIT FOR FOUR HUNDRED PUZZLES...
+
+
+The BOOK measures 5 × 7½ inches. It is well printed, nicely bound,
+and contains 15 chapters, 80 pages, and 128 illustrations. The KEY is
+illustrated. It is bound with the book, and contains the solution of
+every puzzle. The COMPLETE OUTFIT is placed in a neat box with the
+book. It consists of numbers, counters, figures, pictures, etc., for
+doing the puzzles.
+
+    =CONTENTS:= _Chapter_ (1) Secret Writing. (2) Magic
+    Triangles, Squares, Rectangles, Hexagons, Crosses, Circles,
+    etc. (3) Dropped Letter and Dropped Word Puzzles. (4) Mixed
+    Proverbs, Prose and Rhyme. (5) Word Diamonds, Squares,
+    Triangles, and Rhomboids. (6) Numerical Enigmas. (7)
+    Jumbled Writing and Magic Proverbs. (8) Dissected Puzzles.
+    (9) Hidden and Concealed Words. (10) Divided Cakes, Pies,
+    Gardens, Farms, etc. (11) Bicycle and Boat Puzzles. (12)
+    Various Word and Letter Puzzles. (13) Puzzles with Counters.
+    (14) Combination Puzzles. (15) Mazes and Labyrinths.
+
+"Fun With Puzzles" is a book that every boy and girl should have. It
+is amusing, instructive,--educational. It is just the thing to wake up
+boys and girls and make them think. They like it, because it is real
+fun. This sort of educational play should be given in every school-room
+and in every home.
+
+"Fun With Puzzles" will puzzle your friends, as well as yourself; it
+contains some real brain-splitters. Over 300 new and original puzzles
+are given, besides many that are hundreds of years old.
+
+=Secret Writing.= Among the many things that "F. W. P." contains, is
+the key to _secret writing_. It shows you a very simple way to write
+letters to your friends, and it is simply impossible for others to read
+what you have written, unless they know the secret. This, alone is a
+valuable thing for any boy or girl who wants to have some fun.
+
+    _The Book, Key, and Complete Outfit will be sent, postpaid,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+Fun With Soap-Bubbles.
+
+BOOK AND COMPLETE OUTFIT FOR FANCY BUBBLES AND FILMS....
+
+
+[Illustration]
+
+=THE OUTFIT= contains everything necessary for thousands of beautiful
+bubbles and films. All highly colored articles have been carefully
+avoided, as cheap paints and dyes are positively dangerous in
+children's mouths. The outfit contains the following articles:
+
+One Book of Instructions, called "Fun With Soap-Bubbles," 1 Metal Base
+for Bubble Stand, 1 Wooden Rod for Bubble Stand, 3 Large Wire Rings for
+Bubble Stand, 1 Small Wire Ring, 3 Straws, 1 Package of Prepared Soap,
+1 Bubble Pipe, 1 Water-proof Bubble Horn. The complete outfit is placed
+in a neat box with the book. (Extra Horns, Soap, etc., furnished at
+slight cost.)
+
+    =CONTENTS OF BOOK.=--Twenty-one
+    Illustrations.--Introduction.--The Colors of
+    Soap-bubbles.--The Outfit.--Soap Mixture.--Useful
+    Hints.--Bubbles Blown With Pipes.--Bubbles Blown
+    With Straws.--Bubbles Blown With the Horn.--Floating
+    Bubbles.--Baby Bubbles.--Smoke Bubbles.--Bombshell
+    Bubbles.--Dancing Bubbles.--Bubble Games.--Supported
+    Bubbles.--Bubble Cluster.--Suspended Bubbles.--Bubble
+    Lamp Chimney.--Bubble Lenses.--Bubble Basket.--Bubble
+    Bellows.--To Draw a Bubble Through a Ring.--Bubble
+    Acorn.--Bubble Bottle.--A Bubble Within a Bubble.--Another
+    Way.--Bubble Shade.--Bubble Hammock.--Wrestling
+    Bubbles.--A Smoking Bubble.--Soap Films.--The Tennis
+    Racket Film.--Fish-net Film.--Pan-shaped Film.--Bow and
+    Arrow Film.--Bubble Dome.--Double Bubble Dome.--Pyramid
+    Bubbles.--Turtle-back Bubbles.--Soap-bubbles and Frictional
+    Electricity.
+
+
+"There is nothing more beautiful than the airy-fairy soap-bubble with
+its everchanging colors."
+
+    _THE BEST POSSIBLE AMUSEMENT FOR OLD
+    AND YOUNG._
+
+
+    _The Book and Complete Outfit will be sent, =POST-PAID=,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+The Study of Elementary Electricity and
+
+Magnetism by Experiment.
+
+
+    By THOMAS M. ST. JOHN, Met. E.
+
+    The book contains 220 pages and 168 illustrations;
+    it measures 5 × 7½ in. and is bound in green cloth.
+
+    PRICE, POST-PAID, $1.25.
+
+This book is designed as a text-book for amateurs, students, and others
+who wish to take up a systematic course of elementary electrical
+experiments at home or in school. Full directions are given for.......
+
+    _Two Hundred Simple Experiments._
+
+The experiments are discussed by the author, after the student has been
+led to form his own opinion about the results obtained and the points
+learned.
+
+In selecting the apparatus for the experiments in this book, the author
+has kept constantly in mind the fact that the average student will not
+buy the expensive pieces usually described in text-books.
+
+    The two hundred experiments given can be performed with
+    simple apparatus; in fact, the student should make at least
+    a part of his own apparatus, and for the benefit of those
+    who wish to do this, the author has given, throughout the
+    work, explanations that will aid in the construction of
+    certain pieces especially adapted to these experiments. For
+    those who have the author's "How Two Boys Made Their Own
+    Electrical Apparatus," constant references have been made to
+    it as the "Apparatus Book," as this contains full details
+    for making almost all kinds of simple apparatus needed
+    in "The Study of Elementary Electricity and Magnetism by
+    Experiment."
+
+_If you wish to take up a systematic course of experiments--experiments
+that may be performed with simple, inexpensive apparatus,--this book
+will serve as a valuable guide._
+
+
+
+
+Condensed List of Apparatus
+
+FOR
+
+"The Study of Elementary Electricity and Magnetism by Experiment."
+
+
+_Number_ 1. Steel Needles; package of twenty-five.--2. Flat Cork.--3.
+Candle.--4-15. Annealed Iron Wires; assorted lengths.--16. Horseshoe
+Magnet; best quality; English.--17. Iron Filings.--18. Parts for
+Compass.--19, 20. Wire Nails; soft steel.--21, 22. Spring Steel; for
+bar magnets.--23. Iron Ring.--24. Sifter; for iron filings.--25.
+Spring Steel; for flexible magnet.--26, 27. Ebonite Sheets; with
+special surface.--28. Ebonite Rod.--29. Ebonite Rod; short.--30.
+Flannel Cloth.--31. Tissue Paper.--32. Cotton Thread.--33. Silk
+Thread.--34. Support Base.--35. Support Rod.--36. Support Wire.--37.
+Wire Swing.--38. Sheet of Glass.--39. Hairpin.--40. Circular
+Conductor.--41. Circular Conductor.--42. Electrophorus Cover.--43.
+Insulating Table.--44. Insulated Copper Wire.--45. Rubber Band.--46.
+Bent Wire Clamps.--47. Cylindrical Conductor.--48. Discharger; for
+condenser.--49. Aluminum-Leaf.--50. Wires.
+
+51. Dry Cell.--52. Mercury.--53. Insulated Copper Wire; for
+connections.--54. Spring Connectors; two dozen.--55. Parts
+for Key.--56. Metal Connecting Plates.--57. Parts for Current
+Reverser.--58. Parts for Galvanoscope.--59. Parts for Astatic
+Galvanoscope.--60-63. Zinc Strips.--64. Carbon Rod.--65, 66. Glass
+Tumblers.--67, 68. Copper Strips.--69. Galvanized Iron Nail.--70,
+71. Wooden Cross-Pieces.--72. Brass Screws; one dozen.--73. Porous
+Cup.--74. Zinc Rod.--75. Copper Plate.--76. Iron Strip.--77, 78. Lead
+Strips.--79. Parts for Resistance Coil.--80. Parts for Wheatstone's
+Bridge.--81. German-Silver Wire; Size No. 30.--82. German-Silver Wire;
+No. 28.--83--85. Plate Binding-Posts.--86. Copper Sulphate.--87. Copper
+Burs; one dozen.--88. Combination Rule.--89. Coil of Wire; on spool
+for electromagnet.--90. Coil of Wire; on spool for electromagnet.--91.
+Carbon Rod.--92, 93. Soft Iron Cores with Screws.--94. Combined
+Base and Yoke.--95. Combination Connecting Plates.--96. Long Iron
+Core.--97. Round Bar Magnet, 5 × 3/8 in.--98. Thin Electromagnet.--99.
+Degree-Card; for galvanoscope.--100. Scale for Bridge.--101, 102. Soft
+Iron Cores with Heads.--103, 104. Flat Bar Magnets; these are 6 × ½ × ¼
+in.; highly polished steel; poles marked.--105. Compass.
+
+    =_Illustrated Price Catalogue upon Application._=
+
+
+
+
+Electrical Apparatus For Sale
+
+A COMPLETE ELECTRIC AND MAGNETIC CABINET FOR STUDENTS, SCHOOLS AND
+AMATEURS. SIX EXTRAORDINARY OFFERS
+
+
+=This Cabinet of Electrical Experiments= contains three main parts:
+(_A_) Apparatus; (_B_) Text-Book; (_C_) Apparatus List.
+
+(_A_) =The Apparatus= furnished consists of one hundred and five
+pieces. Over three hundred separate articles are used in making up this
+set. Most of it is ready for use when received. Seven pieces, however,
+are not assembled; but the parts can be readily finished and put
+together. (Sold, also, _all_ pieces assembled.)
+
+(_B_) =The Text-Book=--called "The Study of Elementary Electricity
+and Magnetism by Experiment"--gives full directions for two hundred
+experiments. (See table of contents, etc.) Price, post-paid, $1.25.
+
+(_C_) =The Apparatus List= is an illustrated book devoted entirely to
+this special set of apparatus. Not given with first offer.
+
+  _THE APPARATUS IS SIMPLE because the designs and methods of
+     construction have been worked out with great care._
+
+  _THE APPARATUS IS PRACTICAL because it has been designed
+     for real use in "The Study of Elementary Electricity and
+     Magnetism by Experiment."_
+
+  _THE APPARATUS IS CHEAP because the various parts are
+     so designed that they can be turned out in quantity by
+     machinery._
+
+    =1st Offer:= Pieces 1 to 50                     $1.00
+    =2d Offer:= Pieces 51 to 105, with part (_C_)      3.50
+    =3d Offer:= Pieces 1 to 105, with part (_C_)      4.00
+    =4th Offer:= Complete Cabinet, parts (_A_), (_B_), (_C_)      5.00
+    =5th Offer:= Apparatus only, all pieces assembled      4.60
+    =6th Offer:= Complete Cabinet, all pieces assembled      5.60
+
+    =_Express charges must be paid by you. Estimates given._=
+
+A "Special Catalogue," pertaining to the above, with complete
+price-list, will be mailed upon application.
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City=
+
+
+
+
+Fun With Telegraphy
+
+BOOK AND COMPLETE OUTFIT.
+
+
+[Illustration]
+
+=TELEGRAPHY= is of the greatest importance to all civilized nations,
+and upon it depend some of the world's most important enterprises.
+Every boy and girl can make practical use of telegraphy in one way or
+another, and the time it takes to learn it will be well spent.
+
+
+=THE OUTFIT.=--Mr. St. John has worked for a number of years to produce
+a telegraph outfit that would be simple, cheap, and practical for those
+who wish to make a study of telegraphy. After making and experimenting
+with nearly one hundred models, many of which were good, he has at last
+perfected an instrument so simple, original, and effective that it is
+now being made in large quantities.
+
+The sounders are so designed that they will work properly with any dry
+cell of ordinary strength, and this is a great advantage for practice
+lines. Dry batteries are cheap and clean, and there are no dangers from
+acids.
+
+The outfit consists of the following articles, placed in a neat box:
+One Book of Instruction, called "Fun With Telegraphy"; one Telegraph
+"Key"; one Telegraph "Sounder"; Insulated Copper Wires for connections.
+The "key" and "sounder" are mounted, with proper "binding-posts," upon
+a base of peculiar construction, which aids in giving a large volume of
+sound.
+
+
+=CONTENTS OF BOOK.=--Telegraphy.--The Outfit.--A Complete Telegraph
+Line.--Connections.--The Telegraph Key.--The Sounder.--The Battery.--A
+Practice Line.--A Two-instrument Line.--Operation of Line.--The Morse
+Telegraph Alphabet.--Aids to Learning Alphabet.--Cautions.--Office
+Calls.--Receiving Messages.--Remember.--Extra Parts.
+
+
+=ABOUT BATTERIES.=--For those who cannot easily secure batteries, we
+will furnish small dry cells, post-paid, at 15 cents each, in order to
+deliver the outfits complete to our customers. This price barely covers
+the total cost to us, postage alone being 6 cents.
+
+    _=FUN WITH TELEGRAPHY, including Book, Key, Sounder,
+    and Wire (no battery), post-paid, 50 cents, by=_
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+Tool Sets for Students
+
+
+The following tool sets have been arranged especially for those who
+wish to make use of the designs contained in "How Two Boys Made Their
+Own Electrical Apparatus," "Real Electric Toy-Making for Boys,"
+"Electric Instrument-Making," etc. It is very poor economy to waste
+valuable time and energy in order to save the cost of a few extra tools.
+
+=NOTE.=--Save money by buying your tools in sets. We do not pay express
+or freight charges at the special prices below.
+
+=FOR $1.00.=--One _Steel Punch_; round, knurled head.--One light
+_Hammer_; polished, nickel-plated, varnished handle.--One _Iron Clamp_;
+japanned, 2¼ in.--One _Screw-Driver_; tempered and polished blade,
+cherry stained hardwood handle, nickel ferrule.--One _Wrench_; retinned
+skeleton frame, gilt adjusting wheel.--One _Awl_; tempered steel
+point, turned and stained wood handle, with ferrule.--One _Vise_; full
+malleable, nicely retinned, 1-3/8 in. jaws, full malleable screw with
+spring.--One pair _Steel Pliers_; 4 in. long, polished tool steel,
+unbreakable, best grooved jaw.--One pair of _Shears_; carbonized steel
+blades, hardened edge, nickel-plated, heavy brass nut and bolt.--One
+_File_; triangular, good steel.--One _File Handle_; good wood, brass
+ferrule.--One _Foot Rule_; varnished wood, has English and metric
+system.--One _Soldering Set_; contains soldering iron, solder, resin,
+sal ammoniac, and directions. One _Center-Punch_; finely tempered steel.
+
+=FOR $2.00.=--All that is contained in the $1.00 set of tools, together
+with the following: One pair of _Tinner's Shears_; cut, 2¾ in., cast
+iron, hardened, suitable for cutting thin metal.--One _Hollow Handle
+Tool Set_; very useful; polished handle holds 10 tools, gimlet,
+brad-awls, chisel, etc.--One _Try Square_; 6-in. blue steel blade,
+marked in 1/8s, strongly riveted.--One 1-lb. _Hammer_; full size,
+polished head, wedged varnished hardwood handle.--One _Hack Saw_; steel
+frame, 9½-in. polished steel blade, black enamel handle; very useful.
+
+=FOR $3.50.=--Two _Steel Punches_; different sizes, one solid round,
+knurled head, polished; the other, point and head brightly polished,
+full nickel, center part knurled.--One _Light Hammer_; polished and
+nickel plated, varnished handle.--One regular _Machinist's Hammer_;
+ball peen, solid cast steel, with varnished hardwood handle; a
+superior article.--Two _Iron Clamps_; one opens 2¼ in., the other
+3 in., japanned.--One _Screw-Driver_; tempered and polished blade,
+firmly set in cherry stained hardwood handle with nickel ferrule.--One
+_Wrench_; retinned, skeleton frame, gilt adjusting wheel.--One _Awl_;
+tempered steel blade, ground to point, firmly set in turned and stained
+handle with ferrule.--One _Steel Vise_; 2¼-in., jaws, steel screw,
+bright polished jaws and handle; a good strong vise.--One pair of
+_Steel Pliers_; 6 in. long, bright steel, flat nose, 2 wire-cutters,
+practically unbreakable.--One pair of _Shears_; carbonized steel
+blades, hardened edges, nickel plated, heavy brass nut and bolt.--One
+_File_; triangular and of good steel.--One _File Handle_; good wood,
+with brass ferrule.--One _Foot Rule_; varnished wood, has both the
+English and metric systems.--One _Soldering Set_; contains soldering
+iron, solder, resin, sal ammoniac, and directions; a very handy
+article.--One _Center-Punch_; finely tempered steel.--One pair of
+_Tinner's Shears_; these are best grade, inlaid steel cutting edges,
+polished and tempered, japanned handles; thoroughly reliable.--One
+_Hollow Handle Tool Set_; very useful; the polished handle holds 10
+tools, gimlet, chisel, brad-awl, etc.--One _Try Square_; 6-in. blue
+steel blade, marked both sides in 1/8s, strongly riveted with brass
+rivets.--One _Hack Saw_; steel frame, 9½-in. polished steel blade,
+black enamel handle; very useful for sawing small pieces of wood.
+
+=FOR $5.00= will be included everything in the $3.50 offer, and the
+following: One _Glue-Pot_; medium size, with brush and best wood
+glue; inside pot has hinge cover.--One _Ratchet Screw-Driver_; great
+improvement over ordinary screw-drivers; well made and useful.--One
+_Hand Drill_; frame malleable iron; hollow screw top holding 6 drills;
+bores from 1-16 to 3-16-in. holes; solid gear teeth; 3-jawed nickel
+plated chuck; a superior tool, and almost a necessity.
+
+    =GIVE THE BOY A SET OF TOOLS=
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+REAL ELECTRIC TOY-MAKING FOR BOYS
+
+    _By_ THOMAS M. ST. JOHN, Met. E.
+
+
+    This book contains 140 pages and over one hundred
+    original drawings, diagrams, and full-page plates.
+    It measures 5 x 7½ in., and is bound in cloth.
+
+    Price, post-paid, $1.00
+
+
+=CONTENTS:= _Chapter_ I. Toys Operated by Permanent Magnets.--II.
+Toys Operated by Static Electricity.--III. Making Electromagnets for
+Toys.--IV. Electric Batteries.--V. Circuits and Connections.--VI. Toys
+Operated by Electromagnets. VII. Making Solenoids for Toys.--VIII.
+Toys Operated by Solenoids.--IX. Electric Motors.--X. Power,
+Speed, and Gearing.--XI. Shafting and Bearings.--XII. Pulleys and
+Winding-Drums.--XIII. Belts and Cables.--XIV. Toys Operated by
+Electric Motors.--XV. Miscellaneous Electric Toys.--XVI. Tools.--XVII.
+Materials.--XVIII. Various Aids to Construction.
+
+While planning this book, Mr. St. John definitely decided that he would
+not fill it with descriptions of complicated, machine-made instruments
+and apparatus, under the name of "Toy-Making," for it is just as
+impossible for most boys to get the parts for such things as it is
+for them to do the required machine work even after they have the raw
+materials.
+
+Great care has been taken in designing the toys which are described
+in this book, in order to make them so simple that any boy of average
+ability can construct them out of ordinary materials. The author can
+personally guarantee the designs, for there is no guesswork about
+them. Every toy was made, changed, and experimented with until it was
+as simple as possible; the drawings were then made from the perfected
+models.
+
+As the result of the enormous amount of work and experimenting which
+were required to originate and perfect so many new models, the author
+feels that this book may be truly called "Real Electric Toy-Making for
+Boys."
+
+    =Every Boy Should Make Electrical Toys.=
+
+
+
+
+The Electric Shooting Game>
+
+A MOST ORIGINAL AND FASCINATING GAME PATENT APPLIED FOR AND COPYRIGHTED
+
+
+[Illustration]
+
+_=SHOOTING BY ELECTRICITY=_
+
+=The Electric Shooting Game= is an entirely new idea, and one that
+brings into use that most mysterious something--_electricity_. The
+game is so simple that small children can play it, and as there are
+no batteries, acids, or liquids of any kind, there is absolutely no
+danger. The electricity is of such a nature that it is perfectly
+harmless--but very active.
+
+The "_game-preserve_" is neat and attractive, being printed in colors,
+and the birds and animals are well worth hunting. Each has a fixed
+value--and some of them must not be shot at all--so there is ample
+opportunity for a display of skill in bringing down those which count
+most.
+
+"_Electric bullets_" are actually shot from the "_electric gun_" by
+electricity. This instructive game will furnish a vast amount of
+amusement to all.
+
+ _=The "Game-Preserve,"--the "Electric Gun,"--the
+    "Shooting-Box,"--the "Electric Bullets,"--in fact, the
+    entire electrical outfit, together with complete illustrated
+    directions, will be sent in a neat box, Post-Paid, upon
+    receipt of 50 cents, by=_
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+       *       *       *       *       *
+
+
+
+
+Transcriber's note:
+
+Obvious punctuation errors were corrected.
+
+Page 46, "turnnd" changed to "turned" (be turned to 1)
+
+Page 66, word "a" added to text (in a glass jar)
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT
+ELECTRICITY***
+
+
+******* This file should be named 44665-8.txt or 44665-8.zip *******
+
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+<body>
+<h1>The Project Gutenberg eBook, Things a Boy Should Know About Electricity,
+by Thomas M. (Thomas Matthew) St. John</h1>
+<p>This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at <a
+href="http://www.gutenberg.org">www.gutenberg.org</a></p>
+<p>Title: Things a Boy Should Know About Electricity</p>
+<p>       Second Edition</p>
+<p>Author: Thomas M. (Thomas Matthew) St. John</p>
+<p>Release Date: January 14, 2014  [eBook #44665]</p>
+<p>Language: English</p>
+<p>Character set encoding: ISO-8859-1</p>
+<p>***START OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY***</p>
+<p>&nbsp;</p>
+<h4>E-text prepared by Chris Curnow, Emmy,<br />
+    and the Online Distributed Proofreading Team<br />
+    (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br />
+    from page images generously made available by<br />
+    Internet Archive<br />
+    (<a href="https://archive.org">https://archive.org</a>)</h4>
+<p>&nbsp;</p>
+<table border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10">
+  <tr>
+    <td valign="top">
+      Note:
+    </td>
+    <td>
+      Images of the original pages are available through
+      Internet Archive. See
+      <a href="https://archive.org/details/thingsboyshouldk00stjo">
+      https://archive.org/details/thingsboyshouldk00stjo</a>
+    </td>
+  </tr>
+</table>
+<p>&nbsp;</p>
+<hr class="full" />
+<p>&nbsp;</p>
+
+<div class="figcenter" style="width: 509px;">
+<img src="images/i_cover.jpg" width="509" height="800" alt="cover" />
+</div>
+
+
+<p><span class="pagenum"><a name="Page_i" id="Page_i">[i]</a></span></p>
+
+
+<div class="figcenter" style="width: 334px;">
+<img src="images/i_001.jpg" width="334" height="456" alt="Boy holding a flaming torch" />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[ii]</a></span></p>
+
+
+
+
+<div class='adtitle2'><i>BY THE SAME AUTHOR.</i></div>
+
+
+
+
+<div class='hang1'><b>FUN WITH MAGNETISM.</b> A book and complete outfit of
+apparatus for <i>Sixty-One Experiments</i>.</div>
+
+<div class='hang1'><b>FUN WITH ELECTRICITY.</b> A book and complete outfit of
+apparatus for <i>Sixty Experiments</i>.</div>
+
+<div class='hang1'><b>FUN WITH PUZZLES.</b> A book, key and complete outfit for
+<i>Four Hundred Puzzles</i>.</div>
+
+<div class='hang1'><b>FUN WITH SOAP-BUBBLES.</b> A book and complete outfit
+of apparatus for <i>Fancy Bubbles and Films</i>.</div>
+
+<div class='hang1'><b>FUN WITH SHADOWS.</b> Including book of instructions with
+one hundred illustrations and a complete outfit of apparatus
+for <i>Shadow Pictures, Pantomimes, Entertainments, etc., etc.</i></div>
+
+<div class='hang1'><b>HUSTLE-BALL.</b> An American game. Played by means of
+magic wands and polished balls of steel.</div>
+
+<div class='hang1'><b>JINGO.</b> The great war game, including JINGO JUNIOR.</div>
+
+<div class='hang1'><b>HOW TWO BOYS MADE THEIR OWN ELECTRICAL
+APPARATUS.</b> A book containing complete directions for
+making all kinds of simple apparatus for the study of elementary
+electricity.</div>
+
+<div class='hang1'><b>THE STUDY OF ELEMENTARY ELECTRICITY AND
+MAGNETISM BY EXPERIMENT.</b> This book is designed
+as a text-book for amateurs, students, and others who wish
+to take up a systematic course of simple experiments at home
+or in school.</div>
+
+<div class='hang1'><b>THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.</b>
+This book explains, in simple, straightforward
+language, many things about electricity; things in which the
+American boy is intensely interested; things he wants to
+know; things he should know.</div>
+
+<div class='hang1'><b>ANS., OR ACCURACY, NEATNESS AND SPEED.</b> For
+teachers and pupils. Containing study-charts, practice devices
+and special methods for accurate, rapid work with
+figures.</div>
+
+<div class='center'><i><b>Ask Your Bookseller, Stationer, or Toy Dealer for our<br />
+Books, Games, Puzzles, Educational Amusements, Etc.</b></i><br />
+
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+CATALOGUE UPON APPLICATION<br />
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+Thomas M. St. John, 407 West 51st St., New York.<br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_iii" id="Page_iii">[iii]</a></span></p>
+
+
+
+
+<h1>Things A Boy Should<br />
+Know About<br />
+Electricity</h1>
+
+<div class='center'>
+BY<br />
+<span class='author'>THOMAS M. ST. JOHN, Met. E.</span><br />
+
+<div class='authorof'>Author of "Fun With Magnetism," "Fun With Electricity,"<br />
+"How Two Boys Made Their Own Electrical Apparatus,"<br />
+"The Study of Elementary Electricity<br />
+and Magnetism by Experiment," etc.<br />
+</div>
+
+
+<br /><br />
+<i>SECOND</i> <img src="images/i_003.png" width="107" height="115" alt="emblem" />
+<i>EDITION</i><br />
+<br /><br /><br /><br />
+<small>NEW YORK</small><br />
+THOMAS M. ST. JOHN<br />
+407 West 51st Street<br />
+<small>1903</small><br />
+</div>
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_iv" id="Page_iv">[iv]</a></span></p>
+
+
+
+
+<div class='copyright'>
+Copyright, 1900.<br />
+By <span class="smcap">Thomas M. St. John</span>.<br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_v" id="Page_v">[v]</a></span></p>
+
+
+
+
+<h2>THINGS A BOY SHOULD KNOW
+ABOUT ELECTRICITY</h2>
+
+
+
+<hr class="chap" />
+<h2><a id="TABLE_OF_CONTENTS"></a>TABLE OF CONTENTS</h2>
+
+
+
+
+<div class="center">
+<table border="0" cellpadding="1" cellspacing="0" summary="Contents">
+<tr><td align="left" colspan='2'><span class="smcap"><small>Chapter</small></span></td><td align="right"><span class="smcap"><small>Page</small></span></td></tr>
+<tr><td align="right">I.</td><td align="left">About Frictional Electricty</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align="right">II.</td><td align="left">About Magnets and Magnetism</td><td align='right'><a href="#Page_21">21</a></td></tr>
+<tr><td align="right">III.</td><td align="left">How Electricity is Generated by the Voltaic Cell,</td><td align='right'><a href="#Page_32">32</a></td></tr>
+<tr><td align="right">IV.</td><td align="left">Various Voltaic Cells,</td><td align='right'><a href="#Page_36">36</a></td></tr>
+<tr><td align="right">V.</td><td align="left">About Push-Buttons, Switches and Binding-Posts,</td><td align='right'><a href="#Page_43">43</a></td></tr>
+<tr><td align="right">VI.</td><td align="left">Units and Apparatus for Electrical Measurements,</td><td align='right'><a href="#Page_48">48</a></td></tr>
+<tr><td align="right">VII.</td><td align="left">Chemical Effects of the Electric Current,</td><td align='right'><a href="#Page_58">58</a></td></tr>
+<tr><td align="right">VIII.</td><td align="left">How Electroplating and Electrotyping are Done,</td><td align='right'><a href="#Page_60">60</a></td></tr>
+<tr><td align="right">IX.</td><td align="left">The Storage Battery, and How it Works,</td><td align='right'><a href="#Page_63">63</a></td></tr>
+<tr><td align="right">X.</td><td align="left">How Electricity is Generated by Heat,</td><td align='right'><a href="#Page_68">68</a></td></tr>
+<tr><td align="right">XI.</td><td align="left">Magnetic Effects of the Electric Current,</td><td align='right'><a href="#Page_71">71</a></td></tr>
+<tr><td align="right">XII.</td><td align="left">How Electricity is Generated by Induction,</td><td align='right'><a href="#Page_77">77</a></td></tr>
+<tr><td align="right">XIII.</td><td align="left">How the Induction Coil Works,</td><td align='right'><a href="#Page_80">80</a></td></tr>
+<tr><td align="right">XIV.</td><td align="left">The Electric Telegraph, and How it Sends Messages,</td><td align='right'><a href="#Page_84">84</a></td></tr>
+<tr><td align="right">XV.</td><td align="left">The Electric Bell and Some of its Uses,</td><td align='right'><a href="#Page_91">91</a></td></tr>
+<tr><td align="right">XVI.</td><td align="left">The Telephone and How it Transmits Speech,</td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td align="right">XVII.</td><td align="left">How Electricity is Generated by Dynamos,</td><td align='right'><a href="#Page_101">101</a></td></tr>
+<tr><td align="right">XVIII.</td><td align="left">How the Electric Current is Transformed,</td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td align="right">XIX.</td><td align="left">How Electric Currents are Distributed for Use,</td><td align='right'><a href="#Page_114">114</a></td></tr>
+<tr><td align="right">XX.</td><td align="left">How Heat is Produced by the Electric Current,</td><td align='right'><a href="#Page_124">124</a></td></tr>
+<tr><td align="right">XXI.</td><td align="left">How Light is Produced by the Incandescent Lamp,</td><td align='right'><a href="#Page_129">129</a></td></tr>
+<tr><td align="right">XXII.</td><td align="left">How Light is Produced by the Arc Lamp,</td><td align='right'><a href="#Page_135">135</a></td></tr>
+<tr><td align="right">XXIII.</td><td align="left">X-Rays, and How the Bones of the Human Body are Photographed,</td><td align='right'><a href="#Page_141">141</a></td></tr>
+<tr><td align="right">XXIV.</td><td align="left">The Electric Motor, and How it Does Work,</td><td align='right'><a href="#Page_147">147</a></td></tr>
+<tr><td align="right">XXV.</td><td align="left">Electric Cars, Boats and Automobiles,</td><td align='right'><a href="#Page_154">154</a></td></tr>
+<tr><td align="right">XXVI.</td><td align="left">A Word About Central Stations,</td><td align='right'><a href="#Page_162">162</a></td></tr>
+<tr><td align="right">XXVII.</td><td align="left">Miscellaneous Uses of Electricity,</td><td align='right'><a href="#Page_165">165</a></td></tr>
+</table></div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_vi" id="Page_vi">[vi]</a></span></p>
+
+
+
+
+<h2>TO THE READER</h2>
+
+
+<p>For the benefit of those who wish to make their own
+electrical apparatus for experimental purposes, references
+have been made throughout this work to the "Apparatus
+Book;" by this is meant the author's "How Two Boys
+Made Their Own Electrical Apparatus."</p>
+
+<p>For those who wish to take up a course of elementary
+electrical experiments that can be performed with simple,
+home-made apparatus, references have been made to
+"Study;" by this is meant "The Study of Elementary
+Electricity and Magnetism by Experiment."</p>
+
+<div class='sig'>
+<span class="smcap">The Author.</span><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_7" id="Page_7">[7]</a></span></p>
+
+
+
+
+<h2>Things A Boy Should Know About
+Electricity</h2>
+
+
+
+<hr class="chap" />
+<h2><a id="CHAPTER_I"></a>CHAPTER I.<br />
+<small>ABOUT FRICTIONAL ELECTRICITY.</small></h2>
+
+
+<p><b><a id="par_1"></a>1. Some Simple Experiments.</b> Have you ever
+shuffled your feet along over the carpet on a winter's
+evening and then quickly touched your finger to the
+nose of an unsuspecting friend? Did
+he jump when a bright spark leaped
+from your finger and struck him fairly
+on the very tip of his sensitive nasal
+organ?</p>
+
+<div class="figright" style="width: 184px;">
+<img src="images/i_007.png" width="184" height="342" alt="black cat" />
+<div class="caption">Fig. 1.</div>
+</div>
+
+<p>Did you ever succeed in proving to
+the pussy-cat, Fig. 1, that something
+unusual occurs when you thoroughly
+rub his warm fur with your hand? Did
+you notice the bright sparks that passed
+to your hand when it was held just above
+the cat's back? You should be able to
+see, hear, and feel these sparks, especially when the air
+is dry and you are in a dark room.</p>
+
+<p>Did you ever heat a piece of paper before the fire until
+it was real hot, then lay it upon the table and rub it from
+end to end with your hand, and finally see it cling to the
+wall?</p>
+
+<p><span class="pagenum"><a name="Page_8" id="Page_8">[8]</a></span></p>
+
+<p>Were you ever in a factory where there were large
+belts running rapidly over pulleys or wheels, and where
+large sparks would jump to your hands when held near
+the belts?</p>
+
+<p>If you have never performed any of the four experiments
+mentioned, you should try them the first time a
+chance occurs. There are dozens of simple, fascinating
+experiments that may be performed with this kind of
+electricity.</p>
+
+<p><b><a id="par_2"></a>2. Name.</b> As this variety of electricity is made, or
+generated, by the friction of substances upon each other,
+it is called <i>frictional</i> electricity. It is also called <i>static</i>
+electricity, because it generally stands still upon the surface
+of bodies and does not "flow in currents" as easily
+as some of the other varieties. Static electricity may be
+produced by induction
+as well as by friction.</p>
+
+<div class="figleft" style="width: 406px;">
+<img src="images/i_008.png" width="406" height="378" alt="drawing" />
+<div class="caption">Fig. 2.</div>
+</div>
+
+<p><b><a id="par_3"></a>3. History.</b> It has
+been known for over
+2,000 years that certain
+substances act queerly
+when rubbed. Amber
+was the first substance
+upon which electricity
+was produced by friction,
+and as the Greek
+name for amber is
+<i>elektron</i>, bodies so affected were said to be <i>electrified</i>.
+When a body, like ebonite, is rubbed with a flannel
+cloth, we say that it becomes <i>charged with electricity</i>.
+Just what happens to the ebonite is not clearly understood.<span class="pagenum"><a name="Page_9" id="Page_9">[9]</a></span>
+We know, however, that it will attract light
+bodies, and then quickly repel them if they be conductors.
+Fig. 2 shows a piece of tissue-paper jumping toward a
+sheet of ebonite that has been electrified with a flannel
+cloth.</p>
+<div class="figright" style="width: 237px;">
+<img src="images/i_009.png" width="237" height="326" alt="drawing" />
+<div class="caption">Fig. 3.</div>
+</div>
+<p><b><a id="par_4"></a>4. Conductors and Non-Conductors.</b> Electricity
+can be produced upon glass and ebonite because they do
+not carry or conduct it away. If a piece of iron be
+rubbed, the electricity passes from the iron into the
+earth as fast as it is generated, because the iron is a <i>conductor</i>
+of electricity. Glass is an <i>insulator</i> or <i>non-conductor</i>.
+Frictional electricity resides upon the outside,
+only, of conductors. A hollow tin box
+will hold as great a charge as a solid
+piece of metal having the same outside
+size and shape. When frictional
+electricity passes from one place to
+another, sparks are produced. Lightning
+is caused by the passage of
+static electricity from a cloud to the
+earth, or from one cloud to another.
+In this case air forms the conductor.
+(For experiments, see "Study,"
+Chapter VII.)</p>
+
+
+
+<p><b><a id="par_5"></a>5. Electroscopes.</b> A piece of carbon, pith, or even
+a small piece of damp tissue-paper will serve as an electroscope
+to test the presence of static electricity. The pith
+is usually tied to a piece of silk thread which is a non-conductor.
+Fig. 3 shows the ordinary form of <i>pith-ball
+electroscope</i>.</p>
+<div class="figleft" style="width: 308px;">
+<img src="images/i_010.png" width="308" height="476" alt="drawing" />
+<div class="caption">Fig. 4.</div>
+</div>
+
+<p>The <i>leaf electroscope</i> is a very delicate apparatus. Gold-leaf<span class="pagenum"><a name="Page_10" id="Page_10">[10]</a></span>
+is generally used, but aluminum-leaf will stand
+handling and will do for all ordinary purposes. Fig. 4
+shows a common form, the glass being used to keep
+currents of air from the leaves and at the same time to
+insulate them from the earth.</p>
+
+<p>Electroscopes are used to show the presence, relative
+amount, or kind of static
+electricity on a body.
+(See "Study," Chapter
+XI.)</p>
+
+
+<p><b><a id="par_6"></a>6. Two Kinds of
+Electrification.</b> It can
+be shown that the electrification
+produced on all
+bodies by friction is not
+the same; for example,
+that generated with glass
+and silk is not the same
+as that made with ebonite
+and flannel. It has been
+agreed to call that produced
+by glass and silk
+<i>positive</i>, and that by
+ebonite and flannel <i>negative</i>.
+The signs + and - are used for positive and
+negative.</p>
+
+<p><b><a id="par_7"></a>7. Laws of Electrification.</b> (1) Charges of the
+same kind repel each other; (2) charges of unlike kinds
+attract each other; (3) either kind of a charge attracts
+and is attracted by a neutral body.</p>
+
+<p><b><a id="par_8"></a>8. Static Electric Machines.</b> In order to produce<span class="pagenum"><a name="Page_11" id="Page_11">[11]</a></span>
+static electricity in quantities for experiments, some
+device is necessary.</p>
+
+<p>The <i>electrophorus</i> (e-lec-troph´-o-rus) is about the simplest
+form of machine. Fig. 5 shows a simple electrophorus
+in which are two insulators and one conductor.
+The ebonite sheet E S is used with a flannel cloth to generate
+the electricity. The metal cover E C is lifted by the
+insulating handle E R. The cover E C is placed upon
+the thoroughly charged sheet E S, and then it is touched
+for an instant with the finger, before lifting it by E R.
+The charge upon E C can then be
+removed by bringing the hand near
+it. The bright spark that passes
+from E C to the hand indicates that
+E C has discharged itself into the
+earth. The action of the electrophorus
+depends upon induction. (For
+experiments, details of action,
+induced electrification, etc., see
+"The Study of Elementary Electricity and Magnetism
+by Experiment," Chapters VIII. and IX.)</p>
+
+<div class="figcenter" style="width: 264px;">
+<img src="images/i_011.png" width="264" height="265" alt="drawing" />
+<div class="caption">Fig. 5.</div>
+</div>
+
+<p><i>The first electric machine</i> consisted of a ball of sulphur
+fastened to a spindle which could be turned by a crank.
+By holding the hands or a pad of silk upon the revolving
+ball, electricity was produced.</p>
+
+<div class="figcenter" style="width: 558px;">
+<img src="images/i_012a.jpg" width="558" height="374" alt="" />
+<div class="caption">Fig. 6.</div>
+</div>
+
+<div class="figcenter" style="width: 551px;">
+<img src="images/i_012b.jpg" width="551" height="397" alt="" />
+<div class="caption">Fig. 7.</div>
+</div>
+
+<p><b><a id="par_9"></a>9. The Cylinder Electric Machine</b> consists, as
+shown in Fig. 6, of a glass cylinder so mounted that it
+can be turned by a crank. Friction is produced by a
+pad of leather C, which presses against the cylinder as it
+turns. Electric sparks can be taken from the large "conductors"
+which are insulated from the earth. The opposite<span class="pagenum"><a name="Page_12" id="Page_12">[12]</a><br /><a name="Page_13" id="Page_13">[13]</a></span>
+electricities unite with sparks across D and E. If
+use is to be made of the electricity, either the rubber or
+the prime conductor must be connected with the ground.
+In the former case positive electricity is obtained; in the
+latter, negative.</p>
+
+<p><b><a id="par_10"></a>10. The Plate Electrical Machine.</b> Fig. 7 also
+shows an old form of machine. Such machines are made
+of circular plates of glass or ebonite, two rubbing pads
+being usually employed, one on each side of the plate.
+One operator is seen on an insulated stool (Fig. 7), the
+electricity passing through him before entering the earth
+by way of the body of the man at the right.</p>
+
+<div class="figcenter" style="width: 565px;">
+<img src="images/i_013.jpg" width="565" height="385" alt="drawing" />
+<div class="caption">Fig. 8.</div>
+</div>
+
+<p><b><a id="par_11"></a>11. The Toepler-Holtz Machine</b>, in one form, is
+shown in Fig. 8. The electricity is produced by the
+principle of induction, and not by mere friction. This
+machine, used in connection with condensers, produces
+large sparks.</p>
+<div class="figleft" style="width: 274px;">
+<img src="images/i_014a.png" width="274" height="335" alt="drawing" />
+<div class="caption">Fig. 9.</div>
+</div>
+<p><span class="pagenum"><a name="Page_14" id="Page_14">[14]</a></span></p>
+
+<p><b><a id="par_12"></a>12. The Wimshurst Machine</b> is of recent date, and
+not being easily affected by atmospheric changes, is very
+useful for ordinary laboratory work. Fig. 9 shows one
+form of this machine.</p>
+
+<p><b><a id="par_13"></a>13. Influence Machines
+for Medical Purposes</b> are
+made in a large variety of
+forms. A Wimshurst machine
+is generally used as an exciter
+to charge the plates of the
+large machine when they lose
+their charge on account of
+excessive moisture in the
+atmosphere. Fig. 10 shows a
+large machine.</p>
+
+
+
+<p><b><a id="par_14"></a>14. Uses of Electrical
+Machines.</b> Static electricity has been used for many
+years in the laboratory
+for experimental
+purposes, for
+charging condensers,
+for medical purposes,
+etc. It is
+now being used for
+X-ray work, and
+considerable advancement
+has been
+made within a few
+years in the construction
+and efficiency
+of the machines.</p>
+
+<div class="figcenter" style="width: 379px;">
+<img src="images/i_014b.png" width="379" height="355" alt="drawing" />
+<div class="caption">Fig. 10.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_15" id="Page_15">[15]</a></span></p>
+
+<p>With the modern machines large sparks are produced
+by merely turning a crank, enough electricity being produced
+to imitate a small thunderstorm. The sparks of
+home-made lightning will jump several inches.</p>
+
+<p>Do not think that electricity is generated in a commercial
+way by static electric machines. The practical
+uses of static electricity are very few when compared
+with those of current electricity from batteries and
+dynamos.</p>
+
+<p><b><a id="par_15"></a>15. Condensation of Static Electricity.</b> By means
+of apparatus called <i>condensers</i>, a terrific charge of static
+electricity may be stored. Fig. 11 shows the most
+common form of condenser, known as the <i>Leyden jar</i>.
+It consists of a glass jar with an inside and outside coating
+of tin-foil.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="0" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 212px;">
+<img src="images/i_015a.png" width="212" height="353" alt="drawing" />
+<div class="caption">Fig. 11.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 335px;">
+<img src="images/i_015b.png" width="335" height="267" alt="drawing" />
+<div class="caption">Fig. 12.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><i>To charge</i> the jar it is held in the hand so that the outside
+coating shall be connected with the earth, the sparks<span class="pagenum"><a name="Page_16" id="Page_16">[16]</a></span>
+from an electric machine being passed to the knob at the
+top, which is connected by a chain to the inside coating.</p>
+
+<p><i>To discharge</i> the jar, Fig. 12, a conductor with an
+insulating handle is placed
+against the outside coat;
+when the other end of the
+conductor is swung over
+towards the knob, a bright
+spark passes between them.
+This device is called a discharger.
+Fig. 13 shows a
+discharge through ether
+which the spark ignites.</p>
+
+<div class="figcenter" style="width: 294px;">
+<img src="images/i_016a.png" width="294" height="298" alt="drawing" />
+<div class="caption">Fig. 13.</div>
+</div>
+
+<p><b><a id="par_16"></a>16. The Leyden Battery</b>,
+Fig. 14, consists of
+several jars connected in such a way that the area of the
+inner and outer coatings is greatly increased. The battery
+has a larger capacity
+than one of its
+jars. (For Experiments
+in Condensation,
+see "Study,"
+Chapter X.)</p>
+
+<div class="figcenter" style="width: 333px;">
+<img src="images/i_016b.png" width="333" height="278" alt="drawing" />
+<div class="caption">Fig. 14.</div>
+</div>
+<div class="figright" style="width: 263px;">
+<img src="images/i_017a.png" width="263" height="323" alt="drawing of a man" />
+<div class="caption">Fig. 15.</div>
+</div>
+<p><b><a id="par_17"></a>17. Electromotive
+Force of Static
+Electricity.</b> Although
+the sparks of
+static electricity are
+large, the <i>quantity</i> of electricity is very small. It would
+take thousands of galvanic cells to produce a spark an
+inch long. While the quantity of static electricity is<span class="pagenum"><a name="Page_17" id="Page_17">[17]</a></span>
+small, its potential, or electromotive force (E. M. F.),
+is very high. We say that an ordinary gravity cell has
+an E. M. F. of a little over one volt. Five such cells
+joined in the proper way
+would have an E. M. F. of a
+little over five volts. You
+will understand, then, what
+is meant when we say that the
+E. M. F. of a lightning flash
+is millions of volts.</p>
+
+<p><b><a id="par_18"></a>18. Atmospheric Electricity.</b>
+The air is usually
+electrified, even in clear
+weather, although its cause is
+not thoroughly understood.
+In 1752 it was proved by
+Benjamin Franklin (Fig. 15), with his famous kite
+experiment, that atmospheric and frictional electricities
+are of the same nature. By means of a kite, the string
+being wet by the rain, he succeeded, during a thunderstorm,
+in drawing sparks, charging
+condensers, etc.</p>
+
+
+
+<div class="figleft" style="width: 240px;">
+<img src="images/i_017b.png" width="240" height="204" alt="drawing" />
+<div class="caption">Fig. 16.</div>
+</div>
+
+<p><b><a id="par_19"></a>19. Lightning</b> may be produced
+by the passage of electricity
+between clouds, or between a
+cloud and the earth (Fig. 16),
+which, with the intervening air,
+have the effect of a condenser.
+When the attraction between
+the two electrifications gets great enough, a spark passes.
+When the spark has a zigzag motion it is called <i>chain<span class="pagenum"><a name="Page_18" id="Page_18">[18]</a></span>
+lightning</i>. In hot weather flashes are often seen which
+light whole clouds, no thunder being heard. This is
+called <i>heat lightning</i>, and is generally considered to be
+due to distant discharges,
+the light of
+which is reflected by
+the clouds. The
+lightning flash represents
+billions of volts.</p>
+
+<div class="figcenter" style="width: 346px;">
+<img src="images/i_018.jpg" width="346" height="635" alt="drawing street with hole in ground" />
+<div class="caption">Fig. 17.</div>
+</div>
+
+<p><b><a id="par_20"></a>20. Thunder</b> is
+caused by the violent
+disturbances produced
+in the air by lightning.
+Clouds, hills,
+etc., produce echoes,
+which, with the original
+sound, make the
+rolling effect.</p>
+
+<p><b><a id="par_21"></a>21. Lightning-Rods</b>,
+when well
+constructed, often prevent
+violent discharges.
+Their pointed
+prongs at the top
+allow the negative
+electricity of the earth
+to pass quietly into the air to neutralize the positive in
+the cloud above. In case of a discharge, or stroke of
+lightning, the rods aid in conducting the electricity to
+the earth. The ends of the rods are placed deep in the
+earth, Fig. 17.</p>
+
+<p><span class="pagenum"><a name="Page_19" id="Page_19">[19]</a></span></p>
+
+<p><b><a id="par_22"></a>22. St. Elmo's Fire.</b> Electrification from the earth
+is often drawn up from the earth through the masts of
+ships, Fig. 18, to neutralize that in the clouds, and, as it
+escapes from the points of the masts, light is produced.</p>
+
+<div class="figcenter" style="width: 449px;">
+<img src="images/i_019.jpg" width="449" height="600" alt="drawing of a ship" />
+<div class="caption">Fig. 18.</div>
+</div>
+
+<p><b><a id="par_23"></a>23. Aurora Borealis</b>, also called Northern Lights, are<span class="pagenum"><a name="Page_20" id="Page_20">[20]</a></span>
+luminous effects, Fig. 19, often seen in the north. They
+often occur at the same time with magnetic storms, when
+telegraph and telephone work may be disturbed. The
+exact cause of this light is not known, but it is thought
+by many to be due to disturbances in the earth's magnetism
+caused by the action of the sun.</p>
+
+<div class="figcenter" style="width: 474px;">
+<img src="images/i_020.jpg" width="474" height="471" alt="drawing of a sunrise or sunset that is supposed to be the Northern lights" />
+<div class="caption">Fig. 19.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_21" id="Page_21">[21]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_II"></a>CHAPTER II.<br />
+<small>ABOUT MAGNETS AND MAGNETISM.</small></h2>
+
+
+<div class="figright" style="width: 91px;">
+<img src="images/i_021a.png" width="91" height="164" alt="drawing of magnet" />
+<div class="caption">Fig. 20</div>
+</div>
+
+<p><b><a id="par_24"></a>24. Natural Magnets.</b> Hundreds of years ago it
+was discovered that a certain ore of iron, called lodestone,
+had the power of picking up small pieces of iron. It was
+used to indicate the north and south line, and it was discovered
+later that small pieces of steel could be permanently
+magnetized by rubbing them upon the lodestone.</p>
+
+
+<p><b><a id="par_25"></a>25. Artificial Magnets.</b> Pieces of steel, when magnetized,
+are called artificial magnets. They are made in
+many forms. The electromagnet is also an artificial
+magnet; this will be treated separately.</p>
+
+
+<p><b><a id="par_26"></a>26. The Horseshoe Magnet</b>, Fig. 20, is, however,
+the one with which we are the most familiar.
+They are always painted red, but the red paint
+has nothing to do with the magnetism.</p>
+<div class="figcenter" style="width: 513px;">
+<img src="images/i_022a.png" width="513" height="62" alt="drawing" />
+<div class="caption">Fig. 21.</div>
+</div>
+<div class="figleft" style="width: 133px;">
+<img src="images/i_022b.png" width="133" height="361" alt="drawing" />
+<div class="caption">Fig. 22.</div>
+</div>
+<p>The little end-piece is called the keeper, or
+armature; it should always be kept in place
+when the magnet is not in use. The magnet
+itself is made of steel, while the armature is
+made of soft iron. Steel retains magnetism
+for a long time, while soft iron loses it almost instantly.
+The ends of the magnet are called its <i>poles</i>, and nearly
+all the strength of the magnet seems to reside at the
+poles, the curved part having no attraction for outside
+bodies. One of the poles of the magnet is marked with
+a line, or with the letter N. This is called the north
+pole of the magnet, the other being its south pole.</p>
+
+<p><span class="pagenum"><a name="Page_22" id="Page_22">[22]</a></span></p>
+
+
+
+<p><b><a id="par_27"></a>27. Bar Magnets</b> are straight magnets. Fig. 21
+shows a round bar magnet. The screw in the end is for
+use in the telephone, described later.</p>
+
+<p><b><a id="par_28"></a>28. Compound Magnets.</b> When several thin steel
+magnets are riveted together, a compound
+magnet is formed. These can be
+made with considerable strength. Fig.
+22 shows a compound horseshoe magnet.
+Fig. 23 shows a form of compound bar
+magnet used in telephones. The use of
+the coil of wire will be explained later.
+A thick piece of steel can not be magnetized
+through and through. In the compound
+magnet we have the effect of a
+thick magnet practically magnetized
+through and through.</p>
+
+
+
+<div class="figcenter" style="width: 539px;">
+<img src="images/i_022c.png" width="539" height="130" alt="drawing" />
+<div class="caption">Fig. 23.</div>
+</div>
+
+<p><b><a id="par_29"></a>29. Magnetic and Diamagnetic
+Bodies.</b> Iron, and substances containing
+iron, are the ones most readily attracted by a magnet.
+Iron is said to be <i>magnetic</i>. Some substances, like
+nickel, for example, are visibly attracted by very strong<span class="pagenum"><a name="Page_23" id="Page_23">[23]</a></span>
+magnets only. Strange as it may seem, some substances
+are actually repelled by strong magnets; these are called
+<i>diamagnetic</i> bodies. Brass, copper, zinc, etc., are not
+visibly affected by a magnet.
+Magnetism will act through
+paper, glass, copper, lead,
+etc.</p>
+
+<div class="figcenter" style="width: 250px;">
+<img src="images/i_023a.png" width="250" height="93" alt="drawing" />
+<div class="caption">Fig. 24.</div>
+</div>
+
+<p><b><a id="par_30"></a>30. Making Magnets.</b>
+One of the strangest properties
+that a magnet has is its power to give magnetism to
+another piece of steel. If a sewing-needle be properly
+rubbed upon one of the poles of a magnet, it will become
+strongly magnetized
+and will retain its magnetism
+for years. Strong
+permanent magnets are
+made with the aid of
+electromagnets. Any
+number of little magnets
+may be made from a horseshoe
+magnet without injuring
+it.</p>
+
+<div class="figcenter" style="width: 312px;">
+<img src="images/i_023b.jpg" width="312" height="418" alt="drawing" />
+<div class="caption">Fig. 25.</div>
+</div>
+
+<p><b><a id="par_31"></a>31. Magnetic Needles
+and Compasses.</b> If a
+bar magnet be suspended
+by a string, or floated
+upon a cork, which can
+easily be done with the
+magnet made from a sewing-needle, Fig. 24, it will
+swing around until its poles point north and south. Such
+an arrangement is called a <i>magnetic needle</i>. In the regular<span class="pagenum"><a name="Page_24" id="Page_24">[24]</a></span>
+<i>compass</i>, a magnetic needle is supported upon a pivot.
+Compasses have been used for many centuries by mariners
+and others. Fig. 25 shows an ordinary pocket
+compass, and Fig. 26 a form of mariner's compass, in
+which the small bar magnets are fastened to a card
+which floats, the whole being so mounted that it keeps a
+horizontal position, even though the vessel rocks.</p>
+
+<div class="figcenter" style="width: 506px;">
+<img src="images/i_024.jpg" width="506" height="428" alt="drawing" />
+<div class="caption">Fig. 26.</div>
+</div>
+
+<p><b><a id="par_32"></a>32. Action of Magnets Upon Each Other.</b> By
+making two small sewing-needle magnets, you can easily
+study the laws of attraction and repulsion. By bringing
+the two north poles, or the two south poles, near each
+other, a repulsion will be noticed. Unlike poles attract
+each other. The attraction between a magnet and iron
+is mutual; that is, each attracts the other. Either pole
+of a magnet attracts soft iron.</p>
+
+<p><span class="pagenum"><a name="Page_25" id="Page_25">[25]</a></span></p>
+
+<p>In magnetizing a needle, either end may be made a
+north pole at will; in fact, the poles of a weak magnet
+can easily be reversed by properly rubbing it upon a
+stronger magnet.</p>
+
+<p><b><a id="par_33"></a>33. Theory of Magnetism.</b> Each little particle of a
+piece of steel or iron is supposed to be a magnet, even
+before it touches a magnet. When these little magnets
+are thoroughly mixed up in the steel, they pull in all
+sorts of directions upon each other and tend to keep the
+steel from attracting outside bodies. When a magnet is
+properly rubbed upon a bar of steel, the north poles of the
+little molecular magnets of the steel are all made to point
+in the same direction. As the north poles help each
+other, the whole bar can attract outside bodies.</p>
+
+<p>By jarring a magnet its molecules are thoroughly
+shaken up; in fact, most of the magnetism can be
+knocked out of a weak magnet by hammering it.</p>
+
+<p><b><a id="par_34"></a>34. Retentivity.</b> The power that a piece of steel has
+to hold magnetism is called <i>retentivity</i>. Different kinds
+of steel have different retentivities. A sewing-needle of
+good steel will retain magnetism for years, and it is
+almost impossible to knock the magnetism out by
+hammering it. Soft steel has very little retentivity,
+because it does not contain much carbon. Soft iron,
+which contains less carbon than steel, holds magnetism
+very poorly; so it is not used for permanent magnets.
+A little magnetism, however, will remain in the soft iron
+after it is removed from a magnet. This is called <i>residual
+magnetism</i>.</p>
+<div class="figleft" style="width: 313px;">
+<img src="images/i_026a.png" width="313" height="412" alt="drawing" />
+<div class="caption">Fig. 27.</div>
+</div>
+<p><b><a id="par_35"></a>35. Heat and Magnetism.</b> Steel will completely
+lose its magnetism when heated to redness, and a magnet<span class="pagenum"><a name="Page_26" id="Page_26">[26]</a></span>
+will not attract red-hot iron. The molecules of a piece
+of red-hot iron are in such a state of rapid vibration that
+they refuse to be brought into line by the magnet.</p>
+
+<p><b><a id="par_36"></a>36. Induced Magnetism.</b>
+A piece of soft iron
+may be induced to become
+a magnet by holding it
+near a magnet, absolute
+contact not being necessary.
+When the soft iron is removed,
+again, from the influence
+of the magnet, its
+magnetism nearly all disappears.
+It is said to have
+<i>temporary</i> magnetism; it
+had <i>induced</i> magnetism. If
+a piece of soft iron be held
+near the north pole of a
+magnet, as in Fig. 27,
+poles will be produced in the soft iron, the one nearest
+the magnet being the south pole, and the other the north
+pole.</p>
+
+<div class="figcenter" style="width: 263px;">
+<img src="images/i_026b.png" width="263" height="82" alt="drawing" />
+<div class="caption">Fig. 28.</div>
+</div>
+
+<p><b><a id="par_37"></a>37. Magnetic Field.</b> If a bar magnet be laid upon
+the table, and a compass be moved about it, the compass-needle
+will be attracted by
+the magnet, and it will point
+in a different direction for
+every position given to the
+compass. This strange
+power, called magnetism, reaches out on all sides of a
+magnet. The magnet may be said to act by induction<span class="pagenum"><a name="Page_27" id="Page_27">[27]</a></span>
+upon the compass-needle. The space around the magnet,
+in which this inductive action takes place, is called the
+<i>magnetic field</i>. Fig. 28 shows some of the positions
+taken by a compass-needle when moved about on one side
+of a bar magnet.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 228px;">
+<img src="images/i_027a.jpg" width="228" height="600" alt="drawing" />
+<div class="caption">Fig. 29.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 283px;">
+<img src="images/i_027b.jpg" width="283" height="588" alt="drawing" />
+<div class="caption">Fig. 30.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+
+<p><b><a id="par_38"></a>38. Magnetic Figures</b> can be made by sprinkling iron<span class="pagenum"><a name="Page_28" id="Page_28">[28]</a></span>
+filings upon a sheet of paper under which is placed a
+magnet. Fig. 29 shows a magnetic figure made with an
+ordinary bar magnet. The magnet was placed upon the
+table and over this was laid
+a piece of smooth paper.
+Fine iron filings were sifted
+upon the paper, which was
+gently tapped so that the
+filings could arrange themselves.
+As each particle of
+iron became a little magnet,
+by induction, its poles were
+attracted and repelled by
+the magnet; and when the
+paper was tapped they
+swung around to their final
+positions. Notice that the
+filings have arranged themselves
+in lines. These lines
+show the positions of some
+of the <i>lines of magnetic force</i>
+which surrounded the
+magnet.</p>
+
+<p>These lines of force pass
+from the north pole of a
+magnet through the air on
+all sides to its south pole.</p>
+
+<div class="figleft" style="width: 253px;">
+<img src="images/i_028.jpg" width="253" height="600" alt="drawing" />
+<div class="caption">Fig. 31.</div>
+</div>
+
+<p>Fig. 30 shows a magnetic
+figure made from two bar
+magnets placed side by side, their unlike poles being
+next to each other. Fig. 31 shows the magnetic figure<span class="pagenum"><a name="Page_29" id="Page_29">[29]</a></span>
+of a horseshoe magnet with round poles, the poles being
+uppermost.</p>
+
+<p><b><a id="par_39"></a>39. The Use of Armatures.</b> A magnet attracts
+iron most strongly at its poles, because it is at the poles
+that the greatest number of lines of force pass into the
+air. Lines of force pass easily through soft iron, which
+is said to be a good conductor of them. Air is not a
+good conductor of the lines of force; in order, then, for
+the lines of force to pass from the north pole of a magnet
+to its south pole, they must overcome this resistance of
+the air, unless the armature is in place. A magnet will
+gradually grow weaker when its armature is left off.</p>
+
+<p><b><a id="par_40"></a>40. Terrestrial Magnetism.</b> As the compass-needle
+points to the north and south, the earth must act like a
+magnet. There is a place very far north, about a thousand
+miles from the north pole of the earth, which is
+called the earth's north magnetic pole. Compass-needles
+point to this place, and not to the earth's real north pole.
+You can see, then, that if a compass be taken north of
+this magnetic pole, its north pole will point south. Lines
+of force pass from the earth's north magnetic pole
+through the air on all sides of the earth and enter the
+earth's south magnetic pole. The compass-needle, in
+pointing toward the north magnetic pole, merely takes
+the direction of the earth's lines of force, just as the particles
+of iron filings arrange themselves in the magnetic
+figures.</p>
+
+<p><b><a id="par_41"></a>41. Declination.</b> As the magnetic needle does not
+point exactly to the north, an angle is formed between
+the true north and south line and the line of the needle.
+In Fig. 32 the line marked N S is the true north and<span class="pagenum"><a name="Page_30" id="Page_30">[30]</a></span>
+south line. The <i>angle of variation</i>, or the declination, is
+the angle A between the line N S and the compass-needle.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 65px;">
+<img src="images/i_030a.png" width="65" height="148" alt="drawing" />
+<div class="caption">Fig. 32.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 177px;">
+<img src="images/i_030b.png" width="177" height="71" alt="drawing" />
+<div class="caption">Fig. 33.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_42"></a>42. Dip or Inclination.</b> If a piece of steel be carefully
+balanced upon a support, and then magnetized, it
+will be found that it will no longer balance. The north
+pole will <i>dip</i> or point downward. Fig. 33 shows what
+happens to a needle when it is held in different positions
+over a bar magnet. It
+simply takes the directions
+of the lines of force as
+they pass from the north
+to the south pole of the
+magnet. As the earth's
+lines of force pass in curves
+from the north to the south
+magnetic pole, you can
+see why the magnetic
+needle dips, unless its
+south pole is made heavier
+than its north. Magnetic
+needles are balanced after they are magnetized.</p>
+
+<div class="figright" style="width: 306px;">
+<img src="images/i_030c.png" width="306" height="347" alt="drawings" />
+<div class="caption">Fig. 34.</div>
+</div>
+
+<p>Fig. 34 shows a simple form of dipping needle. These
+are often used by geologists and miners. In the hands<span class="pagenum"><a name="Page_31" id="Page_31">[31]</a></span>
+of the prospector, the miner's compass, or dipping
+needle, proves a serviceable guide to the discovery and
+location of magnetic iron ore. In this instrument the
+magnetic needle is carefully balanced upon a horizontal
+axis within a graduated circle, and in which the needle
+will be found to assume a position inclined to the horizon.
+This angle of deviation is called the <i>inclination</i> or <i>dip</i>,
+and varies in different latitudes, and even at different
+times in the same place.</p>
+
+<p><b><a id="par_43"></a>43. The Earth's Inductive Influence.</b> The earth's
+magnetism acts inductively upon pieces of steel or iron
+upon its surface. If a piece of steel or iron, like a stove
+poker, for example, be held in a north and south line
+with its north end dipping considerably, it will be in the
+best position for the magnetism of the earth to act upon
+it; that is, it will lie in the direction taken by the earth's
+lines of force. If the poker be struck two or three times
+with a hammer to shake up its molecules, we shall find,
+upon testing it, that it has become magnetized. By this
+method we can pound magnetism right out of the air with
+a hammer. If the magnetized poker be held level, in an
+east and west direction, it will no longer be acted upon to
+advantage by the inductive influence of the earth, and
+we can easily hammer the magnetism out of it again.
+(For experiments on magnets and magnetism see
+"Study," Part I.)</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_32" id="Page_32">[32]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_III"></a>CHAPTER III.<br />
+<small>HOW ELECTRICITY IS GENERATED BY THE VOLTAIC
+CELL.</small></h2>
+
+<div class="figright" style="width: 371px;">
+<img src="images/i_032.png" width="371" height="420" alt="drawing" />
+<div class="caption">Fig. 35.</div>
+</div>
+<p><b><a id="par_44"></a>44. Early Experiments.</b> In 1786 Galvani, an
+Italian physician, made experiments to study the effect of
+static electricity upon the nervous excitability of animals,
+and especially upon the frog. He found that electric
+machines were not
+necessary to produce
+muscular contractions
+or kicks of the
+frog's legs, and that
+they could be produced
+when two different
+metals, Fig.
+35, like iron and
+copper, for example,
+were placed in proper
+contact with a nerve
+and a muscle and
+then made to touch
+each other. Galvani
+first thought that the
+frog generated the electricity instead of the metals.</p>
+
+<p>Volta proved that the electricity was caused by the
+contact of the metals. He used the condensing electroscope
+as one means of proving that two dissimilar metals
+become charged differently when in contact. Volta also<span class="pagenum"><a name="Page_33" id="Page_33">[33]</a></span>
+carried out his belief by constructing
+what is called a <i>Voltaic Pile</i>. He
+thought that by making several pairs
+of metals so arranged that all the
+little currents would help each other,
+a strong current could be generated.
+Fig. 36 shows a <i>pile</i>, it being made by
+placing a pair of zinc and copper discs
+in contact with one another, then laying
+on the copper disc a piece of
+flannel soaked in brine, then on top of
+this another pair, etc., etc. By connecting
+the first zinc and the last
+copper, quite a little current was produced.
+This was a start from which
+has been built our present knowledge
+of electricity. Strictly speaking,
+electricity is not generated by combinations
+of metals or by cells; they
+really keep up a difference of potential,
+as will be seen.</p>
+
+<div class="figcenter" style="width: 193px;">
+<img src="images/i_033a.png" width="193" height="575" alt="drawing" />
+<div class="caption">Fig. 36.</div>
+</div>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 114px;">
+<img src="images/i_033b.png" width="114" height="122" alt="drawing" />
+<div class="caption">Fig. 37.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 130px;">
+<img src="images/i_033c.png" width="130" height="168" alt="drawing" />
+<div class="caption">Fig. 38.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_45"></a>45. The Simple Cell.</b> It has been stated that two
+different kinds of electrifications may be produced by
+friction; one positive, the other negative. Either can be
+produced, at will, by using proper materials.
+Fig. 37 shows a
+section of a <i>simple cell</i>;
+Fig. 38 shows another view.
+Cu is a piece of copper,
+and Zn a piece of zinc.
+When they are placed in<span class="pagenum"><a name="Page_34" id="Page_34">[34]</a></span>
+dilute sulphuric acid, it can be shown by delicate apparatus
+that they become charged differently, because the
+acid acts differently upon the plates. They become
+charged by chemical action, and not by friction. The
+zinc is gradually dissolved, and it is this chemical burning
+of the zinc that furnishes energy for the electric current
+in the simple cell. The electrification, or charge, on
+the plates tends to flow from the place of higher to the
+place of lower potential, just as water tends to flow down
+hill. If a wire be joined to the two metals, a constant
+current of electricity will flow through it, because the
+acid continues to act upon the plates. The simple cell
+is a <i>single-fluid</i> cell, as but one liquid is used in its construction.</p>
+
+<p><b>45a. Plates and Poles.</b> The metal strips used in
+voltaic cells are called <i>plates</i> or <i>elements</i>. The one most
+acted upon by the acid is called the positive (+) plate.
+In the simple cell the zinc is the + plate, and the copper
+the negative (-) plate. The end of a wire attached to
+the - plate is called the + pole, or electrode. Fig. 37
+shows the negative (-) electrode as the end of the wire
+attached to the + plate.</p>
+
+<p><b><a id="par_46"></a>46. Direction of Current.</b> In the cell the current
+passes from the zinc to the copper; that is, from the positive
+to the negative plate, where bubbles of hydrogen
+gas are deposited. In the wire connecting the plates,
+the current passes from the copper to the zinc plate. In
+most cells, carbon takes the place of copper. (See
+"Study," § 268.)</p>
+
+<p><b><a id="par_47"></a>47. Local Currents; Amalgamation.</b> Ordinary
+zinc contains impurities such as carbon, iron, etc., and<span class="pagenum"><a name="Page_35" id="Page_35">[35]</a></span>
+when the acid comes in contact with these, they form
+with the zinc a small cell. This tends to eat away the
+zinc without producing useful currents. The little currents
+in the cell from this cause are called <i>local currents</i>.
+(See "Study," Exp. 111, § 273.) This is largely overcome
+by coating the zinc with mercury. This process is
+called <i>amalgamation</i>. It makes the zinc act like pure
+zinc, which is not acted upon by dilute sulphuric acid
+when the current does not pass. (See "Study," § 257,
+274.)</p>
+
+<p><b><a id="par_48"></a>48. Polarization of Cells.</b> Bubbles of hydrogen gas
+are formed when zinc is dissolved by an acid. In the
+ordinary simple cell these bubbles collect on the copper
+plate, and not on the zinc plate, as might be expected.
+The hydrogen is not a conductor of electricity, so this
+film of gas holds the current back. The hydrogen acts
+like a metal and sets up a current that opposes the zinc
+to the copper current. Several methods are employed to
+get rid of the hydrogen. (See "Study," § 278, 279,
+280.)</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_36" id="Page_36">[36]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_IV"></a>CHAPTER IV.<br />
+<small>VARIOUS VOLTAIC CELLS.</small></h2>
+
+
+<p><b><a id="par_49"></a>49. Single-Fluid and Two-Fluid Cells.</b> The simple
+cell (§ 45) is a single-fluid cell. The liquid is called
+the <i>electrolyte</i>, and this must act upon one of the plates;
+that is, chemical action must take place in order to produce
+a current. The simple cell polarizes rapidly, so
+something must be used with the dilute sulphuric acid to
+destroy the hydrogen bubbles. This is done in the
+<i>bichromate of potash cell</i>.</p>
+
+<p>In order to get complete depolarization&mdash;that is, to
+keep the carbon plate almost perfectly free from hydrogen,
+it is necessary to use <i>two-fluid cells</i>, or those to
+which some solid depolarizer is added to the one fluid.</p>
+
+<p><b><a id="par_50"></a>50. Open and Closed Circuit Cells.</b> If we consider
+a voltaic cell, the wires attached to it, and perhaps some
+instrument through which the current passes, we have an
+<i>electric circuit</i>. When the current passes, the circuit is
+<i>closed</i>, but when the wire is cut, or in any way disconnected
+so that the current can not pass, the circuit is
+<i>open</i> or <i>broken</i>. (See "Study," § 266.)</p>
+
+<p><i>Open Circuit Cells</i> are those which can give momentary
+currents at intervals, such as are needed for bells, telephones,
+etc. These must have plenty of time to rest, as
+they polarize when the circuit is closed for a long time.
+The <i>Leclanché</i> and <i>dry</i> cells are the most common open
+circuit cells.</p>
+
+<p><i>Closed Circuit Cells.</i> For telegraph lines, motors, etc.,<span class="pagenum"><a name="Page_37" id="Page_37">[37]</a></span>
+where a current is needed for some time, the cell must be
+of such a nature that it will not polarize quickly; it must
+give a strong and constant current. The <i>bichromate</i> and
+<i>gravity cells</i> are examples of this variety. (See "Study,"
+§ 286.)</p>
+<div class="figleft" style="width: 215px;">
+<img src="images/i_037.png" width="215" height="395" alt="drawing" />
+<div class="caption">Fig. 39.</div>
+</div>
+<p><b><a id="par_51"></a>51. Bichromate of Potash Cells</b> are very useful for
+general laboratory work. They are especially useful for
+operating induction coils, small
+motors, small incandescent lamps,
+for heating platinum wires, etc.
+These cells have an E.M.F. of
+about 2 volts. Dilute sulphuric
+acid is used as the exciting fluid,
+and in this is dissolved the bichromate
+of potash which keeps
+the hydrogen bubbles from the
+carbon plate. (See "Apparatus
+Book," § 26.) Zinc and carbon
+are used for the plates, the +
+pole being the wire attached to
+the carbon.</p>
+
+
+
+<p>Fig. 39 shows one form of bichromate
+cell. It furnishes a large quantity of current,
+and as the zinc can be raised from the fluid, it may be
+kept charged ready for use for many months, and can be
+set in action any time when required by lowering the
+zinc into the liquid. Two of these cells will burn a one
+candle-power miniature incandescent lamp several hours.
+The carbon is indestructible.</p>
+
+<blockquote>
+
+<p><b>Note.</b> For various forms of home-made cells, see "Apparatus
+Book," Chapter I., and for battery fluids see Chapter II.</p></blockquote>
+
+<p><span class="pagenum"><a name="Page_38" id="Page_38">[38]</a></span></p>
+
+<p><b><a id="par_52"></a>52. The Grenet Cell.</b> Fig. 40 is another form of
+bichromate cell. The carbon plates are left in the fluid
+constantly. The zinc plate should be raised when the
+cell is not in use, to keep it from being uselessly dissolved.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 210px;">
+<img src="images/i_038a.png" width="210" height="409" alt="drawing" />
+<div class="caption">Fig. 40.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 208px;">
+<img src="images/i_038b.png" width="208" height="347" alt="drawing" />
+<div class="caption">Fig. 41.</div>
+</div>
+</td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_53"></a>53. Plunge Batteries.</b> Two or more cells are often
+arranged so that their elements can be quickly lowered
+into the acid solution. Such a combination, Fig. 41, is
+called a <i>plunge battery</i>. The binding-posts are so arranged
+that currents of different strengths can be taken from the
+combination. The two binding-posts on the right of the
+battery will give the current of one cell; the two binding-posts
+on the left of the battery will give the current of
+two cells, and the two end binding-posts will give the
+current of all three cells. When not in use the elements
+must always be hung on the hooks and kept out of the
+solution.</p>
+
+<p><span class="pagenum"><a name="Page_39" id="Page_39">[39]</a></span></p>
+
+<p><b><a id="par_54"></a>54. Large Plunge Batteries</b>. Fig. 42, are arranged
+with a winch and a bar above the cells; these afford a
+ready and convenient means of lifting or lowering the
+elements and avoiding waste. In the battery shown,
+Fig. 42, the zincs are 4×6 inches; the carbons have the
+same dimensions, but there are two carbon plates to each
+zinc, thus giving double the carbon surface.</p>
+
+<div class="figcenter" style="width: 419px;">
+<img src="images/i_039a.png" width="419" height="263" alt="drawing" />
+<div class="caption">Fig. 42.</div>
+</div>
+
+<p><b><a id="par_55"></a>55. The Fuller Cell</b>, Fig. 43, is another type of
+bichromate cell, used largely for long-distance telephone
+service, for telephone exchange
+and switch service, for running
+small motors, etc. It consists of a
+glass jar, a carbon plate, with
+proper connections, a clay porous
+cup, containing the zinc, which is
+made in the form of a cone. A
+little mercury is placed in the
+porous cup to keep the zinc well
+amalgamated. Either bichromate
+of potash or bichromate of soda can
+be used as a depolarizer.</p>
+
+<div class="figcenter" style="width: 211px;">
+<img src="images/i_039b.png" width="211" height="313" alt="drawing" />
+<div class="caption">Fig. 43.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_40" id="Page_40">[40]</a></span></p>
+
+<div class="figcenter" style="width: 204px;">
+<img src="images/i_040a.png" width="204" height="289" alt="drawing" />
+<div class="caption">Fig. 44.</div>
+</div>
+
+<p><b><a id="par_56"></a>56. The Gravity Cell</b>, sometimes called the <i>bluestone</i>
+or <i>crowfoot</i> cell, is used largely for telegraph, police, and
+fire-alarm signal service, laboratory
+and experimental work, or whenever
+a closed circuit cell is required. The
+E.M.F. is about one volt. This is a
+modified form of the Daniell cell. Fig.
+44 shows a home-made gravity cell.</p>
+
+<p>A copper plate is placed at the
+bottom of the glass jar, and upon this
+rests a solution of copper sulphate
+(bluestone). The zinc plate is supported
+about four inches above the
+copper, and is surrounded by a solution
+of zinc sulphate which floats upon the top of the
+blue solution. An insulated wire reaches from the copper
+to the top of the cell and forms
+the positive pole. (See "Apparatus
+Book," § 11 to 15, for home-made
+gravity cell, its regulation,
+etc. For experiments with two-fluid
+Daniell cell, see "Study,"
+Exp. 113, § 281 to 286.)</p>
+
+<div class="figcenter" style="width: 225px;">
+<img src="images/i_040b.png" width="225" height="431" alt="drawing" />
+<div class="caption">Fig. 45.</div>
+</div>
+
+<p><b>56a. Bunsen Cells,</b> Fig. 45, are
+used for motors, small incandescent
+lamps, etc. A carbon rod is inclosed
+in a porous cup, on the
+outside of which is a cylinder of
+zinc that stands in dilute sulphuric
+acid, the carbon being in
+nitric acid.</p>
+
+<p><span class="pagenum"><a name="Page_41" id="Page_41">[41]</a></span></p>
+
+<p><b><a id="par_57"></a>57. The Leclanché Cell</b> is an open circuit cell. Sal
+ammoniac is used as the exciting fluid, carbon and zinc
+being used for plates. Manganese dioxide is used as the
+depolarizer; this surrounds the
+carbon plate, the two being
+either packed together in a
+porous cup or held together in the form of cakes. The
+porous cup, or pressed cake, stands in the exciting fluid.
+The E. M. F. is about 1.5 volts.</p>
+
+<div class="figcenter" style="width: 170px;">
+<img src="images/i_041a.png" width="170" height="238" alt="drawing" />
+<div class="caption">Fig. 46.</div>
+</div>
+
+<div class="figcenter" style="width: 221px;">
+<img src="images/i_041b.png" width="221" height="334" alt="drawing" />
+<div class="caption">Fig. 47.</div>
+</div>
+
+<div class="figcenter" style="width: 556px;">
+<img src="images/i_041.png" width="556" height="347" alt="drawings" />
+<div class="caption">Fig. 48.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_42" id="Page_42">[42]</a></span></p>
+
+
+<p>Fig. 46 shows a form with porous cup. The binding-post
+at the top of the carbon plate forms the + electrode,
+the current
+leaving the cell at this
+point.</p>
+<div class="figcenter" style="width: 362px;">
+<img src="images/i_042a.png" width="362" height="326" alt="drawing" />
+<div class="caption">Fig. 49.</div>
+</div>
+
+<p><i>The Gonda Prism
+Cell</i> (Fig. 47), is a
+form of Leclanché in
+which the depolarizer
+is in the form of a
+cake.</p>
+<div class="figright" style="width: 245px;">
+<img src="images/i_042b.png" width="245" height="336" alt="drawing" />
+<div class="caption">Fig. 50.</div>
+</div>
+<p><b><a id="par_58"></a>58. Dry Cells</b> are
+open circuit cells, and
+can be carried about,
+although they are
+moist inside. The + pole is the end of the carbon plate.
+Zinc is used as the outside case and + plate. Fig. 48
+shows the ordinary forms.</p>
+
+<p>Fig. 49 shows a number of
+dry cells arranged in a box
+with switch in front, so that the
+current can be regulated at will.</p>
+
+
+
+<p><b><a id="par_59"></a>59. The Edison-Lelande
+Cells</b>, Fig. 50, are made in
+several sizes and types. Zinc
+and copper oxide, which is
+pressed into plates, form the
+elements. The exciting fluid
+consists of a 25 per cent. solution
+of caustic potash in water.
+They are designed for both open and closed circuit work.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_43" id="Page_43">[43]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_V"></a>CHAPTER V.<br />
+<small>ABOUT PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.</small></h2>
+
+
+<p><b><a id="par_60"></a>60. Electrical Connections.</b> In experimental work,
+as well as in the everyday work of the electrician, electrical
+connections must constantly be made. One wire
+must be joined to another, just for a moment, perhaps,
+or one piece of apparatus must be put in an electric circuit
+with other apparatus, or the current must be turned
+on or off from motors, lamps, etc. In order to conveniently
+and quickly make such connections, apparatus
+called push-buttons, switches and binding-posts are used.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 185px;">
+<img src="images/i_043a.png" width="185" height="74" alt="drawing" />
+<div class="caption">Fig. 51.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 340px;">
+<img src="images/i_043b.png" width="340" height="108" alt="drawing" />
+<div class="caption">Fig. 52.</div>
+</div></td></tr>
+</table></div>
+
+
+<p><b><a id="par_61"></a>61. Push-Buttons.</b> The simple act of pressing your
+finger upon a movable button, or knob, may ring a bell
+a mile away, or do some other equally wonderful thing.
+Fig. 51 shows a simple push-button, somewhat like a
+simple key in construction. If we cut a wire, through
+which a current is passing, then join one of the free ends
+to the screw A and the other end to screw C, we shall be
+able to let the current pass at any instant by pressing the
+spring B firmly upon A.</p>
+
+<p>Push-buttons are made in all sorts of shapes and sizes.
+Fig. 52 gives an idea of the general internal construction.<span class="pagenum"><a name="Page_44" id="Page_44">[44]</a></span>
+The current enters A by one wire, and leaves by another
+wire as soon as the button is pushed and B is forced
+down to A. The bottom of the little button rests upon
+the top of B.</p>
+
+<p>Fig. 53 shows a <i>Table Clamp-Push</i> for use on dining-tables,
+card-tables, chairs, desks, and other movable furniture.
+Fig. 54 shows a combination of push-button,
+speaking-tube, and letter-box used in city apartment
+houses. Fig. 55 shows an <i>Indicating Push</i>. The buzzer
+indicates, by the sound, whether the call has been heard;
+that is, the person called answers back.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 381px;">
+<img src="images/i_044a.png" width="381" height="408" alt="drawing" />
+<div class="caption">Fig. 53.</div>
+</div></td><td align="left">
+<div class="figcenter" style="width: 140px;">
+<img src="images/i_044b.png" width="140" height="452" alt="drawing" />
+<div class="caption">Fig. 54.</div>
+</div></td></tr>
+</table></div>
+
+
+<p><i>Modifications</i> of ordinary push-buttons are used for
+floor push-buttons, on doors, windows, etc., for burglar-alarms,
+for turning off or on lights, etc., etc. (See<span class="pagenum"><a name="Page_45" id="Page_45">[45]</a></span>
+"Apparatus Book,"
+Chapter III., for home-made
+push-buttons.)</p>
+
+<div class="figcenter" style="width: 301px;">
+<img src="images/i_045a.png" width="301" height="422" alt="draiwng" />
+<div class="caption">Fig. 55.</div>
+</div>
+
+<p><b><a id="par_62"></a>62. Switches</b> have a
+movable bar or plug of
+metal, moving on a pivot,
+to make or break a circuit,
+or transfer a current from
+one conductor to another.</p>
+
+<p>Fig. 56 shows a <i>single
+point switch</i>. The current
+entering the pivoted
+arm can go no farther
+when the switch is open,
+as shown. To close the
+circuit, the arm is pushed
+over until it presses down upon the contact-point. For
+neatness, both wires are joined to the under side of the
+switch or to binding-posts.</p>
+
+<div class="figcenter" style="width: 340px;">
+<img src="images/i_045b.png" width="340" height="281" alt="drawing" />
+<div class="caption">Fig. 56.</div>
+</div>
+
+<p>Fig. 57 shows a <i>knife switch</i>. Copper blades are
+pressed down between copper spring clips to close the
+circuit. The handle is
+made of insulating material.</p>
+
+<p><i>Pole-changing
+switches</i>, Fig. 58, are
+used for changing or
+reversing the poles of
+batteries, etc.</p>
+
+<p>Fig. 59 shows a
+home-made switch, useful<span class="pagenum"><a name="Page_46" id="Page_46">[46]</a></span>
+in connection with
+resistance coils. By joining
+the ends of the coils
+A, B, C, D, with the
+contact-points 1, 2, 3,
+etc., more or less resistance
+can be easily thrown
+in by simply swinging
+the lever E around to
+the left or right. If E
+be turned to 1, the
+current will be
+obliged to pass
+through all the
+coils A, B, etc.,
+before it can pass
+out at Y. If E
+be moved to 3,
+coils A and B will
+be cut out of the circuit,
+thus decreasing the resistance
+to the current on its
+way from X to Y. Current
+regulators are made
+upon this principle. (See
+"Apparatus Book," Chapter
+IV., for home-made
+switches.)</p>
+
+<div class="figcenter" style="width: 328px;">
+<img src="images/i_046a.png" width="328" height="288" alt="drawing" />
+<div class="caption">Fig. 57.</div>
+</div>
+
+<div class="figcenter" style="width: 398px;">
+<img src="images/i_046b.png" width="398" height="223" alt="drawing" />
+<div class="caption">Fig. 58.</div>
+</div>
+
+<div class="figcenter" style="width: 291px;">
+<img src="images/i_046c.png" width="291" height="331" alt="drawing" />
+<div class="caption">Fig. 59.</div>
+</div>
+
+<p><i>Switchboards</i> are made
+containing from two or
+three to hundreds of<span class="pagenum"><a name="Page_47" id="Page_47">[47]</a></span>
+switches, and are used in telegraph and telephone work,
+in electric light stations, etc., etc. (See Chapter on
+Central Stations.) Fig. 60 shows a switch used for incandescent
+lighting
+currents.</p>
+
+<div class="figcenter" style="width: 375px;">
+<img src="images/i_047a.jpg" width="375" height="360" alt="drawing" />
+<div class="caption">Fig. 60.</div>
+</div>
+
+<div class="figcenter" style="width: 551px;">
+<img src="images/i_047b.jpg" width="551" height="246" alt="drawing" />
+<div class="caption">Fig. 61.</div>
+</div>
+
+<p><b><a id="par_63"></a>63. Binding-Posts</b>
+are used to
+make connections
+between two pieces
+of apparatus, between
+two or more
+wires, between a
+wire and any apparatus,
+etc., etc.
+They allow the
+wires to be quickly
+fastened or unfastened
+to the apparatus. A large part of the apparatus
+shown in this book has binding-posts attached. Fig. 61
+shows a few of the common forms used. (See "Apparatus
+Book," Chapter V., for home-made binding-posts.)</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_48" id="Page_48">[48]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_VI"></a>CHAPTER VI.<br />
+<small>UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.</small></h2>
+
+
+<p><b><a id="par_64"></a>64. Electrical Units.</b> In order to measure electricity
+for experimental or commercial purposes, standards or
+units are just as necessary as the inch or foot for measuring
+distances.</p>
+
+<p><b><a id="par_65"></a>65. Potential; Electromotive Force.</b> If water in
+a tall tank be allowed to squirt from two holes, one near
+the bottom, the other near the top, it is evident that the
+force of the water that comes from the hole at the bottom
+will be the greater. The pressure at the bottom is greater
+than that near the top, because the "head" is greater.</p>
+
+<p>When a spark of static electricity jumps a long distance,
+we say that the charge has a high <i>potential</i>; that is, it
+has a high electrical pressure. Potential, for electricity,
+means the same as pressure, for water. The greater the
+potential, or <i>electromotive force</i> (E.M.F.) of a cell, the
+greater its power to push a current through wires. (See
+"Study," § 296 to 305, with experiments.)</p>
+
+<p><b><a id="par_66"></a>66. Unit of E.M.F.; the Volt.</b>&mdash;In speaking of
+water, we say that its pressure is so many pounds to the
+square inch, or that it has a fall, or head, of so many
+feet. We speak of a current as having so many volts;
+for example, we say that a wire is carrying a 110-volt
+current. The volt is the unit of E.M.F. An ordinary
+gravity cell has an E.M.F. of about one volt. This
+name was given in honor of Volta.</p>
+
+<p><span class="pagenum"><a name="Page_49" id="Page_49">[49]</a></span></p>
+
+<p><b><a id="par_67"></a>67. Measurement of Electromotive Force.</b> There
+are several ways by which the E.M.F. of a cell, for
+example, can be
+measured. It is
+usually measured
+<i>relatively</i>, by comparison
+with the
+E. M. F. of some
+standard cell. (See
+"Study," Exp.
+140, for measuring
+the E. M. F. of a
+cell by comparison
+with the two-fluid cell.)</p>
+
+<div class="figcenter" style="width: 386px;">
+<img src="images/i_049a.jpg" width="386" height="292" alt="drawing" />
+<div class="caption">Fig. 62.</div>
+</div>
+
+<p><i>Voltmeters</i> are instruments by means of which E. M. F.
+can be read on a printed scale. They are a variety of
+galvanometer, and are made with coils of such high
+resistance, compared with the resistance of a cell or
+dynamo, that the E. M. F.
+can be read direct. The
+reason for this will be seen
+by referring to Ohm's law
+("Study," § 356); the
+resistance is so great that
+the strength of the current
+depends entirely upon
+the E. M. F.</p>
+
+<div class="figcenter" style="width: 312px;">
+<img src="images/i_049b.jpg" width="312" height="297" alt="drawing" />
+<div class="caption">Fig. 63.</div>
+</div>
+
+<p>Voltmeters measure
+electrical pressure just as
+steam gauges measure the pressure of steam. Fig. 62
+shows one form of voltmeter. Fig. 63 shows a voltmeter<span class="pagenum"><a name="Page_50" id="Page_50">[50]</a></span>
+with illuminated dial. An electrical bulb behind the
+instrument furnishes light so that the readings can be
+easily taken.</p>
+
+<p><b><a id="par_68"></a>68. Electrical Resistance.</b> Did you ever ride down
+hill on a hand-sled? How easily the sled glides over the
+snow! What happens, though, when you strike a bare
+place, or a place where some evil-minded person has
+sprinkled ashes? Does the sled pass easily over bare
+ground or ashes? Snow offers
+very little <i>resistance</i> to the sled,
+while ashes offer a great resistance.</p>
+
+<div class="figleft" style="width: 243px;">
+<img src="images/i_050.png" width="243" height="303" alt="drawing" />
+<div class="caption">Fig. 64.</div>
+</div>
+
+<p>All substances do not allow the
+electric current to pass through
+them with the same ease. Even
+the liquid in a cell tends to hold
+the current back and offers <i>internal
+resistance</i>. The various
+wires and instruments connected
+to a cell offer <i>external resistance</i>.
+(See "Study," Chapter XVIII., for experiments,
+etc.)</p>
+
+<p><b><a id="par_69"></a>69. Unit of Resistance.</b> <b>The Ohm</b> is the name given
+to the unit of resistance. About 9 ft. 9 in. of No. 30
+copper wire, or 39 feet 1 in. of No. 24 copper wire, will
+make a fairly accurate ohm.</p>
+
+<p><i>Resistance coils</i>, having carefully measured resistances,
+are made for standards. (See "Apparatus Book,"
+Chapter XVII., for home-made resistance coils.) Fig.
+64 shows a commercial form of a standard resistance coil.
+The coil is inclosed in a case and has large wires leading<span class="pagenum"><a name="Page_51" id="Page_51">[51]</a></span>
+from its ends for connections. Fig. 65 gives an idea of
+the way in which coils are wound and used with plugs to
+build up <i>resistance boxes</i>, Fig. 66.</p>
+
+<p><b><a id="par_70"></a>70. Laws of Resistance.</b> 1. The resistance of a
+wire is directly proportional
+to its length, provided its
+cross-section, material, etc.,
+are uniform.</p>
+
+<p>2. The resistance of a wire
+is inversely proportional to its
+area of cross-section; or, in
+other words, inversely proportional
+to the square of its
+diameter, other things being
+equal.</p>
+
+<div class="figcenter" style="width: 273px;">
+<img src="images/i_051a.png" width="273" height="255" alt="drawing" />
+<div class="caption">Fig. 65.</div>
+</div>
+
+<p>3. The resistance of a wire depends upon its material,
+as well as upon its length, size, etc.</p>
+
+<p>4. The resistance of a wire increases as its temperature
+rises. (See "Study," Chapters XVIII. and XIX., for
+experiments on
+resistance, its
+measurement,
+etc.)</p>
+
+<div class="figcenter" style="width: 373px;">
+<img src="images/i_051b.png" width="373" height="237" alt="drawing" />
+<div class="caption">Fig. 66.</div>
+</div>
+
+<p><b><a id="par_71"></a>71. Current
+Strength.</b> The
+strength of a current
+at the end of
+a circuit depends
+not only upon the
+<i>electrical pressure</i>, or E. M. F., which drives the current,
+but also upon the <i>resistance</i> which has to be overcome.<span class="pagenum"><a name="Page_52" id="Page_52">[52]</a></span>
+The greater the resistance the weaker the current at the
+end of its journey.</p>
+
+<p><b><a id="par_72"></a>72. Unit of Current Strength; The Ampere.</b> A
+current having an E. M. F. of <i>one volt</i>, pushing its way
+through a resistance of <i>one ohm</i>, would have a unit of
+strength, called <i>one ampere</i>. This current, one ampere
+strong, would deposit, under proper conditions, .0003277
+gramme of copper in
+<i>one second</i> from a solution
+of copper sulphate.</p>
+
+<p><b><a id="par_73"></a>73. Measurement
+of Current Strength.</b>
+A magnetic needle is
+deflected when a current
+passes around it,
+as in instruments like
+the galvanometer. The
+<i>galvanoscope</i> merely indicates
+the presence of
+a current. <i>Galvanometers</i>
+measure the
+strength of a current,
+and they are made in many forms, depending upon the
+nature and strength of the currents to be measured.
+Galvanometers are standardized, or calibrated, by special
+measurements, or by comparison with some standard instrument,
+so that when the deflection is a certain number
+of degrees, the current passing through it is known to
+be of a certain strength.</p>
+
+<div class="figcenter" style="width: 348px;">
+<img src="images/i_052a.jpg" width="348" height="405" alt="drawing" />
+<div class="caption">Fig. 67.</div>
+</div>
+
+<p>Fig. 67 shows an <i>astatic galvanometer</i>. Fig. 68 shows
+a <i>tangent galvanometer</i>, in which the strength of the current<span class="pagenum"><a name="Page_53" id="Page_53">[53]</a></span>
+is proportional to the tangent of the angle of deflection.
+Fig. 69 shows a <i>D'Arsonval galvanometer</i>, in which
+a coil of wire is suspended between the
+poles of a permanent horseshoe magnet.
+The lines of force are concentrated
+by the iron core of the coil.
+The two thin suspending wires convey
+the current to the coil. A ray of light
+is reflected from the small mirror and
+acts as a pointer as in other forms of
+reflecting galvanometers.</p>
+
+<div class="figcenter" style="width: 182px;">
+<img src="images/i_053a.png" width="182" height="259" alt="drawing" />
+<div class="caption">Fig. 68.</div>
+</div>
+
+<p><b><a id="par_74"></a>74. The Ammeter</b>, Fig. 70, is a
+form of galvanometer in which the strength of a current,
+in amperes, can be read. In these the strength of current
+is proportional to the angular deflections. The coils are
+made with a small resistance,
+so that the current
+will not be greatly reduced
+in strength in passing
+through them.</p>
+
+
+<div class="figcenter" style="width: 307px;">
+<img src="images/i_053.png" width="307" height="449" alt="drawing" />
+<div class="caption">Fig. 69.</div>
+</div>
+
+<p><b><a id="par_75"></a>75. Voltameters</b>
+measure the strength of a
+current by chemical means,
+the quantity of metal deposited
+or gas generated
+being proportional to the
+time that the current flows
+and to its strength. In
+the <i>water voltameter</i>, Fig.
+71, the hydrogen and
+oxygen produced in a<span class="pagenum"><a name="Page_54" id="Page_54">[54]</a></span>
+given time are
+measured. (See
+"Study," Chapter
+XXI.)</p>
+
+<div class="figcenter" style="width: 379px;">
+<img src="images/i_054a.png" width="379" height="304" alt="drawing" />
+<div class="caption">Fig. 70.</div>
+</div>
+
+<p>The <i>copper voltameter</i>
+measures the
+amount of copper
+deposited in a given
+time by the current.
+Fig. 72 shows one
+form. The copper
+cathode is weighed
+before and after the current flows. The weight of
+copper deposited and the time taken are used to calculate
+the current strength.</p>
+
+<div class="figcenter" style="width: 551px;">
+<img src="images/i_054b.png" width="551" height="402" alt="drawing" />
+<div class="caption">Fig. 71.</div>
+</div>
+
+<p><b><a id="par_76"></a>76. Unit of Quantity</b>; <b>The Coulomb</b> is the quantity
+of electricity given, in <i>one second</i>, by a current having a<span class="pagenum"><a name="Page_55" id="Page_55">[55]</a></span>
+strength of one
+ampere. Time is
+an important element
+in considering
+the work a current
+can do.</p>
+
+<div class="figcenter" style="width: 368px;">
+<img src="images/i_055a.jpg" width="368" height="295" alt="drawing" />
+<div class="caption">Fig. 72.</div>
+</div>
+
+<p><b><a id="par_77"></a>77. Electrical
+Horse-power</b>;
+<b>The Watt</b> is the
+unit of electrical
+power. A current
+having the strength of one ampere, and an E. M. F. of
+one volt has a unit of power. 746 watts make one electrical
+horse-power. Watts = amperes × volts. Fig. 73
+shows a direct reading wattmeter based on the international
+volt and ampere. They save taking simultaneous
+ammeter and voltmeter readings, which are otherwise
+necessary to
+get the product
+of volts and amperes,
+and are also
+used on alternating
+current
+measurements.</p>
+
+<div class="figcenter" style="width: 391px;">
+<a href="images/i_055b-big.jpg"><img src="images/i_055b.jpg" width="391" height="353" alt="drawing" /></a>
+<div class="caption">Fig. 73.</div>
+</div>
+
+<p>There are also
+forms of wattmeters,
+Fig. 74,
+in which the watts
+are read from
+dials like those on
+an ordinary gas-meter, the records being permanent.</p>
+
+<p><span class="pagenum"><a name="Page_56" id="Page_56">[56]</a></span></p>
+
+<p>Fig. 75 shows a voltmeter V, and ammeter A, so placed
+in the circuit that readings can be taken. D represents
+a dynamo. A is placed so that the whole current passes
+through it, while V is placed
+between the main wires to
+measure the difference in
+potential. The product of the
+two readings in volts and
+amperes gives the number of
+watts.</p>
+
+<div class="figcenter" style="width: 277px;">
+<img src="images/i_056a.jpg" width="277" height="260" alt="drawing" />
+<div class="caption">Fig. 74.</div>
+</div>
+
+<p><b><a id="par_78"></a>78. Chemical Meters</b> also
+measure the quantity of current
+that is used; for example,
+one may be placed in the cellar
+to measure the quantity of current used to light the
+house.</p>
+
+<div class="figcenter" style="width: 414px;">
+<img src="images/i_056b.png" width="414" height="160" alt="drawing" />
+<div class="caption">Fig. 75.</div>
+</div>
+
+<p>Fig. 76 shows a chemical meter, a part of the current
+passing through a jar containing zinc plates and a solution
+of zinc sulphate. Metallic zinc is dissolved from
+one plate and deposited upon the other. The increase in
+weight shows the amount of chemical action which is
+proportional to the ampere hours. Knowing the relation
+between the quantity of current that can pass through
+the solution to that which can pass through the meter by<span class="pagenum"><a name="Page_57" id="Page_57">[57]</a></span>
+another conductor, a calculation can be made which will
+give the current used. A lamp is so arranged that it
+automatically lights before the meter gets to the freezing-point;
+this warms it up to the proper temperature, at
+which point the light goes out again.</p>
+
+<div class="figcenter" style="width: 469px;">
+<a href="images/i_057-big.jpg"><img src="images/i_057.jpg" width="469" height="303" alt="drawing" /></a>
+<div class="caption">Fig. 76.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_58" id="Page_58">[58]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_VII"></a>CHAPTER VII.<br />
+<small>CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.</small></h2>
+
+
+<p><b><a id="par_79"></a>79. Electrolysis.</b> It has been seen that in the voltaic
+cell electricity is generated by chemical action. Sulphuric
+acid acts upon zinc and dissolves it in the cell,
+hydrogen is produced, etc. When this process is reversed,
+that is, when the electric current is passed
+through some solutions, they are decomposed, or broken
+up into their constituents. This process is called <i>electrolysis</i>,
+and the compound decomposed is the <i>electrolyte</i>.
+(See "Study," § 369, etc., with experiments.)</p>
+
+<div class="figcenter" style="width: 533px;">
+<img src="images/i_058.jpg" width="533" height="393" alt="drawing" />
+<div class="caption">Fig. 77.</div>
+</div>
+
+<p>Fig. 77 shows how water can be decomposed into its
+two constituents, hydrogen and oxygen, there being
+twice as much hydrogen formed as oxygen.</p>
+
+<p>Fig. 78 shows a glass jar in which are placed two metal<span class="pagenum"><a name="Page_59" id="Page_59">[59]</a></span>
+strips, A and C, these being connected with two cells.
+In this jar may be placed various conducting solutions to
+be tested. If, for example, we use a solution of copper
+sulphate, its chemical formula being CuSO<sub>4</sub>, the current
+will break it up into Cu (copper) and SO<sub>4</sub>. The Cu will
+be deposited upon C as the current passes from A to C
+through the solution. A is called the <i>anode</i>, and C the
+<i>cathode</i>.</p>
+
+<div class="figcenter" style="width: 374px;">
+<img src="images/i_059a.jpg" width="374" height="302" alt="drawing" />
+<div class="caption">Fig. 78.</div>
+</div>
+
+<p>Fig. 79 shows another form of jar used to study the
+decomposition of solutions by the electric
+current.</p>
+
+<div class="figcenter" style="width: 140px;">
+<img src="images/i_059b.jpg" width="140" height="183" alt="drawing" />
+<div class="caption">Fig 79.</div>
+</div>
+
+<p><b><a id="par_80"></a>80. Ions.</b> When a solution is decomposed
+into parts by a current, the parts are
+called the <i>Ions</i>. When copper sulphate
+(Cu SO<sub>4</sub>) is used, the ions are Cu, which is
+a metal, and SO<sub>4</sub>, called an acid radical.
+When silver nitrate (Ag NO<sub>3</sub>) is used, Ag
+and NO<sub>3</sub> are the ions. The metal part of the compound
+goes to the cathode.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_60" id="Page_60">[60]</a></span></p>
+
+
+
+
+<h2>CHAPTER VIII.<br />
+<small>HOW ELECTROPLATING AND ELECTROTYPING ARE DONE.</small></h2>
+
+
+<p><b><a id="par_81"></a>81. Electricity and Chemical Action.</b> We have
+just seen, Chapter VII., that the electric current has the
+power to decompose certain compounds when they are in
+solution. By choosing the right solutions, then, we shall
+be able to get copper, silver, and other metals set free by
+electrolysis.</p>
+
+<p><b><a id="par_82"></a>82. Electroplating</b> consists in coating substances
+with metal with the aid of the electric current. If we
+wish to electroplate a piece of metal with copper, for
+example, we can use the arrangement shown in Fig. 78,
+in which C is the cathode plate to be covered, and A is a
+copper plate. The two are in a solution of copper sulphate,
+and, as explained in § 79, the solution will be
+decomposed. Copper will be deposited upon C, and the
+SO<sub>4</sub> part of the solution will go to the anode A, which it
+will attack and gradually dissolve. The SO<sub>4</sub>, acting upon
+the copper anode, makes CuSO<sub>4</sub> again, and this keeps the
+solution at a uniform strength. The amount of copper
+dissolved from the copper anode equals, nearly, the
+amount deposited upon the cathode. The metal is carried
+in the direction of the current.</p>
+
+<p>If we wish to plate something with silver or gold, it
+will be necessary to use a solution of silver or gold for
+the electrolyte, a plate of metallic silver or gold being
+used for the anode, as the case may be.</p>
+
+<p><span class="pagenum"><a name="Page_61" id="Page_61">[61]</a></span></p>
+
+<p>Great care is used in cleaning substances to be plated,
+all dirt and grease being carefully removed.</p>
+
+<p>Fig. 80 shows a plating bath in which several articles
+can be plated at the same time by hanging them upon a
+metal bar which really forms a part of the cathode. If,
+for example, we wish to plate knives, spoons, etc., with
+silver, they would be hung from the bar shown, each
+being a part of the cathode. The vat would contain a
+solution of silver, and from the other bar would be hung
+a silver plate having a surface about equal to that of the
+combined knives, etc.</p>
+
+<div class="figcenter" style="width: 486px;">
+<img src="images/i_061.jpg" width="486" height="265" alt="drawing" />
+<div class="caption">Fig. 80.</div>
+</div>
+
+<p>Most metals are coated with copper before they are
+plated with silver or gold. When plating is done on a
+large scale, a current from a dynamo is used. For
+experimental purposes a Gravity cell will do very well.
+(See "Study," § 374 to 380 with experiments.)</p>
+
+<p><b><a id="par_83"></a>83. Electrotyping.</b> It was observed by De La Rue
+in 1836 that in the Daniell cell an even coating of copper
+was deposited upon the copper plate. From this was
+developed the process of electrotyping, which consists in<span class="pagenum"><a name="Page_62" id="Page_62">[62]</a></span>
+making a copy in metal of a wood-cut, page of type, etc.
+A mould or impression of the type or coin is first
+made in wax, or other suitable material. These moulds
+are, of course, the reverse of the original, and as they do
+not conduct electricity, have to be coated with graphite.
+This thin coating lines the mould with conducting
+material so that the current can get to every part of the
+mould. These are then hung upon the cathode in a bath
+of copper sulphate as described in § 82. The electric
+current which passes through the vat deposits a thin
+layer of metallic copper next to the graphite. When this
+copper gets thick enough, the wax is melted away from
+it, leaving a thin shell of copper, the side next to the
+graphite being exactly alike in shape to the type, but
+made of copper. These thin copper sheets are too thin
+to stand the pressure necessary on printing presses, so
+they are strengthened by backing them with soft metal
+which fills every crevice, making solid plates about ¼ in.
+thick. These plates or <i>electrotypes</i> are used to print
+from, the original type being used to set up another page.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_63" id="Page_63">[63]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_IX"></a>CHAPTER IX.<br />
+<small>THE STORAGE BATTERY, AND HOW IT WORKS.</small></h2>
+
+
+<p><b><a id="par_84"></a>84. Polarization.</b> It has been stated that a simple
+cell polarizes rapidly on account of hydrogen bubbles that
+form upon the copper plate. They tend to send a current
+in the opposite direction to that of the main current,
+which is thereby weakened.</p>
+
+<div class="figcenter" style="width: 356px;">
+<img src="images/i_063.jpg" width="356" height="243" alt="drawing" />
+<div class="caption">Fig. 81.</div>
+</div>
+
+<p><b><a id="par_85"></a>85. Electromotive Force of Polarization.</b> It has
+been shown, Fig. 71, that water can be decomposed by
+the electric current.
+Hydrogen and
+oxygen have a
+strong attraction or
+chemical affinity for
+each other, or they
+would not unite to
+form water. This
+attraction has to be
+overcome before the
+water can be decomposed. As soon as the decomposing
+current ceases to flow, the gases formed try to rush together
+again; in fact, if the water voltameter be disconnected
+from the cells and connected with a galvanoscope,
+the presence of a current will be shown. This voltameter
+will give a current with an E. M. F. of nearly 1.5
+volts; so it is evident that we must have a current with a
+higher voltage than this to decompose water. This<span class="pagenum"><a name="Page_64" id="Page_64">[64]</a></span>
+E. M. F., due to polarization, is called the E. M. F. of
+polarization.</p>
+
+<p><b><a id="par_86"></a>86. Secondary or Storage Batteries</b>, also called
+<i>accumulators</i>, do not really store electricity. They must
+be charged by a current before they can give out any
+electricity. Chemical changes are produced in the storage
+cells by the charging current just as they are in voltameters,
+electroplating solutions, etc.; so it is potential
+chemical energy that is really
+stored. When the new products
+are allowed to go back
+to their original state, by
+joining the electrodes of the
+charged cell, a current is produced.</p>
+
+<p>Fig. 81 shows two lead
+plates, A and B, immersed in
+dilute sulphuric acid, and
+connected with two ordinary
+cells. A strong current will
+pass through the liquid between
+A and B at first, but it
+will quickly become weaker, as chemical changes take
+place in the liquid. This may be shown by a galvanometer
+put in the circuit before beginning the experiment.
+By disconnecting the wires from the cells and
+joining them to the galvanometer, it will be shown that
+a current comes from the lead plates. This arrangement
+may be called a simple storage cell. Regular storage cells
+are charged with the current from a dynamo. (See
+"Study," Exp. 151.)</p>
+
+<div class="figcenter" style="width: 272px;">
+<img src="images/i_064.jpg" width="272" height="349" alt="drawing" />
+<div class="caption">Fig. 82.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_65" id="Page_65">[65]</a></span></p>
+
+<p>The first storage cells were made of plain lead plates,
+rolled up in such a way that they were close to each
+other, but did not touch. These were placed in dilute
+sulphuric acid. They were charged in alternate directions
+several times, until the lead became properly acted
+upon, at which time the cell would furnish a current.</p>
+
+<p>A great improvement was made in 1881, by Faure, who
+coated the plates with red lead.</p>
+
+<div class="figcenter" style="width: 507px;">
+<img src="images/i_065.jpg" width="507" height="346" alt="drawing" />
+<div class="caption">Fig. 83.</div>
+</div>
+
+<p>The method now generally practiced is to cast a frame
+of lead, with raised right-angled ribs on each side, thus
+forming little depressed squares, or to punch a lead plate
+full of holes, which squares or holes are then filled with
+a pasty mixture of red oxide of lead in positive plates,
+and with litharge in negatives. In a form called the
+chloride battery, instead of cementing lead oxide paste
+into or against a lead framing in order to obtain the
+necessary active material, the latter is obtained by a
+strictly chemical process.</p>
+
+<p><span class="pagenum"><a name="Page_66" id="Page_66">[66]</a></span></p>
+
+<p>Fig. 82 shows a storage cell with plates, etc., contained
+in a glass jar. Fig. 83 shows a cell of 41 plates, set up in
+a lead-lined wood tank. Fig. 84 shows three cells joined
+in series. Many storage cells are used in central electric
+light stations to help the dynamos during the "rush"
+hours at night. They are charged during the day when
+the load on the dynamos is not heavy.</p>
+
+<p>Fig. 85 shows another form of storage cell containing
+a number of plates.</p>
+
+<div class="figcenter" style="width: 466px;">
+<img src="images/i_066.jpg" width="466" height="321" alt="drawing" />
+<div class="caption">Fig. 84.</div>
+</div>
+
+<p><b><a id="par_87"></a>87. The Uses of Storage Batteries</b> are almost
+numberless. The current can be used for nearly everything
+for which a constant current is adapted, the following
+being some of its applications: Carriage propulsion;
+electric launch propulsion; train lighting; yacht lighting;
+carriage lighting; bicycle lighting; miners' lamps; dental,
+medical, surgical, and laboratory work; phonographs;
+kinetoscopes; automaton pianos; sewing-machine motors;
+fan motors; telegraph; telephone; electric bell; electric<span class="pagenum"><a name="Page_67" id="Page_67">[67]</a></span>
+fire-alarm; heat regulating; railroad switch and signal
+apparatus.</p>
+
+<p>By the installing of a storage plant many natural but
+small sources of power may be utilized in furnishing light
+and power; sources which otherwise are not available,
+because not large enough to supply maximum demands.
+The force of the tides, of small water powers from irrigating
+ditches, and even of the wind, come under this
+heading.</p>
+
+<div class="figcenter" style="width: 306px;">
+<img src="images/i_067.jpg" width="306" height="364" alt="drawing" />
+<div class="caption">Fig. 85.</div>
+</div>
+
+<p>As a regulator of pressure, in case of fluctuations in
+the load, the value of a storage plant is inestimable.
+These fluctuations of load are particularly noticeable in
+electric railway plants, where the demand is constantly
+rising and falling, sometimes jumping from almost nothing
+to the maximum, and <i>vice versa</i>, in a few seconds.
+If for no other reason than the prevention of severe
+strain on the engines and generators, caused by these
+fluctuations of demand, a storage plant will be valuable.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_68" id="Page_68">[68]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_X"></a>CHAPTER X.<br />
+<small>HOW ELECTRICITY IS GENERATED BY HEAT.</small></h2>
+
+
+<p><b><a id="par_88"></a>88. Thermoelectricity</b> is the name given to electricity
+that is generated by heat. If a strip of iron, I, be connected
+between two strips of copper, C C, these being
+joined by a copper wire, C W, we shall have an arrangement
+that will generate a current when heated at either
+of the junctions between C and I. When it is heated
+at A the current will
+flow as shown by
+arrows, from C to I.
+If we heat at B, the
+current will flow in
+the opposite direction
+through the metals,
+although it will still
+go from C to I as before. Such currents are called
+<i>thermoelectric currents</i>.</p>
+
+<div class="figleft" style="width: 345px;">
+<img src="images/i_068.jpg" width="345" height="196" alt="drawing" />
+<div class="caption">Fig. 86.</div>
+</div>
+
+<p>Different pairs of metals produce different results.
+Antimony and bismuth are generally used, because the
+greatest effect is produced by them. If the end of a strip
+of bismuth be soldered to the end of a similar strip of
+antimony, and the free ends be connected to a galvanometer
+of low resistance, the presence of a current will be
+shown when the point of contact becomes hotter than the
+rest of the circuit. The current will flow from bismuth<span class="pagenum"><a name="Page_69" id="Page_69">[69]</a></span>
+to antimony across the joint. By cooling the juncture
+below the temperature of the rest of the circuit, a current
+will be produced in the opposite direction to the above.
+The energy of the current is kept up by the heat absorbed,
+just as it is kept up by chemical action in the voltaic
+cell.</p>
+
+<p><b><a id="par_89"></a>89. Peltier Effect.</b> If an electric current be passed
+through pairs of metals, the parts at the junction become
+slightly warmer or cooler than before, depending upon
+the direction of the current. This action is really the
+reverse of that in which currents are produced by heat.</p>
+
+<div class="figcenter" style="width: 309px;">
+<img src="images/i_069.jpg" width="309" height="201" alt="drawing" />
+<div class="caption">Fig. 87.</div>
+</div>
+
+<p><b><a id="par_90"></a>90. Thermopiles.</b> As the E.M.F. of the current
+produced by a single pair of metals is very small, several
+pairs are usually joined in series, so that the different
+currents will help each other by flowing in the same direction.
+Such combinations are called thermoelectric piles,
+or simply <i>thermopiles</i>.</p>
+
+<p>Fig. 87 shows such an arrangement, in which a large
+number of elements are placed in a small space. The
+junctures are so arranged that the alternate ones come
+together at one side.</p>
+
+<p>Fig. 88 shows a thermopile connected with a galvanometer.<span class="pagenum"><a name="Page_70" id="Page_70">[70]</a></span>
+The heat of a match, or the cold of a piece of ice,
+will produce a current, even if held at some distance from
+the thermopile. The galvanometer should be a short-coil
+astatic one. (See "Study," Chapter XXIV., for
+experiments and home-made thermopile.)</p>
+
+<div class="figcenter" style="width: 570px;">
+<img src="images/i_070.jpg" width="570" height="400" alt="drawing" />
+<div class="caption">Fig. 88.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_71" id="Page_71">[71]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XI"></a>CHAPTER XI.<br />
+<small>MAGNETIC EFFECTS OF THE ELECTRIC CURRENT.</small></h2>
+
+
+<p><b><a id="par_91"></a>91. Electromagnetism</b> is the name given to magnetism
+that is developed by electricity. We have seen that
+if a magnetic needle be placed in the field of a magnet, its
+N pole will point in the direction taken by the lines of
+force as they pass from the N to the S pole of the magnet.</p>
+
+<div class="figcenter" style="width: 398px;">
+<img src="images/i_071.jpg" width="398" height="294" alt="drawing" />
+<div class="caption">Fig. 89.</div>
+</div>
+
+<p><b><a id="par_92"></a>92. Lines of Force about a Wire.</b> When a current
+passes through a wire, the magnetic needle placed over or
+under it tends to take a position at right angles to the
+wire. Fig. 89 shows such a wire and needle, and how
+the needle is deflected; it twists right around from its N
+and S position as soon as the current begins to flow.
+This shows that the lines of force pass <i>around</i> the wire
+and not in the direction of its length. The needle does
+not swing entirely perpendicular to the wire, that is, to<span class="pagenum"><a name="Page_72" id="Page_72">[72]</a></span>
+the E and W line, because the earth is at the same time
+pulling its N pole toward the N.</p>
+
+<p>Fig. 90 shows a bent wire through which a current
+passes from C to Z. If you look along the wire from C
+toward the points A and B, you will see that <i>under</i> the
+wire the lines of force pass to the left. Looking along
+the wire from Z toward D you will see that the lines of
+force pass opposite to the above, as the current comes
+<i>toward</i> you. This is learned by experiment. (See
+"Study," Exp. 152, § 385, etc.)</p>
+
+<div class="figcenter" style="width: 269px;">
+<img src="images/i_072a.png" width="269" height="203" alt="drawing" />
+<div class="caption">Fig. 90.</div>
+</div>
+
+<div class="figcenter" style="width: 185px;">
+<img src="images/i_072b.png" width="185" height="94" alt="drawing" />
+<div class="caption">Fig. 91.</div>
+</div>
+
+<p><i>Rule.</i> Hold the right hand with the thumb extended
+(Fig. 89) and with the fingers pointing in the direction of
+the current, the palm being toward the needle and on
+the opposite side of the wire from the needle. The north-seeking
+pole will then be deflected in the direction in
+which the thumb points.</p>
+
+<p><b><a id="par_93"></a>93. Current Detectors.</b> As there is a magnetic field
+about a wire when a current passes through it, and as the
+magnetic needle is affected, we have a means of detecting
+the presence of a current. When the current is strong it
+is simply necessary to let it pass once over or under a
+needle; when it is weak, the wire must pass several
+times above and below the needle, Fig. 91, to give the
+needle motion. (See "Apparatus Book," Chapter XIII.,
+for home-made detectors.)</p>
+
+<p><span class="pagenum"><a name="Page_73" id="Page_73">[73]</a></span></p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/i_073a.png" width="336" height="209" alt="drawing" />
+<div class="caption">Fig. 92.</div>
+</div>
+
+<p><b><a id="par_94"></a>94. Astatic Needles and Detectors.</b> By arranging
+two magnetized needles with their poles opposite each
+other, Fig. 92, an <i>astatic needle</i> is formed. The pointing-power
+is almost nothing, although their magnetic
+fields are retained. This combination
+is used to detect feeble
+currents. In the ordinary detector,
+the tendency of the needle
+to point to the N and S has to be
+overcome by the magnetic field
+about the coil before the needle
+can be moved; but in the <i>astatic detector</i> and <i>galvanoscope</i>
+this pointing-power is done away with. Fig. 93
+shows a simple <i>astatic galvanoscope</i>. Fig. 67 shows an
+astatic galvanometer for measuring weak currents.</p>
+
+<div class="figcenter" style="width: 352px;">
+<img src="images/i_073b.png" width="352" height="437" alt="drawing" />
+<div class="caption">Fig. 93.</div>
+</div>
+
+<p><b><a id="par_95"></a>95. Polarity of Coils.</b> When a current of electricity
+passes through a coil of wire, the
+coil acts very much like a magnet,
+although no iron enters into its
+construction. The coil becomes
+magnetized by the electric current,
+lines of force pass from it
+into the air, etc. Fig. 94 shows a
+coil connected to copper and zinc
+plates, so arranged with cork that
+the whole can float in a dish of
+dilute sulphuric acid. The current
+passes as shown by the
+arrows, and when the N pole of a magnet is brought
+near the right-hand end, there is a repulsion, showing
+that that end of the coil has a N pole.</p>
+
+<p><span class="pagenum"><a name="Page_74" id="Page_74">[74]</a></span></p>
+
+<p><i>Rule.</i> When you face the right-hand end of the coil,
+the current is seen to pass around it in an anti-clockwise
+direction; this produces a N pole. When the current
+passes in a clockwise direction a S pole is produced.</p>
+
+<div class="figcenter" style="width: 228px;">
+<img src="images/i_074.png" width="228" height="333" alt="drawing" />
+<div class="caption">Fig. 94.</div>
+</div>
+
+<p><b><a id="par_96"></a>96. Electromagnets.</b>
+A coil of wire has a stronger
+field than a straight wire
+carrying the same current,
+because each turn adds its
+field to the fields of the
+other turns. By having the
+central part of the coil
+made of iron, or by having
+the coil of insulated wire
+wound upon an iron <i>core</i>,
+the strength of the magnetic
+field of the coil is
+greatly increased.</p>
+
+<p>Lines of force do not
+pass as readily through air
+as through iron; in fact,
+lines of force will go out of their way to go through
+iron. With a coil of wire the lines of force pass from its
+N pole through the air on all sides of the coil to its S
+pole; they then pass through the inside of the coil and
+through the air back to the N pole. When the resistance
+to their passage through the coil is decreased by the
+core, the magnetic field is greatly strengthened, and we
+have an <i>electromagnet</i>.</p>
+
+<p>The coil of wire temporarily magnetizes the iron core;
+it can permanently magnetize a piece of steel used as<span class="pagenum"><a name="Page_75" id="Page_75">[75]</a></span>
+a core. (See "Study," Chapter XXII., for experiments.)</p>
+
+
+<div class="figcenter" style="width: 520px;">
+<img src="images/i_075.png" width="520" height="389" alt="drawing" />
+<div class="caption">Fig. 95.</div>
+</div>
+
+<p><b><a id="par_97"></a>97. Forms of Electromagnets.</b> Fig. 95 shows a
+<i>straight, or bar electromagnet</i>. Fig. 96 shows a simple
+form of <i>horseshoe electromagnet</i>. As this form is not easily
+wound, the coils are generally wound on two separate
+cores which are then joined by a <i>yoke</i>. The yoke
+merely takes the place of the curved part shown in Fig.
+96. In Fig. 97 is shown the ordinary form of horseshoe
+electromagnet used for all sorts of electrical instruments.
+(See "Apparatus Book," Chapter IX., for home-made
+electromagnets.)</p>
+
+<p><b><a id="par_98"></a>98. Yokes and Armatures.</b> In the horseshoe magnet
+there are two poles to attract and two to induce. The
+lines of force pass through the yoke on their way from
+one core to the other, instead of going through the air.<span class="pagenum"><a name="Page_76" id="Page_76">[76]</a></span>
+This reduces the resistance to them. If we had no yoke
+we should simply have two straight electromagnets, and
+the resistance to the lines of force would be so great that
+the total strength would be much reduced. Yokes are
+made of soft iron, as well as the cores and armature. The
+<i>armature</i>, as with permanent horseshoe magnets, is
+strongly drawn toward the poles. As soon as the current
+ceases to flow, the attraction also ceases.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 192px;">
+<img src="images/i_076a.jpg" width="192" height="215" alt="drawing" />
+<div class="caption">Fig. 96</div>
+</div></td><td align="left"><div class="figcenter" style="width: 272px;">
+<img src="images/i_076b.jpg" width="272" height="177" alt="drawings" />
+<div class="caption">Fig. 97.</div>
+</div></td></tr>
+</table></div>
+
+<div class="figcenter" style="width: 263px;">
+<img src="images/i_076c.png" width="263" height="98" alt="drawing" />
+<div class="caption">Fig. 98.</div>
+</div>
+
+<p>Beautiful magnetic figures can be made with horseshoe
+magnets. Fig. 98 shows that the coils must be joined so
+that the current can pass around the cores in opposite
+directions to make unlike poles. (See "Study," Exp.
+164 to 173.)</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_77" id="Page_77">[77]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XII"></a>CHAPTER XII.<br />
+<small>HOW ELECTRICITY IS GENERATED BY INDUCTION.</small></h2>
+
+
+<p><b><a id="par_99"></a>99. Electromagnetic Induction.</b> We have seen that
+a magnet has the power to act through space and induce
+another piece of iron or steel to become a magnet. A
+charge of static electricity can induce a charge upon
+another conductor. We have now to see how a <i>current</i>
+of electricity in one conductor can induce a current in
+another conductor, not in any way connected with the
+first, and how a magnet and a coil can generate a current.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 362px;">
+<img src="images/i_077a.png" width="362" height="187" alt="drawing" />
+<div class="caption">Fig. 99.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 109px;">
+<img src="images/i_077b.png" width="109" height="265" alt="drawing" />
+<div class="caption">Fig. 100.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_100"></a>100. Current from Magnet and Coil.</b> If a bar magnet,
+Fig. 99, be suddenly thrust into a hollow coil of
+wire, a momentary current of electricity will be generated
+in the coil. No current passes when the magnet and coil
+are still; at least one of them must be in motion. Such a
+current is said to be <i>induced</i>, and is an <i>inverse</i> one when<span class="pagenum"><a name="Page_78" id="Page_78">[78]</a></span>
+the magnet is inserted, and a <i>direct</i> one when the magnet
+is withdrawn from the coil.</p>
+
+<p><b><a id="par_101"></a>101. Induced Currents and Lines of Force.</b> Permanent
+magnets are constantly sending out thousands of
+lines of force. Fig. 100 shows a bar magnet entering a
+coil of wire; the number of lines of force is increasing,
+and the induced current passes in an anti-clockwise direction
+when looking down into the coil along the lines of
+force. This produces an indirect current. If an iron
+core be used in the coil, the induced current will be
+greatly strengthened.</p>
+
+<div class="figcenter" style="width: 538px;">
+<img src="images/i_078.jpg" width="538" height="315" alt="drawing" />
+<div class="caption">Fig. 101.</div>
+</div>
+
+<p>It takes force to move a magnet through the center of
+a coil, and it is this work that is the source of the induced
+current. We have, in this simple experiment, the key to
+the action of the dynamo and other electrical machines.</p>
+
+<p><b><a id="par_102"></a>102. Current from two Coils.</b> Fig. 101 shows two
+coils of wire, the smaller being connected to a cell, the
+larger to a galvanometer. By moving the small coil up<span class="pagenum"><a name="Page_79" id="Page_79">[79]</a></span>
+and down inside of the large one, induced currents are
+generated, first in one direction and then in the opposite.
+We have here two entirely separate circuits, in no way
+connected. The <i>primary</i> current comes from the cell,
+while the <i>secondary</i> current is an induced one. By placing
+a core in the small coil of Fig. 101, the induced current
+will be greatly strengthened.</p>
+
+<p>It is not necessary to have the two coils so that one or
+both of them can move. They may be wound on the
+same core, or otherwise arranged as in the induction coil.
+(See "Study," Chapter XXV., for experiments on
+induced currents.)</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_80" id="Page_80">[80]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XIII"></a>CHAPTER XIII.<br />
+<small>HOW THE INDUCTION COIL WORKS.</small></h2>
+
+
+<p><b><a id="par_103"></a>103. The Coils.</b> We saw, § 102, that an induced
+current was generated when a current-carrying coil, Fig.
+101, was thrust into another coil connected with a galvanometer.
+The galvanometer was used merely to show the
+presence of the current. The <i>primary coil</i> is the one
+connected with the cell; the other one is called the <i>secondary
+coil</i>.</p>
+
+<div class="figcenter" style="width: 459px;">
+<img src="images/i_080.jpg" width="459" height="289" alt="drawing" />
+<div class="caption">Fig. 102.</div>
+</div>
+
+<p>When a current suddenly begins to flow through a coil,
+the effect upon a neighboring coil is the same as that produced
+by suddenly bringing a magnet near it; and when
+the current stops, the opposite effect is produced. It is
+evident, then, that we can keep the small coil of Fig. 101
+with its core inside of the large coil, and generate induced
+currents by merely making and breaking the primary
+circuit.</p>
+
+<p><span class="pagenum"><a name="Page_81" id="Page_81">[81]</a></span></p>
+
+<p>We may consider that when the primary circuit is
+closed, the lines of force shoot out through the turns of
+the secondary coil just as they do when a magnet or a
+current-carrying coil is thrust into it. Upon opening the
+circuit, the lines of force cease to exist; that is, we may
+imagine them drawn in again.</p>
+
+<p><b><a id="par_104"></a>104. Construction.</b> Fig. 102 shows one form of
+home-made induction coil, given here merely to explain
+the action and connections. Nearly all induction coils
+have some form of automatic current interrupter, placed
+in the primary circuit, to rapidly turn the current off
+and on.</p>
+<div class="figleft" style="width: 202px;">
+<img src="images/i_081.png" width="202" height="153" alt="drawing" />
+<div class="caption">Fig. 103.</div>
+</div>
+<p><i>Details of Figs. 102 and 103.</i> Wires 5 and 6 are the
+ends of the primary coil, while
+wires 7 and 8 are the terminals of
+the secondary coil. The primary
+coil is wound on a bolt which
+serves as the core, and on this
+coil is wound the secondary which
+consists of many turns of fine wire.
+The wires from a battery should be joined to binding-posts
+W and X, and the handles, from which the shock is
+felt, to Y and Z. Fig. 103 shows the details of the interrupter.</p>
+
+
+
+<p>If the current from a cell enters at W, it will pass
+through the primary coil and out at X, after going
+through 5, R, F, S I, B, E and C. The instant the
+current passes, the bolt becomes magnetized; this attracts
+A, which pulls B away from the end of S I, thus automatically
+opening the circuit. B at once springs back to
+its former position against SI, as A is no longer attracted;<span class="pagenum"><a name="Page_82" id="Page_82">[82]</a></span>
+the circuit being closed, the operation is rapidly
+repeated.</p>
+
+<p>A <i>condenser</i> is usually connected to commercial forms.
+It is placed under the wood-work and decreases sparking
+at the interrupter. (See "Apparatus Book," Chapter
+XI., for home-made induction coils.)</p>
+
+<div class="figcenter" style="width: 384px;">
+<img src="images/i_082.jpg" width="384" height="241" alt="drawing" />
+<div class="caption">Fig. 104.</div>
+</div>
+
+<p>Fig. 104 shows one form of coil. The battery wires
+are joined to the binding-posts at the left. The secondary
+coil ends in two rods, and the spark jumps from one to
+the other. The interrupter and a switch are shown at
+the left.</p>
+
+<p>Fig. 105 shows a small coil for medical purposes. A
+dry cell is placed under the coil and all is included in
+a neat box. The handles form the terminals of the
+secondary coil.</p>
+
+<p><b><a id="par_105"></a>105. The Currents.</b> It should be noted that the
+current from the cell does not get into the secondary coil.
+The coils are thoroughly insulated from each other. The
+secondary current is an induced one, its voltage depending
+upon the relative number of turns of wire there are<span class="pagenum"><a name="Page_83" id="Page_83">[83]</a></span>
+in the two coils. (See Transformers.) The secondary
+current is an alternating one; that is, it flows in one
+direction for an instant and then immediately reverses its
+direction. The rapidity of the alternations depends upon
+the speed of the interrupter. Coils are made that give a
+secondary current with an enormous voltage; so high, in
+fact, that the spark will pass many inches, and otherwise
+act like those produced by static electric machines.</p>
+
+<div class="figcenter" style="width: 328px;">
+<img src="images/i_083.jpg" width="328" height="267" alt="drawing" />
+<div class="caption">Fig. 105.</div>
+</div>
+
+<p><b><a id="par_106"></a>106. Uses of Induction Coils.</b> Gas-jets can be
+lighted at a distance with the spark from a coil, by extending
+wires from the secondary coil to the jet. Powder
+can be fired at a distance, and other things performed,
+when a high voltage current is needed. Its use in medicine
+has been noted. It is largely used in telephone work.
+Of late, great use has been made of the secondary current
+in experiments with vacuum-tubes, X-ray work, etc.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_84" id="Page_84">[84]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XIV"></a>CHAPTER XIV.<br />
+<small>THE ELECTRIC TELEGRAPH, AND HOW IT SENDS
+MESSAGES.</small></h2>
+
+
+<p><b><a id="par_107"></a>107. The Complete Telegraph Line</b> consists of
+several instruments, switches, etc., etc., but its essential
+parts are: The <i>Line</i>, or wire, which connects the different
+stations; the <i>Transmitter</i> or <i>Key</i>; the <i>Receiver</i> or
+<i>Sounder</i>, and the <i>Battery</i> or <i>Dynamo</i>.</p>
+
+<p><b><a id="par_108"></a>108. The Line</b> is made of strong copper, iron, or soft
+steel wire. To keep the current in the line it is insulated,
+generally upon poles, by glass insulators. For
+very short lines two wires
+can be used, the line wire
+and the return; but for long
+lines the earth is used as a
+return, a wire from each
+end being joined to large metal plates sunk in the earth.</p>
+
+<div class="figcenter" style="width: 269px;">
+<img src="images/i_084.jpg" width="269" height="91" alt="drawing" />
+<div class="caption">Fig. 106.</div>
+</div>
+
+<p><b><a id="par_109"></a>109. Telegraph Keys</b> are merely instruments by
+which the circuit can be conveniently and rapidly opened
+or closed at the will of the operator. An ordinary push-button
+may be used to turn the current off and on, but it
+is not so convenient as a key.</p>
+
+<p>Fig. 106 shows a side view of a simple key which can
+be put anywhere in the circuit, one end of the cut wire
+being attached to X and the other to Y. By moving the
+lever C up and down according to a previously arranged
+set of signals, a current will be allowed to pass to a distant<span class="pagenum"><a name="Page_85" id="Page_85">[85]</a></span>
+station. As X and Y are insulated from each other,
+the current can pass only when C presses against Y.</p>
+
+<p>Fig. 107 shows a regular key, with switch, which is
+used to allow the current to pass through the instrument
+when receiving a message.</p>
+
+<div class="figcenter" style="width: 432px;">
+<img src="images/i_085a.jpg" width="432" height="167" alt="drawing" />
+<div class="caption">Fig. 107.</div>
+</div>
+
+<p><b><a id="par_110"></a>110. Telegraph Sounders</b> receive the current from
+some distant station, and with its electromagnet produce
+sounds that can be translated into messages.</p>
+
+<div class="figcenter" style="width: 491px;">
+<img src="images/i_085b.jpg" width="491" height="119" alt="drawing" />
+<div class="caption">Fig. 108.</div>
+</div>
+
+<p>Fig. 108 shows simply an electromagnet H, the coil
+being connected in series with a key K and a cell D C.
+The key and D C are shown by a top view. The lever
+of K does not touch the other metal strap until it is
+pressed down. A little above the core of H is held a
+strip of iron, on armature I. As soon as the circuit is
+closed at K, the current rushes through the circuit, and
+the core attracts I making a distinct <i>click</i>. As soon as
+K is raised, I springs away from the core, if it has been<span class="pagenum"><a name="Page_86" id="Page_86">[86]</a></span>
+properly held. In regular instruments a click is also
+made when the armature springs back again.</p>
+
+<p>The time between the two clicks can be short or long,
+to represent <i>dots</i> or <i>dashes</i>,
+which, together with <i>spaces</i>,
+represent letters. (For
+Telegraph Alphabet and
+complete directions for
+home-made keys, sounders,
+etc., see "Apparatus
+Book," Chapter XIV.)</p>
+
+<div class="figcenter" style="width: 275px;">
+<img src="images/i_086a.jpg" width="275" height="225" alt="" />
+<div class="caption">Fig. 109.</div>
+</div>
+
+<div class="figcenter" style="width: 501px;">
+<img src="images/i_086b.jpg" width="501" height="331" alt="drawing" />
+<div class="caption">Fig. 110.</div>
+</div>
+
+<p>Fig. 109 shows a form of
+home-made sounder. Fig.
+110 shows one form of telegraph sounder. Over the poles
+of the horseshoe electromagnet is an armature fixed to a
+metal bar that can rock up and down. The instant the
+current passes through the coils the armature comes
+down until a stop-screw strikes firmly upon the metal
+frame, making the down click. As soon as the distant<span class="pagenum"><a name="Page_87" id="Page_87">[87]</a></span>
+key is raised, the armature is firmly pulled back and
+another click is made. The two clicks differ in sound,
+and can be readily recognized by the operator.</p>
+
+<p><b><a id="par_111"></a>111. Connections for Simple Line.</b> Fig. 111 shows
+complete connections for a home-made telegraph line.
+The capital letters are used for the right side, R, and
+small letters for the left side, L. Gravity cells, B and b,
+are used. The <i>sounders</i>, S and s, and the <i>keys</i>, K and k,
+are shown by a top view. The broad black lines of S and
+s represent the armatures which are directly over the
+electromagnets. The keys have switches, E and e.</p>
+
+<p>The two stations, R and L, may be in the same room,
+or in different houses.
+The <i>return wire</i>, R W,
+passes from the copper
+of b to the zinc of B.
+This is important, as the
+cells must help each
+other; that is, they are
+in series. The <i>line wire</i>,
+L W, passes from one
+station to the other, and the return may be through the
+wire, R W, or through the earth; but for short lines a
+wire is best.</p>
+
+<div class="figcenter" style="width: 323px;">
+<img src="images/i_087.jpg" width="323" height="206" alt="drawing" />
+<div class="caption">Fig. 111.</div>
+</div>
+
+<p><b><a id="par_112"></a>112. Operation of Simple Line.</b> Suppose two boys,
+R (right) and L (left) have a line. Fig. 111 shows that
+R's switch, E, is open, while e is closed. The entire
+circuit, then, is broken at but one point. As soon as R
+presses his key, the circuit is closed, and the current from
+both cells rushes around from B, through K, S, L W, s,
+k, b, R W, and back to B. This makes the armatures of<span class="pagenum"><a name="Page_88" id="Page_88">[88]</a></span>
+S and s come down with a click at the same time. As
+soon as the key is raised, the armatures lift and make
+the up-click. As soon as R has finished, he closes his
+switch E. As the armatures are then held down, L
+knows that R has finished, so he opens his switch e, and
+answers R. Both E and e are closed when the line is not
+in use, so that either can open his switch at any time and
+call up the other. Closed circuit cells must be used for
+such lines. On very large lines dynamos are used to
+furnish the current.</p>
+
+<p><b><a id="par_113"></a>113. The Relay.</b> Owing to the large resistance of
+long telegraph lines, the current is weak when it reaches
+a distant station, and not strong enough to work an
+ordinary sounder. To get around this, relays are used;
+these are very delicate instruments that replace the
+sounder in the line wire circuit. Their coils are usually
+wound with many turns of fine wire, so that a feeble
+current will move its nicely adjusted armature. The
+relay armature merely acts as an automatic key to open
+and close a local circuit which includes a battery and
+sounder. The line current does not enter the sounder; it
+passes back from the relay to the sending station through
+the earth.</p>
+
+<div class="figcenter" style="width: 564px;">
+<img src="images/i_088.jpg" width="564" height="95" alt="drawing" />
+<div class="caption">Fig. 112.</div>
+</div>
+
+<p>Fig. 112 gives an idea of simple relay connections.
+The key K, and cell D C, represent a distant sending
+station. E is the electromagnet of the relay, and R A is<span class="pagenum"><a name="Page_89" id="Page_89">[89]</a></span>
+its armature. L W and R W represent the line and
+return wires. R A will vibrate toward E every time K
+is pressed, and close the local circuit, which includes a
+local battery, L B, and a sounder. It is evident that as
+soon as K is pressed the sounder will work with a good
+strong click, as the local battery can be made as strong
+as desired.</p>
+
+<p>Fig. 113 shows a regular instrument which opens and
+closes the local circuit at the top of the armature.</p>
+
+<div class="figcenter" style="width: 403px;">
+<img src="images/i_089.jpg" width="403" height="235" alt="drawing" />
+<div class="caption">Fig. 113.</div>
+</div>
+
+<p><b><a id="par_114"></a>114. Ink Writing Registers</b> are frequently used
+instead of sounders. Fig. 114 shows a writing register
+that starts itself promptly at the opening of the circuit,
+and stops automatically as soon as the circuit returns to
+its normal condition. A strip of narrow paper is slowly
+pulled from the reel by the machine, a mark being made
+upon it every time the armature of an inclosed electromagnet
+is attracted. When the circuit is simply closed
+for an instant, a short line, representing a <i>dot</i>, is made.</p>
+
+<p>Registers are built both single pen and double pen.
+In the latter case, as the record of one wire is made with<span class="pagenum"><a name="Page_90" id="Page_90">[90]</a></span>
+a fine pen, and the other with a coarse pen, they can
+always be identified. The record being blocked out upon
+white tape in solid black color, in a series of clean-cut
+dots and dashes, it can be read at a glance, and as it is
+indelible, it may be read years afterward. Registers are
+made for local circuits, for use in connection with relays,
+or for direct use on main lines, as is usually desirable in
+fire-alarm circuits.</p>
+
+<div class="figcenter" style="width: 508px;">
+<img src="images/i_090.jpg" width="508" height="423" alt="drawing" />
+<div class="caption">Fig. 114.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_91" id="Page_91">[91]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XV"></a>CHAPTER XV.<br />
+<small>THE ELECTRIC BELL AND SOME OF ITS USES.</small></h2>
+
+
+<p><b><a id="par_115"></a>115. Automatic Current Interrupters</b> are used on
+most common bells, as well as on induction coils, etc.
+(See § 104.) Fig. 115
+shows a simple form
+of interrupter. The
+wire 1, from a cell D
+C, is joined to an iron
+strip I a short distance
+from its end. The
+other wire from D C passes to one end of the electromagnet
+coil H. The remaining end of H is placed in
+contact with I as shown, completing the circuit. As soon
+as the current passes, I is pulled down and away from
+the upper wire 2, breaking the circuit. I, being held by
+its left-hand end firmly in the hand, immediately springs
+back to its former position, closing the circuit again.
+This action is repeated, the rapidity of the vibrations
+depending somewhat upon the position of the wires on I.
+In regular instruments a platinum point is used where<span class="pagenum"><a name="Page_92" id="Page_92">[92]</a></span>
+the circuit is broken; this stands
+the sparking when the armature
+vibrates.</p>
+
+<div class="figcenter" style="width: 346px;">
+<img src="images/i_091a.jpg" width="346" height="151" alt="drawing" />
+<div class="caption">Fig. 115.</div>
+</div>
+
+<div class="figcenter" style="width: 528px;">
+<img src="images/i_091b.jpg" width="528" height="143" alt="drawing" />
+<div class="caption">Fig. 116.</div>
+</div>
+
+<p><b><a id="par_116"></a>116. Electric Bells</b> may be
+illustrated by referring to Fig.
+116, which shows a circuit similar
+to that described in § 115,
+but which also contains a key
+K, in the circuit. This allows
+the circuit to be opened and
+closed at a distance from the
+vibrating armature. The circuit
+must not be broken at two
+places at the same time, so wires
+should touch at the end of I before
+pressing K. Upon pressing K the armature I will
+vibrate rapidly. By placing a small bell near the end of
+the vibrating armature, so
+that it will be struck by I at
+each vibration, we should
+have a simple electric bell.
+This form of electric bell is
+called a <i>trembling</i> bell, on
+account of its vibrating armature.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 248px;">
+<img src="images/i_092a.jpg" width="248" height="401" alt="drawing" />
+<div class="caption">Fig. 117.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 286px;">
+<img src="images/i_092b.jpg" width="286" height="303" alt="drawing" />
+<div class="caption">Fig. 118.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p>Fig. 117 shows a form of
+trembling bell with cover
+removed. Fig. 118 shows a
+<i>single-stroke</i> bell, used for
+fire-alarms and other signal work. In this the armature
+is attracted but once each time the current passes. As<span class="pagenum"><a name="Page_93" id="Page_93">[93]</a></span>
+many taps of the bell can be
+given as desired by pressing
+the push-button. Fig. 119
+shows a gong for railway
+crossings, signals, etc. Fig.
+120 shows a circuit including
+cell, push-button, and bell,
+with extra wire for lengthening
+the line.</p>
+
+<div class="figcenter" style="width: 260px;">
+<img src="images/i_093a.jpg" width="260" height="399" alt="drawing" />
+<div class="caption">Fig. 119.</div>
+</div>
+
+<p><i>Electro-Mechanical Gongs</i> are
+used to give loud signals for
+special purposes. The mechanical
+device is started by
+the electric current when the
+armature of the electromagnet
+is attracted. Springs, weights, etc., are used as the
+power. Fig. 121 shows a small bell of this kind.</p>
+
+<div class="figcenter" style="width: 218px;">
+<img src="images/i_093b.jpg" width="218" height="316" alt="drawing" />
+<div class="caption">Fig. 120.</div>
+</div>
+
+<p><b><a id="par_117"></a>117. Magneto Testing Bells</b>,
+Fig. 122, are really small hand-power
+dynamos. The armature is
+made to revolve between the poles
+of strong permanent magnets, and
+it is so wound that it gives a current
+with a large E. M. F., so that
+it can ring through the large resistance
+of a long line to test it.</p>
+
+<p><i>Magneto Signal Bells</i>, Fig. 123,
+are used as generator and bell in
+connection with telephones. The
+generator, used to ring a bell at a
+distant station, stands at the bottom of the box. The<span class="pagenum"><a name="Page_94" id="Page_94">[94]</a></span>
+bell is fastened to the lid, and receives current from a
+distant bell.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 166px;">
+<img src="images/i_094a.jpg" width="166" height="329" alt="drawing" />
+<div class="caption">Fig. 121.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 308px;">
+<img src="images/i_094b.jpg" width="308" height="315" alt="drawing" />
+<div class="caption">Fig. 122.</div>
+</div></td></tr>
+<tr><td align="left"><div class="figcenter" style="width: 317px;">
+<img src="images/i_094c.jpg" width="317" height="325" alt="drawing" />
+<div class="caption">Fig. 123.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 242px;">
+<img src="images/i_094d.jpg" width="242" height="293" alt="drawing" />
+<div class="caption">Fig. 124.</div>
+</div></td></tr>
+</table></div>
+
+
+<p><b><a id="par_118"></a>118. Electric Buzzers</b> have the same general construction
+as electric bells; in fact, you will have a buzzer
+by removing the bell from an ordinary electric bell.
+Buzzers are used in places where the loud sound of a bell
+would be objectionable. Fig. 124 shows the usual form
+of buzzers, the cover being removed.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_95" id="Page_95">[95]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XVI"></a>CHAPTER XVI.<br />
+<small>THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.</small></h2>
+
+
+<p><b><a id="par_119"></a>119. The Telephone</b> is an instrument for reproducing
+sounds at a distance, and electricity is the agent by
+which this is generally accomplished. The part spoken
+to is called the <i>transmitter</i>, and the part which gives
+sound out again is called the <i>receiver</i>. Sound itself does
+not pass over the line. While the same apparatus can be
+used for both transmitter and receiver, they are generally
+different in construction to get the best results.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 108px;">
+<img src="images/i_095.jpg" width="108" height="195" alt="drawing" />
+<div class="caption">Fig. 125.</div>
+</div>
+</td><td align="left"><div class="figcenter" style="width: 166px;">
+<img src="images/i_095b.jpg" width="166" height="120" alt="drawing" />
+<div class="caption">Fig. 126.</div>
+</div></td></tr>
+</table></div>
+
+<div class="figcenter" style="width: 397px;">
+<img src="images/i_095c.jpg" width="397" height="182" alt="drawing" />
+<div class="caption">Fig. 127.</div>
+</div>
+
+<p><b><a id="par_120"></a>120. The Bell or Magneto-transmitter</b> generates
+its own current, and is, strictly speaking, a dynamo that<span class="pagenum"><a name="Page_96" id="Page_96">[96]</a></span>
+is run by the voice. It depends upon induction for its
+action.</p>
+
+<div class="figcenter" style="width: 542px;">
+<img src="images/i_096a.jpg" width="542" height="133" alt="drawing" />
+<div class="caption">Fig. 128.</div>
+</div>
+
+
+<div class="figright" style="width: 202px;">
+<img src="images/i_096b.jpg" width="202" height="205" alt="drawing" />
+<div class="caption">Fig. 129.</div>
+</div>
+
+<p>Fig. 125 shows a coil of wire, H, with soft iron core,
+the ends of the wires being connected to a delicate galvanoscope.
+If one pole of the magnet H M be suddenly
+moved up and down near the core, an alternating current
+will be generated in the coil, the circuit being completed
+through the galvanoscope. As H M approaches the core
+the current will flow in one direction, and as H M is
+withdrawn it will pass in the opposite direction. The
+combination makes a miniature alternating dynamo.</p>
+
+
+<p>If we imagine the soft iron core of H, Fig. 125, taken
+out, and one pole of H M, or preferably that of a bar
+magnet stuck through the coil, a feeble current will also
+be produced by moving the soft iron
+back and forth near the magnet's pole.
+This is really what is done in the
+Bell transmitter, soft iron in the shape
+of a thin disc (D, Fig. 126) being
+made to vibrate by the voice immediately
+in front of a coil having a permanent
+magnet for a core. The disc,
+or <i>diaphragm</i>, as it is called, is fixed
+near, but it does not touch, the magnet. It is under a
+constant strain, being attracted by the magnet, so its<span class="pagenum"><a name="Page_97" id="Page_97">[97]</a></span>
+slightest movement changes the strength of the magnetic
+field, causing more or less lines of force to shoot through
+the turns of the coil and induce a current. The coil consists
+of many turns of fine, insulated wire. The current
+generated is an alternating one, and although exceedingly
+small can force its way through a long length of wire.</p>
+
+<div class="figcenter" style="width: 538px;">
+<img src="images/i_097a.jpg" width="538" height="84" alt="drawing" />
+<div class="caption">Fig. 130.</div>
+</div>
+
+<p>Fig. 127 shows a section of a regular transmitter, and
+Fig. 128 a form of compound magnet frequently used in
+the transmitter.
+Fig. 129 shows
+a transmitter
+with cords which
+contain flexible
+wires.</p>
+
+<div class="figcenter" style="width: 418px;">
+<img src="images/i_097b.jpg" width="418" height="354" alt="drawing" />
+<div class="caption">Fig. 131.</div>
+</div>
+
+<p><b><a id="par_121"></a>121. The Receiver</b>,
+for short
+lines, may have
+the same construction
+as the
+Bell transmitter.
+Fig. 130 shows
+a diagram of two
+Bell receivers, either being used as the transmitter and
+the other as the receiver. As the alternating current
+goes to the distant receiver, it flies through the coil
+first in one direction and then in the other. This alternately<span class="pagenum"><a name="Page_98" id="Page_98">[98]</a></span>
+strengthens and weakens the magnetic field
+near the diaphragm, causing it to vibrate back and forth
+as the magnet pulls more or less. The receiver diaphragm
+repeats the vibrations in the transmitter.
+Nothing but the induced electric current passes over the
+wires.</p>
+
+<div class="figcenter" style="width: 538px;">
+<img src="images/i_098a.jpg" width="538" height="82" alt="drawing" />
+<div class="caption">Fig. 132.</div>
+</div>
+
+<p><b><a id="par_122"></a>122. The Microphone.</b> If a current of electricity be
+allowed to pass through a circuit like that shown in Fig.
+131, which includes a battery, a Bell receiver, and a
+microphone, any slight sound near the microphone will
+be greatly magnified in the receiver. The microphone
+consists of pieces of carbon so fixed that they form loose
+contacts. Any slight movement of the carbon causes the
+resistance to the current to be greatly changed. The
+rapidly varying resistance allows more or less current to
+pass, the result being that this pulsating current causes
+the diaphragm to vibrate. The diaphragm has a constantly
+varying pull upon it when the carbons are in any
+way disturbed by the voice, or by the ticking of a watch,
+etc. This principle has been made use of in carbon
+transmitters, which are made in a large variety of forms.</p>
+
+<div class="figcenter" style="width: 537px;">
+<img src="images/i_098b.jpg" width="537" height="83" alt="drawing" />
+<div class="caption">Fig. 133.</div>
+</div>
+
+<p><b><a id="par_123"></a>123. The Carbon Transmitter</b> does not, in itself,<span class="pagenum"><a name="Page_99" id="Page_99">[99]</a></span>
+generate a current like the magneto-transmitter; it merely
+produces changes in the strength of a current that flows
+through it and that comes from
+some outside source. In Fig.
+132, X and Y are two carbon
+buttons, X being attached to
+the diaphragm D. Button Y
+presses gently against X, allowing
+a little current to pass
+through the circuit which includes
+a battery, D C, and a receiver,
+R. When D is caused to
+vibrate by the voice, X is made
+to press more or less against Y,
+and this allows more or less
+current to pass through the circuit.
+This direct undulating
+current changes the pull upon
+the diaphragm of R, causing it to vibrate and reproduce
+the original sounds spoken into the transmitter. In
+regular lines, of course, a receiver and transmitter are
+connected at each end, together with bells, etc., for
+signaling.</p>
+
+<div class="figcenter" style="width: 250px;">
+<img src="images/i_099.jpg" width="250" height="427" alt="drawing" />
+<div class="caption">Fig. 134.</div>
+</div>
+
+<p><b><a id="par_124"></a>124. Induction Coils in Telephone Work.</b> As the
+resistance of long telephone lines is great, a high electrical
+pressure, or E.M.F. is desired. While the current
+from one or two cells is sufficient to work the transmitter
+properly, and cause undulating currents in the short line,
+it does not have power enough to force its way over a
+long line.</p>
+
+<p>To get around this difficulty, an induction coil, Fig. 133,<span class="pagenum"><a name="Page_100" id="Page_100">[100]</a></span>
+is used to transform the battery
+current, that flows through the
+carbon transmitter and primary
+coil, into a current with a high
+E. M. F. The battery current
+in the primary coil is undulating,
+but always passes in the
+same direction, making the
+magnetic field around the core
+weaker and stronger. This
+causes an alternating current
+in the secondary coil and main
+line. In Fig. 133 P and S represent
+the primary and secondary
+coils. P is joined in series
+with a cell and carbon transmitter;
+S is joined to the distant
+receiver. One end of S can be
+grounded, the current completing
+the circuit through the earth
+and into the receiver through another
+wire entering the earth.</p>
+
+<div class="figcenter" style="width: 251px;">
+<img src="images/i_100a.jpg" width="251" height="568" alt="drawing" />
+<div class="caption">Fig. 135.</div>
+</div>
+
+<p><b><a id="par_125"></a>125. Various forms</b> of
+telephones are shown in Figs.
+134, 135, 136. Fig. 134
+shows a form of desk telephone;
+Fig. 135 shows a
+common form of wall telephone;
+Fig. 136 shows head-telephones
+for switchboard
+operators.</p>
+
+<div class="figcenter" style="width: 272px;">
+<img src="images/i_100b.jpg" width="272" height="264" alt="drawing" />
+<div class="caption">Fig. 136.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_101" id="Page_101">[101]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XVII"></a>CHAPTER XVII.<br />
+<small>HOW ELECTRICITY IS GENERATED BY DYNAMOS.</small></h2>
+
+
+<p><b><a id="par_126"></a>126. The Dynamo</b>, <i>Dynamo-Electric Machine</i> or <i>Generator</i>,
+is a machine for converting mechanical energy into
+an electric current, through electromagnetic induction.
+The dynamo is a machine that will convert steam power,
+for example, into an electric current. Strictly speaking,
+a dynamo creates electrical pressure, or electromotive
+force, and not electricity, just as a force-pump creates
+water-pressure, and not water. They are generally run
+by steam or water power.</p>
+
+<div class="figcenter" style="width: 357px;">
+<img src="images/i_101a.jpg" width="357" height="184" alt="drawing" />
+<div class="caption">Fig. 137.</div>
+</div>
+
+<p><b><a id="par_127"></a>127. Induced Currents.</b> We have already spoken
+about currents being induced by moving a coil of wire in
+a magnetic field. We shall now see how this principle
+is used in the dynamo which is a generator of induced
+currents.</p>
+
+<div class="figcenter" style="width: 102px;">
+<img src="images/i_101b.jpg" width="102" height="199" alt="drawing" />
+<div class="caption">Fig. 138.</div>
+</div>
+
+<p>Fig. 137 shows how a current can be generated by a
+bar magnet and a coil of wire. Fig. 138 shows how a
+current can be generated by a horseshoe magnet and a
+coil of wire having an iron core. The ends of the coil are<span class="pagenum"><a name="Page_102" id="Page_102">[102]</a></span>
+to be connected to an astatic galvanoscope; this forms a
+closed circuit. The coil may be moved past the magnet,
+or the magnet past the coil.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 182px;">
+<img src="images/i_102a.jpg" width="182" height="234" alt="drawing" />
+<div class="caption">Fig. 139.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 141px;">
+<img src="images/i_102b.jpg" width="141" height="174" alt="drawing" />
+<div class="caption">Fig. 140.</div>
+</div></td></tr>
+<tr><td align="left"><div class="figcenter" style="width: 169px;">
+<img src="images/i_102c.jpg" width="169" height="172" alt="drawing" />
+<div class="caption">Fig. 141.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 169px;">
+<img src="images/i_102d.jpg" width="169" height="173" alt="drawing" />
+<div class="caption">Fig. 142.</div>
+</div></td></tr>
+</table></div>
+
+<p>Fig. 139 shows how a current can be generated by two
+coils, H being connected to an astatic galvanoscope and
+E to a battery. By suddenly bringing E toward H or
+the core of E past that of H, a current is produced. We
+have in this arrangement the main features of a dynamo.
+We can reverse the operation, holding E in one position
+and moving H rapidly toward it. In this case H would
+represent the armature and E the field-magnet. When
+H is moved toward E, the induced current in H flows in
+one direction, and when H is suddenly withdrawn from<span class="pagenum"><a name="Page_103" id="Page_103">[103]</a></span>
+E the current is reversed in H. (See "Study," Chapter
+XXV., for experiments.)</p>
+
+<div class="figcenter" style="width: 294px;">
+<img src="images/i_103a.jpg" width="294" height="364" alt="drawing" />
+<div class="caption">Fig. 143.</div>
+</div>
+
+<p><b><a id="par_128"></a>128. Induced Currents
+by Rotary Motion.</b> The
+motions of the coils in
+straight lines are not suitable
+for producing currents
+strong enough for commercial
+purposes. In order
+to generate currents of
+considerable strength and
+pressure, the coils of wire
+have to be pushed past
+magnets, or electromagnets,
+with great speed. In the
+dynamo the coils are so wound that they can be given
+a rapid rotary motion as they fly past strong electromagnets.
+In this way the
+coil can keep on passing
+the same magnets, in the
+same direction, as long as
+force is applied to the shaft
+that carries them.</p>
+
+<div class="figcenter" style="width: 298px;">
+<img src="images/i_103b.jpg" width="298" height="364" alt="drawing" />
+<div class="caption">Fig. 144.</div>
+</div>
+
+<p><b><a id="par_129"></a>129. Field-Magnets;
+Armature; Commutator.</b>
+What we need then,
+to produce an induced
+current by a rotary motion,
+is a strong magnetic field,
+a rotating coil of wire
+properly placed in the<span class="pagenum"><a name="Page_104" id="Page_104">[104]</a></span>
+field, and some means of leading the current from the
+machine.</p>
+
+<div class="figcenter" style="width: 259px;">
+<img src="images/i_104a.jpg" width="259" height="303" alt="drawing" />
+<div class="caption">Fig. 145.</div>
+</div>
+
+<div class="figcenter" style="width: 351px;">
+<img src="images/i_104b.jpg" width="351" height="252" alt="drawing" />
+<div class="caption">Fig. 146.</div>
+</div>
+
+<p>If a loop of wire, Fig. 140, be so arranged on bearings
+at its ends that it can be made
+to revolve, a current will flow
+through it in one direction
+during one-half of the revolution,
+and in the opposite direction
+during the other half, it
+being insulated from all external
+conductors. This
+agrees with the experiments
+suggested in § 127, when the
+current generated in a coil
+passed in one direction during
+its motion <i>toward</i> the strongest part of the field, and
+in the opposite direction when the coil passed <i>out</i> of
+it. A coil must be cut by lines of force to generate a
+current. A current
+inside of the machine,
+as in Fig. 140, would
+be of no value; it must
+be led out to external
+conductors where it
+can do work. Some
+sort of sliding contact
+is necessary to connect
+a revolving conductor
+with outside stationary ones. The magnet, called the <i>field-magnet</i>,
+is merely to furnish lines of magnetic force. The
+one turn of wire represents the simplest form of <i>armature</i>.</p>
+
+<p><span class="pagenum"><a name="Page_105" id="Page_105">[105]</a></span></p>
+
+<p>Fig. 141 shows the ends of a coil joined to two rings, X,
+Y, insulated from each other, and rotating with the coil.
+The two stationary pieces of carbon, A, B, called <i>brushes</i>,
+press against the rings, and to these are joined wires,
+which complete the circuit, and which lead out where the
+current can do work. The arrows show the direction of
+the current during one-half of a revolution. The rings
+form a <i>collector</i>, and this arrangement gives an <i>alternating
+current</i>.</p>
+
+<div class="figcenter" style="width: 390px;">
+<img src="images/i_105.jpg" width="390" height="352" alt="drawing" />
+<div class="caption">Fig. 147.</div>
+</div>
+
+<p>In Fig. 142 the ends of the coil are joined to the two
+halves of a cylinder. These halves, X and Y, are insulated
+from each other, and from the axis. The current
+flows from X onto the brush A, through some external
+circuit, to do the work, and thence back through brush
+B onto Y. By the time that Y gets around to A, the
+direction of the current in the loop has reversed, so that
+it passes toward Y, but it still enters the outside circuit<span class="pagenum"><a name="Page_106" id="Page_106">[106]</a></span>
+through A, because Y is then in contact with A. This
+device is called a <i>commutator</i>, and it allows a constant or
+<i>direct current</i> to leave the machine.</p>
+
+<div class="figcenter" style="width: 434px;">
+<img src="images/i_106.jpg" width="434" height="359" alt="drawing" />
+<div class="caption">Fig. 148.</div>
+</div>
+
+<p>In regular machines, the field-magnets are electromagnets,
+the whole or a part of the current from the dynamo
+passing around them on its way out, to excite them and
+make a powerful field between the poles. To lessen the
+resistance to the lines of force on their way from the N to
+the S pole of the field-magnets, the armature coils are
+wound on an iron core; this greatly increases the strength
+of the field, as the lines of force have to jump across but
+two small air-gaps. There are many loops of wire on
+regular armatures, and many segments to the commutator,
+carefully insulated from each other, each getting its
+current from the coil attached to it.</p>
+
+<p><b><a id="par_130"></a>130. Types of Dynamos.</b> While there is an almost
+endless number of different makes and shapes of dynamos,<span class="pagenum"><a name="Page_107" id="Page_107">[107]</a></span>
+they may be divided into two great types; the <i>continuous</i>
+or <i>direct current</i>, and the <i>alternating current</i> dynamo.
+Direct current machines give out a current which constantly
+flows in one direction, and this is because a commutator
+is used. Alternating currents come from collectors
+or rings, as shown in Fig. 141; and as an alternating
+current cannot be used to excite the fields, an outside
+current from a small direct current machine must be
+used. These are called exciters.</p>
+
+<div class="figcenter" style="width: 339px;">
+<img src="images/i_107.jpg" width="339" height="246" alt="drawing" />
+<div class="caption">Fig. 149.</div>
+</div>
+
+<p>In direct current machines enough residual magnetism
+is left in the field to induce a slight current in the armature
+when the machine is started. This immediately
+adds strength to the field-magnets, which, in turn, induce
+a stronger current in the armature.</p>
+
+<p><b><a id="par_131"></a>131. Winding of Dynamos.</b> There are several ways
+of winding dynamos, depending upon the special uses to
+be made of the current.</p>
+
+<p>The <i>series wound</i> dynamo, Fig. 143, is so arranged that
+the entire current passes around the field-magnet cores
+on its way from the machine. In the <i>shunt wound</i> dynamo,
+Fig. 144, a part, only, of the current from the<span class="pagenum"><a name="Page_108" id="Page_108">[108]</a></span>
+machine is carried around the field-magnet cores through
+many turns of fine wire. The <i>compound wound</i> dynamo
+is really a combination of the two methods just given.
+In <i>separately-excited</i> dynamos, the current from a separate
+machine is used to excite the field-magnets.</p>
+
+<p><b><a id="par_132"></a>132. Various Machines.</b> Fig. 145 shows a hand
+power dynamo which produces a current for experimental
+work. Fig. 146 shows a magneto-electrical generator
+which produces a current for medical use. Figs. 147,
+148 show forms of dynamos, and Fig. 149 shows how arc
+lamps are connected in series to dynamos.</p>
+
+<div class="figcenter" style="width: 210px;">
+<img src="images/i_108.jpg" width="210" height="216" alt="drawing" />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_109" id="Page_109">[109]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XVIII"></a>CHAPTER XVIII.<br />
+<small>HOW THE ELECTRIC CURRENT IS TRANSFORMED.</small></h2>
+
+
+<p><b><a id="par_133"></a>133. Electric Current and Work.</b> The amount of
+work a current can do depends upon two factors; the
+strength (amperes), and the pressure, or E. M. F. (volts).
+A current of 10 amperes with a pressure of 1,000 volts =
+10 × 1,000 = 10,000 watts. This furnishes the same
+amount of energy as a current of 50 amperes at 200 volts;
+50 × 200 = 10,000 watts.</p>
+
+<p><b><a id="par_134"></a>134. Transmission of Currents.</b> It is often necessary
+to carry a current a long distance before it is used.
+A current of 50 amperes would need a copper conductor
+25 times as large (sectional area) as one to carry the 10
+ampere current mentioned in § 133. As copper conductors
+are very expensive, electric light companies, etc.,
+generally try to carry the current on as small a wire as
+possible. To do this, the voltage is kept high, and the
+amperage low. Thus, as seen in § 133, the current of
+1,000 volts and 10 amperes could be carried on a much
+smaller wire than the other current of equal energy. A
+current of 1,000 volts, however, is not adapted for lights,
+etc., so it has to be changed to lower voltage by some
+form of transformer before it can be used.</p>
+
+<p><b><a id="par_135"></a>135. Transformers</b>, like induction coils, are instruments
+for changing the E. M. F. and strength of currents.
+There is very little loss of energy in well-made
+transformers. They consist of two coils of wire on one<span class="pagenum"><a name="Page_110" id="Page_110">[110]</a></span>
+core; in fact, an induction coil may be considered a transformer,
+but in this a direct current has to be interrupted.
+If the secondary coil has 100 times as many turns of wire
+as the primary, a current of 100 volts can be taken from
+the secondary coil when the primary current is but 1
+volt; but the <i>strength</i> (amperes) of this new current will
+be but one-hundredth that of the primary current.</p>
+
+<p>By using the coil of fine wire as the primary, we can
+lower the voltage and increase the strength in the same
+proportion.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 176px;">
+<img src="images/i_110a.jpg" width="176" height="282" alt="drawing" />
+<div class="caption">Fig. 150.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 316px;">
+<img src="images/i_110b.jpg" width="316" height="292" alt="drawing" />
+<div class="caption">Fig. 151.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p>Fig. 150 shows about the simplest form of transformer
+with a solid iron core, on which are wound two coils, the
+one, P, being the primary, and the other, S, the secondary.
+Fig. 151 shows the general appearance of one make of
+transformer. The operation of this apparatus, as already
+mentioned, is to reduce the high pressure alternating
+current sent out over the conductors from the dynamo,
+to a potential at which it can be employed with convenience
+and safety, for illumination and other purposes.<span class="pagenum"><a name="Page_111" id="Page_111">[111]</a></span>
+They consist of two or more coils of wire most carefully
+insulated from one another. A core or magnetic circuit
+of soft iron, composed of very thin punchings, is then
+formed around these coils, the purpose of the iron core
+being to reduce the magnetic resistance and increase the
+inductive effect. One set of these coils is connected with
+the primary or high-pressure wires, while the other set,
+which are called the secondary coils, is connected to the
+house or low-pressure wires, or wherever the current is
+required for use. The rapidly alternating current impulses
+in the primary or high-pressure wires induce secondary
+currents similar in form but opposite in direction
+in the secondary coils. These current impulses are of a
+much lower pressure, depending upon the ratio of the
+number of turns of wire in the respective coils, it being
+customary to wind transformers in such a manner as to
+reduce from 1,000 or 2,000-volt primaries to 50 or 100-volt
+secondaries, at which voltage the secondary current is
+perfectly harmless.</p>
+
+<div class="figcenter" style="width: 373px;">
+<img src="images/i_111.jpg" width="373" height="249" alt="drawing" />
+<div class="caption">Fig. 152.</div>
+</div>
+
+<p><b><a id="par_136"></a>136. Motor-Dynamos.</b> Fig. 152. These consist<span class="pagenum"><a name="Page_112" id="Page_112">[112]</a></span>
+essentially of two belt-type machines on a common base,
+direct coupled together, one machine acting as a motor to
+receive current at a certain voltage, and the other acting
+as a dynamo to give out the current usually at a different
+voltage. As they transform current from one voltage to
+another, motor-dynamos are sometimes called Double
+Field Direct Current Transformers. The larger sizes
+have three bearings, one bearing being between the two
+machines, while the smaller sizes have but two bearings,
+the two armatures being fastened to a common
+spider.</p>
+
+<div class="figcenter" style="width: 379px;">
+<img src="images/i_112.jpg" width="379" height="293" alt="drawing" />
+<div class="caption">Fig. 153.</div>
+</div>
+
+<p><i>Applications.</i> The uses to which motor-dynamos are
+put are very various. They are extensively used in the
+larger sizes as "Boosters," for giving the necessary extra
+force on long electric supply circuits to carry the current
+to the end with the same pressure as that which reaches
+the ends of the shorter circuits from the station.</p>
+
+<p>Motor-dynamos have the advantage over dynamotors,
+described later, of having the secondary voltage easily<span class="pagenum"><a name="Page_113" id="Page_113">[113]</a></span>
+and economically varied over wide ranges by means of a
+regulator in the dynamo field.</p>
+
+<p><b><a id="par_137"></a>137. Dynamotors.</b> Fig. 153. In Dynamotors the
+motor and dynamo armatures are combined in one, thus
+requiring a single field only. The primary armature
+winding, which operates as a motor to drive the machine,
+and the secondary or dynamo winding, which operates as
+a generator to produce a new current, are upon the same
+armature core, so that the armature reaction of one winding
+neutralizes that of the other. They therefore have
+no tendency to spark, and do not require shifting of the
+brushes with varying load. Having but one field and
+two bearings, they are also more efficient than motor-dynamos.</p>
+
+<p><i>Applications.</i> They have largely displaced batteries for
+telegraph work. The size shown, occupying a space of
+about 8-inch cube, and having an output of 40 watts, will
+displace about 800 gravity cells, occupying a space of
+about 10 feet cube. The cost of maintenance of such a
+battery per year, exclusive of rent, is about $800, whereas
+the 1-6 dynamotor can be operated at an annual expense
+of $150.</p>
+
+<p>Dynamotors are largely used by telephone companies
+for charging storage batteries, and for transforming from
+direct to alternating current, for ringing telephone bells.
+Electro-cautery, electroplating, and electric heating also
+give use to dynamotors.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_114" id="Page_114">[114]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XIX"></a>CHAPTER XIX.<br />
+<small>HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR
+USE.</small></h2>
+
+
+<div class="figcenter" style="width: 371px;">
+<img src="images/i_114a.jpg" width="371" height="152" alt="drawing" />
+<div class="caption">Fig. 154.</div>
+</div>
+
+<div class="figcenter" style="width: 222px;">
+<img src="images/i_114b.jpg" width="222" height="131" alt="drawing" />
+<div class="caption">Fig. 155.</div>
+</div>
+
+<div class="figcenter" style="width: 525px;">
+<img src="images/i_114c.jpg" width="525" height="199" alt="drawing" />
+<div class="caption">Fig. 156.</div>
+</div>
+
+<p><b><a id="par_138"></a>138. Conductors and Insulators.</b> To carry the
+powerful current from the generating station to distant
+places where it is to give heat, power, or light, or even
+to carry the small current of a single cell from one
+room to another, <i>conductors</i> must be used. To keep the
+current from passing into the earth before it reaches its
+destination <i>insulators</i> must be used. The form of conductors
+and insulators used will depend upon the current and
+many other conditions. It should be remembered that
+the current has to be carried to the lamp or motor,<span class="pagenum"><a name="Page_115" id="Page_115">[115]</a></span>
+through which it passes, and then back again to the
+dynamo, to form a complete circuit. A break anywhere
+in the circuit stops the current. Insulators are as important
+as conductors.</p>
+
+<div class="figcenter" style="width: 543px;">
+<a href="images/i_115a-big.jpg"><img src="images/i_115a.jpg" width="543" height="404" alt="drawing" /></a>
+<div class="caption">Fig. 157.</div>
+</div>
+
+<div class="figcenter" style="width: 340px;">
+<img src="images/i_115b.jpg" width="340" height="159" alt="drawing" />
+<div class="caption">Fig. 158.</div>
+</div>
+
+<p><b><a id="par_139"></a>139. Mains, Service Wires, etc.</b> From the switchboard
+the current flows out through the streets in large
+conductors, or <i>mains</i>,
+the supply being kept
+up by the dynamos,
+just as water-pressure
+is kept up by the constant
+working of
+pumps. Branches,
+called <i>service wires</i>, are
+led off from the mains to supply houses or factories, one
+wire leading the current into the house from one main,<span class="pagenum"><a name="Page_116" id="Page_116">[116]</a></span>
+and a similar one leading it out of the house again to the
+other main.</p>
+
+<div class="figcenter" style="width: 331px;">
+<img src="images/i_116a.jpg" width="331" height="201" alt="drawing" />
+<div class="caption">Fig. 159.</div>
+</div>
+
+<div class="figcenter" style="width: 418px;">
+<img src="images/i_116b.jpg" width="418" height="254" alt="drawing" />
+<div class="caption">Fig. 160.</div>
+</div>
+
+<p>In large buildings, pairs of wires, called <i>risers</i>, branch
+out from the service
+wires and carry the current
+up through the
+building. These have
+still other branches&mdash;<i>floor
+mains</i>, <i>etc.</i>, that
+pass through halls, etc.,
+smaller branches finally
+reaching the lamps. The
+sizes of all of these wires depend upon how much current
+has to pass through them. The mains in large cities are
+usually placed underground. In some places they are
+carried on poles.</p>
+
+<div class="figcenter" style="width: 433px;">
+<img src="images/i_116c.jpg" width="433" height="99" alt="drawing" />
+<div class="caption">Fig. 161.</div>
+</div>
+
+<p><b><a id="par_140"></a>140. Electric Conduits</b> are underground passages for<span class="pagenum"><a name="Page_117" id="Page_117">[117]</a></span>
+electric wires, cables, etc. There are several ways of insulating
+the conductors. Sometimes they are placed in
+earthenware or iron
+tubes, or in wood
+that has been treated
+to make it water-proof.
+At short distances
+are placed
+man-holes, where the
+different lengths are joined, and where branches are
+attached.</p>
+
+<div class="figcenter" style="width: 368px;">
+<img src="images/i_117a.jpg" width="368" height="160" alt="drawing" />
+<div class="caption">Fig. 162.</div>
+</div>
+
+<p>Fig. 154 shows creosoted
+wooden pipes; Fig.
+155 shows another form of
+wooden pipe. Fig. 156
+shows a coupling-box used
+to join Edison tubes. The
+three wires, used in the
+three-wire system, are insulated
+from each other,
+the whole being surrounded
+by an iron pipe
+of convenient length for
+handling. Fig. 157
+shows sections of man-holes
+and various devices
+used in conduit work.</p>
+
+<div class="figcenter" style="width: 309px;">
+<img src="images/i_117b.jpg" width="309" height="505" alt="drawing" />
+<div class="caption">Fig. 163.</div>
+</div>
+
+<p><b><a id="par_141"></a>141. Miscellaneous
+Appliances.</b> When the
+current enters a house
+for incandescent lighting purposes, for example, quite a<span class="pagenum"><a name="Page_118" id="Page_118">[118]</a></span>
+number of things are necessary. To measure the current
+a meter is usually placed in the cellar. In new
+houses the insulated conductors are usually run through
+some sort of tube which
+acts as a double protection,
+all being hidden
+from view. Fig. 158
+shows a short length of
+iron tube with a lining
+of insulating material.
+Wires are often run
+through tubes made of
+rubber and various
+other insulating materials.</p>
+
+<p>Where the current is to be put into houses after the
+plastering has been done, the wires are usually run
+through <i>mouldings</i> or supported by <i>cleats</i>. Fig. 159
+shows a cross-section of moulding. The insulated wires
+are placed in the slots, which are then covered.</p>
+
+<div class="figcenter" style="width: 339px;">
+<img src="images/i_118a.jpg" width="339" height="225" alt="drawing" />
+<div class="caption">Fig. 164.</div>
+</div>
+
+<div class="figcenter" style="width: 226px;">
+<img src="images/i_118b.jpg" width="226" height="222" alt="drawing" />
+<div class="caption">Fig. 165.</div>
+</div>
+
+<div class="figcenter" style="width: 348px;">
+<img src="images/i_118c.jpg" width="348" height="95" alt="drawing" />
+<div class="caption">Fig. 166.</div>
+</div>
+
+<div class="figcenter" style="width: 352px;">
+<img src="images/i_118d.jpg" width="352" height="112" alt="drawing" />
+<div class="caption">Fig. 167.</div>
+</div>
+
+<p>Fig. 160 shows a form of porcelain cleat. These are
+fastened to ceilings or walls, and firmly hold the insulated
+wires in place. Fig. 161 shows a wood cleat. Fig. 162<span class="pagenum"><a name="Page_119" id="Page_119">[119]</a></span>
+shows small porcelain <i>insulators</i>. These may be screwed
+to walls, etc., the wire being then fastened to them. Fig.
+163 shows how telegraph wires are supported and
+insulated. Fig. 164 shows how wires may be carried by
+tree and insulated from
+them.</p>
+
+<div class="figcenter" style="width: 388px;">
+<img src="images/i_119a.jpg" width="388" height="297" alt="drawing" />
+<div class="caption">Fig. 168.</div>
+</div>
+
+<div class="figcenter" style="width: 286px;">
+<img src="images/i_119b.jpg" width="286" height="156" alt="drawing" />
+<div class="caption">Fig. 169.</div>
+</div>
+
+<div class="figcenter" style="width: 475px;">
+<img src="images/i_119c.jpg" width="475" height="86" alt="drawing" />
+<div class="caption">Fig. 170.</div>
+</div>
+
+<p><b><a id="par_142"></a>142. Safety Devices.</b>
+We have seen that when
+too large a current passes
+through a wire, the wire
+becomes heated and may
+even be melted. Buildings
+are wired to use certain currents, and if from any cause
+much more current than the regular amount should
+suddenly pass through the service wires into the house,<span class="pagenum"><a name="Page_120" id="Page_120">[120]</a></span>
+the various smaller wires would become overheated, and
+perhaps melt or start a fire. An accidental short circuit,
+for example, would so reduce resistance that too much
+current would suddenly rush through the wires. There
+are several devices by which the over-heating of wires
+is obviated.</p>
+
+<div class="figcenter" style="width: 507px;">
+<img src="images/i_120.jpg" width="507" height="309" alt="drawing" />
+<div class="caption">Figs. 171 to 175.</div>
+</div>
+
+<p>Fig. 165 shows a <i>safety fuse</i>, or <i>safety cut-out</i>, which
+consists of a short length of easily fusible wire, called
+<i>fuse wire</i>, placed in the circuit and supported by a porcelain
+block. These wires are tested, different sizes being
+used for different currents. As soon as there is any tendency
+toward over-heating, the fuse <i>blows</i>; that is, it
+promptly melts and opens the circuit before any damage
+can be done to the regular conductors. Fig. 166 shows
+a cross-section of a <i>fuse plug</i> that can be screwed into an
+ordinary socket. The fuse wire is shown black.</p>
+
+<p>Fig. 167 shows a <i>fuse link</i>. These are also of fusible
+material, and so made that they can be firmly held under
+screw-heads. For heavy currents <i>fuse ribbons</i> are used,<span class="pagenum"><a name="Page_121" id="Page_121">[121]</a></span>
+or several wires or links may be used side by side. Fig.
+168 shows a <i>fusible rosette</i>. Fig. 169 shows two fuse
+wires fixed between screw-heads, the current passing
+through them in opposite directions,
+both sides of the circuit being included.
+Fig. 170 shows various
+forms of cut-outs.</p>
+
+<div class="figcenter" style="width: 185px;">
+<img src="images/i_121a.jpg" width="185" height="181" alt="drawing" />
+<div class="caption">Fig. 176.</div>
+</div>
+
+<p><b><a id="par_143"></a>143. Wires and Cables</b> are made
+in many sizes. Figs. 171 to 175 show
+various ways of making small conductors.
+They are made very flexible,
+for some purposes, by twisting many small copper wires
+together, the whole being then covered with insulating
+material.</p>
+
+<div class="figcenter" style="width: 274px;">
+<img src="images/i_121b.jpg" width="274" height="270" alt="drawing" />
+<div class="caption">Fig. 177.</div>
+</div>
+
+<p>Figs. 176, 177, show sections of submarine cables.
+Such cables consist of copper conductors insulated with
+pure gutta-percha. These are then surrounded by hempen
+yarn or other elastic material,
+and around the whole are
+placed galvanized iron armor
+wires for protection. Each
+core, or conductor, contains a
+conductor consisting of a
+single copper wire or a strand
+of three or more twisted
+copper wires.</p>
+
+<p><b><a id="par_144"></a>144. Lamp Circuits.</b> As
+has been noted before, in
+order to have the electric current do its work, we must
+have a complete circuit. The current must be brought
+back to the dynamo, much of it, of course, having been<span class="pagenum"><a name="Page_122" id="Page_122">[122]</a></span>
+used to produce light, heat, power, etc. For lighting
+purposes this is accomplished in two principal ways.</p>
+
+<div class="figcenter" style="width: 420px;">
+<img src="images/i_122a.jpg" width="420" height="153" alt="drawing" />
+<div class="caption">Fig. 178.</div>
+</div>
+
+<p>Fig. 178 shows a number of lamps so arranged, "in
+series," that the same current passes through them all,
+one after the other. The total resistance of the circuit
+is large, as all of the lamp resistances are added together.</p>
+
+<div class="figcenter" style="width: 421px;">
+<img src="images/i_122b.jpg" width="421" height="160" alt="drawing" />
+<div class="caption">Fig. 179.</div>
+</div>
+
+<p>Fig. 179 shows lamps arranged side by side, or "in
+parallel," between the two main wires. The current
+divides, a part going through each lamp that operates.
+The total resistance of the circuit is not as large as in
+the series arrangement, as the current has many small
+paths in going from one main wire to the other. Fig. 179
+also shows the ordinary <i>two-wire system</i> for incandescent
+lighting, the two main wires having usually a difference
+of potential equal to 50 or 110 volts. These comparatively
+small pressures require fairly large conductors.</p>
+
+<p><i>The Three-Wire System</i>, Fig. 180, uses the current<span class="pagenum"><a name="Page_123" id="Page_123">[123]</a></span>
+from two dynamos, arranged with three main wires.
+While the total voltage is 220, one of the wires being
+neutral, 110 volts can be had for ordinary lamps. This
+voltage saves in the cost of conductors.</p>
+
+<div class="figcenter" style="width: 397px;">
+<img src="images/i_123a.jpg" width="397" height="183" alt="drawing" />
+<div class="caption">Fig. 180.</div>
+</div>
+
+<div class="figcenter" style="width: 458px;">
+<img src="images/i_123b.jpg" width="458" height="255" alt="drawing" />
+<div class="caption">Fig. 181.</div>
+</div>
+
+<p><i>The Alternating System</i>, Fig. 181, uses transformers.
+The high potential of the current allows small main wires,
+from which branches can be run to the primary coil of
+the transformer. The secondary coil sends out an induced
+current of 50 or 110 volts, while that in the primary
+may be 1,000 to 10,000 volts.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_124" id="Page_124">[124]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XX"></a>CHAPTER XX.<br />
+<small>HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.</small></h2>
+
+
+<p><b><a id="par_145"></a>145. Resistance and Heat.</b> We have seen that all
+wires and conductors offer resistance to the electric current.
+The smaller the wire the greater its resistance.
+Whenever resistance is offered to the current, heat is produced.
+By proper appliances, the heat of resistance can
+be used to advantage for many commercial enterprises.
+Dynamos are used to
+generate the current
+for heating and lighting
+purposes.</p>
+
+<div class="figleft" style="width: 321px;">
+<img src="images/i_124.jpg" width="321" height="219" alt="drawing" />
+<div class="caption">Fig. 182.</div>
+</div>
+
+<p>Fig. 182 shows how
+the current from two
+strong cells can be
+used to heat a short
+length of very fine
+platinum or German-silver wire. The copper conductors
+attached to the cells do not offer very much resistance.</p>
+
+<p>It will be seen from the above that in all electrical work
+the sizes of the wires used have to be such that they do
+not overheat. The coils of dynamos, motors, transformers,
+ampere-meters, etc., etc., become somewhat heated
+by the currents passing through them, great care being
+taken that they are properly designed and ventilated so
+that they will not burn out.</p>
+
+<div class="figcenter" style="width: 462px;">
+<img src="images/i_125a.jpg" width="462" height="442" alt="drawing" />
+<div class="caption">Fig. 183.</div>
+</div>
+
+<div class="figcenter" style="width: 241px;">
+<img src="images/i_125b.jpg" width="241" height="359" alt="drawing" />
+<div class="caption">Fig. 184.</div>
+</div>
+
+<p><b><a id="par_146"></a>146. Electric Welding.</b> Fig. 183 shows one form of
+electric welding machine. The principle involved in<span class="pagenum"><a name="Page_125" id="Page_125">[125]</a></span>
+the art of electric welding is that of causing currents of
+electricity to pass through the abutting ends of the pieces
+of metal which are to be welded,
+thereby generating heat at the
+point of contact, which also becomes
+the point of greatest resistance,
+while at the same time
+mechanical pressure is applied to
+force the parts together. As
+the current heats the metal at
+the junction to the welding
+temperature, the pressure follows
+up the softening surface until a
+complete union or weld is
+effected; and, as the heat is first<span class="pagenum"><a name="Page_126" id="Page_126">[126]</a></span>
+developed in the interior of the parts to be welded, the
+interior of the joint is as efficiently united as the visible
+exterior. With such a method and apparatus, it is
+found possible to accomplish not only the common kinds
+of welding of iron and steel, but also of metals which
+have heretofore resisted attempts at welding, and have
+had to be brazed or soldered.</p>
+
+<div class="figcenter" style="width: 555px;">
+<img src="images/i_126.jpg" width="555" height="582" alt="drawing" />
+<div class="caption">Figs. 185 to 189.</div>
+</div>
+
+<p>The introduction of the electric transformer enables
+enormous currents to be so applied to the weld as to spend
+their energy just at the point where heating is required.<span class="pagenum"><a name="Page_127" id="Page_127">[127]</a></span>
+They need, therefore, only to be applied for a few seconds,
+and the operation is completed before the heat generated
+at the weld has had time to escape by conduction to any
+other part.</p>
+
+<p>Although the quantity of the current so employed in
+the pieces to be welded is enormous, the potential at
+which it is applied is extremely low, not much exceeding
+that of the batteries of cells used for ringing electric bells
+in houses.</p>
+
+<div class="figcenter" style="width: 299px;">
+<img src="images/i_127.jpg" width="299" height="231" alt="drawing" />
+<div class="caption">Fig. 190.</div>
+</div>
+
+<p><b><a id="par_147"></a>147. Miscellaneous Applications.</b> Magneto Blasting
+Machines are now in very common use for blasting
+rocks, etc. Fig. 184 shows one, it being really a small
+hand dynamo, occupying less than one-half a cubic foot
+of space. The armature is made to revolve rapidly
+between the poles of the field-magnet by means of a handle
+that works up and down. The current is carried by
+wires from the binding-posts to fuses. The heat generated
+by resistance in the fuse ignites the powder or other
+explosive.</p>
+
+<p><i>Electric soldering irons</i>, <i>flat-irons</i>, <i>teakettles</i>, <i>griddles</i>,<span class="pagenum"><a name="Page_128" id="Page_128">[128]</a></span>
+<i>broilers</i>, <i>glue pots</i>, <i>chafing-dishes</i>, <i>stoves</i>, etc., etc., are now
+made. Figs. 185 to 189 show some of these applications.
+The coils for producing the resistance are inclosed in the
+apparatus.</p>
+
+<div class="figcenter" style="width: 320px;">
+<img src="images/i_128.jpg" width="320" height="227" alt="drawing" />
+<div class="caption">Fig. 191.</div>
+</div>
+
+<p>Fig. 190 shows a complete electric kitchen. Any kettle
+or part of the outfit can be made hot by simply turning
+a switch. Fig. 191 shows an electric heater placed
+under a car seat. Many large industries that make use
+of the heating effects of the current are now being
+carried on.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_129" id="Page_129">[129]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXI"></a>CHAPTER XXI.<br />
+<small>HOW LIGHT IS PRODUCED BY THE INCANDESCENT
+LAMP.</small></h2>
+
+
+<div class="figleft" style="width: 277px;">
+<img src="images/i_129a.jpg" width="277" height="628" alt="drawing" />
+<div class="caption">Fig. 192.</div>
+</div>
+
+<div class="figright" style="width: 169px;">
+<img src="images/i_129b.jpg" width="169" height="270" alt="drawing" />
+<div class="caption">Fig. 193.</div>
+</div>
+
+<p><b><a id="par_148"></a>148. Incandescence.</b> We have just seen that the
+electric current produces heat
+when it flows through a conductor
+that offers considerable
+resistance to it. As soon as
+this was discovered men began
+to experiment to find whether
+a practical light could also be
+produced. It was found that
+a wire could be kept hot by
+constantly passing a current
+through it, and that the light
+given out from it became
+whiter and whiter as the wire became hotter. The wire
+was said to be <i>incandescent</i>, or glowing with heat. As<span class="pagenum"><a name="Page_130" id="Page_130">[130]</a></span>
+metal wires are good conductors of electricity, they had
+to be made extremely fine to offer enough resistance;
+too fine, in fact, to be properly handled.</p>
+
+<p><b><a id="par_149"></a>149. The Incandescent Lamp.</b> Many substances
+were experimented upon to find a proper material out of
+which could be made a <i>filament</i> that would
+give the proper resistance and at the same
+time be strong and lasting. It was found
+that hair-like pieces of carbon offered the
+proper resistance to the current. When
+heated in the air, however, carbon burns; so
+it became necessary to
+place the carbon filaments
+in a globe from
+which all the air had
+been pumped before
+passing the current through them. This proved to be a
+success.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="lightbulbs">
+<tr><td align="left"><div class="figcenter" style="width: 125px;">
+<img src="images/i_130a.jpg" width="125" height="559" alt="drawing" />
+<div class="caption">Fig. 194.</div>
+</div>
+</td><td align="left" valign='bottom'><div class="figcenter" style="width: 107px;">
+<img src="images/i_130b.jpg" width="107" height="352" alt="drawing" />
+<div class="caption">Fig. 195.</div>
+</div></td><td align="left" valign='bottom'><div class="figcenter" style="width: 146px;">
+<img src="images/i_130c.jpg" width="146" height="176" alt="drawing" />
+<div class="caption">Fig. 196.</div>
+</div></td></tr>
+</table></div>
+
+<p>Fig. 192 shows the ordinary form of lamp. The <i>carbon
+filament</i> is attached, by carbon paste, to short platinum
+wires that are sealed in the glass, their lower ends being
+connected to short copper wires that are joined to the<span class="pagenum"><a name="Page_131" id="Page_131">[131]</a></span>
+terminals of the lamp. When the lamp is screwed into
+its socket, the current can pass up one side of the filament
+and down the other. The filaments used have been
+made of every form of carbonized vegetable matter.
+Bamboo has been largely used, fine strips being cut by
+dies and then heated in air-tight boxes containing fine
+carbon until they were thoroughly carbonized. This
+baking of the bamboo produces a tough fiber of carbon.
+Various forms of thread have been carbonized and used.
+Filaments are now made by pressing finely pulverized
+carbon, with a binding material, through small dies. The
+filaments are made of such sizes and lengths that will
+adapt them to the particular current with which they are
+to be used. The longer the filament, the greater its
+resistance, and the greater the voltage necessary to push
+the current through it.</p>
+
+<div class="figcenter" style="width: 463px;">
+<img src="images/i_131a.jpg" width="463" height="125" alt="drawing" />
+<div class="caption">Fig. 197.</div>
+</div>
+
+<div class="figcenter" style="width: 342px;">
+<img src="images/i_131b.jpg" width="342" height="86" alt="drawing" />
+<div class="caption">Fig. 198.</div>
+</div>
+
+<p>After the filaments are properly attached, the air is
+pumped from the bulb or globe. This is done with some
+form of mercury pump, and the air is so thoroughly removed
+from the bulb that about one-millionth only of the<span class="pagenum"><a name="Page_132" id="Page_132">[132]</a></span>
+original air remains. Before sealing off the lamp, a current
+is passed through the filament to drive out absorbed
+air and gases, and these are carried
+away by the pump. By proper treatment
+the filaments have a uniform
+resistance throughout, and glow uniformly
+when the current passes.</p>
+
+<div class="figcenter" style="width: 490px;">
+<img src="images/i_132a.jpg" width="490" height="288" alt="drawing" />
+<div class="caption">Fig. 199.</div>
+</div>
+
+<div class="figcenter" style="width: 188px;">
+<img src="images/i_132b.jpg" width="188" height="527" alt="drawing" />
+<div class="caption">Fig. 200.</div>
+</div>
+
+<p><b><a id="par_150"></a>150. Candle-Power.</b> A lamp is
+said to have 4, 8, 16 or more candle-power.
+A 16-candle-power lamp, for
+example, means one that will give as
+much light as sixteen standard
+candles. A standard sperm candle
+burns two grains a minute. The
+candle-power of a lamp can be increased
+by forcing a strong current
+through it, but this shortens its life.</p>
+
+<p><i>The Current</i> used for incandescent
+lamps has to be strong enough to
+force its way through the filament and
+produce a heat sufficient to give a<span class="pagenum"><a name="Page_133" id="Page_133">[133]</a></span>
+good light. The usual current has 50 or 110 volts,
+although small lamps are made that can be run by two
+or three cells. If the voltage of the current is less than
+that for which the lamp was made, the light will be dim.
+The filament can be instantly burned
+out by passing a current of too high
+pressure through it.</p>
+
+<p>Even with the proper current,
+lamps soon begin to deteriorate, as
+small particles of carbon leave the
+filament and cling to the glass.
+This is due to the evaporation,
+and it makes the filament smaller, and a higher pressure
+is then needed to force the current through the increased
+resistance; besides this, the darkened bulb does not properly
+let the light out. The current may be direct or
+alternating.</p>
+
+<div class="figcenter" style="width: 182px;">
+<img src="images/i_133a.jpg" width="182" height="194" alt="drawing" />
+<div class="caption">Fig. 201.</div>
+</div>
+
+<div class="figcenter" style="width: 529px;">
+<img src="images/i_133b.jpg" width="529" height="192" alt="drawing" />
+<div class="caption">Fig. 202.</div>
+</div>
+
+<p><b><a id="par_151"></a>151. The Uses</b> to which incandescent lamps are put
+are almost numberless. Fig. 193 shows a decorative
+lamp. Fancy lamps are made in all colors. Fig. 194
+shows a conic candle lamp, to imitate a candle. What
+corresponds to the body of the candle (see figure B to C)<span class="pagenum"><a name="Page_134" id="Page_134">[134]</a></span>
+is a delicately tinted opal glass tube surmounted (see
+figure A to B) by a finely proportioned conic lamp with
+frosted globe. C to D in the figure represents
+the regular base, and thus the relative
+proportions of the parts are shown. Fig.
+195 shows another form of candelabra lamp. Fig. 196
+shows small dental lamps. Fig. 197 shows a small lamp
+with mirror for use in the throat.
+Fig. 198 shows lamp with half
+shade attached, used for library
+tables. Fig. 199 shows an
+electric pendant for several
+lamps, with shade. Fig. 200
+shows a lamp guard. Fig. 201
+shows a lamp socket, into which
+the lamp is screwed. Fig. 202
+shows incandescent bulbs joined
+in parallel to the + and - mains.
+Fig. 203 shows how the lamp
+cord can be adjusted to desired
+length. Fig. 204 shows a lamp with reflector placed on
+a desk. Fig. 205 shows a form of shade and reflector.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 128px;">
+<img src="images/i_134a.jpg" width="128" height="321" alt="drawing" />
+<div class="caption">Fig. 203</div>
+</div></td><td align="left"><div class="figcenter" style="width: 378px;">
+<img src="images/i_134b.jpg" width="378" height="217" alt="drawing" />
+<div class="caption">Fig. 204.</div>
+</div></td></tr>
+</table></div>
+
+<div class="figcenter" style="width: 246px;">
+<img src="images/i_134c.jpg" width="246" height="357" alt="drawing" />
+<div class="caption">Fig. 205.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_135" id="Page_135">[135]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXII"></a>CHAPTER XXII.<br />
+<small>HOW LIGHT IS PRODUCED BY THE ARC LAMP.</small></h2>
+
+<div class="figright" style="width: 292px;">
+<img src="images/i_135.jpg" width="292" height="571" alt="drawing" />
+<div class="caption">Fig. 206.</div>
+</div>
+<p><b><a id="par_152"></a>152. The Electric Arc.</b> When a strong current
+passes from one carbon rod to another across an air-space,
+an <i>electric arc</i> is produced.
+When the ends of
+two carbon rods touch, a
+current can pass from one
+to the other, but the imperfect
+contact causes resistance
+enough to heat the
+ends red-hot. If the rods
+be separated slightly, the
+current will continue to
+flow, as the intensely heated
+air and flying particles of
+carbon reduce the resistance
+of the air-space.</p>
+
+<p>Fig. 206 shows two carbon
+rods which are joined
+to the two terminals of a
+dynamo. The upper, or
+positive, carbon gradually
+wears away and becomes
+slightly hollow. The
+heated <i>crater</i>, as it is called, is the hottest part. The
+negative carbon becomes pointed. The arc will pass in
+a vacuum, and even under water.</p>
+
+
+
+<p><span class="pagenum"><a name="Page_136" id="Page_136">[136]</a></span></p>
+
+<p>As the electric arc is extremely hot, metals are easily
+vaporized in it; in fact, even the carbon rods themselves
+slowly melt and vaporize. This extreme heat is used for
+many industrial purposes.</p>
+
+<p>"The phenomenon of the electric arc was first noticed by
+Humphrey Davy in 1800, and its explanation
+appears to be the following:
+Before contact the difference of potential
+between the points is insufficient
+to permit a spark to leap across even
+1/10000 of an inch of air-space, but when
+the carbons are made to touch, a current is established.
+On separating the carbons, the momentary extra current
+due to self-induction of the circuit, which possesses a high
+electromotive force, can leap the short distance, and in
+doing so volatilizes a small quantity of carbon between<span class="pagenum"><a name="Page_137" id="Page_137">[137]</a></span>
+the points. Carbon vapor, being a partial conductor,
+allows the current to continue to flow across the gap,
+provided it be not too wide; but as
+the carbon vapor has a very high resistance
+it becomes intensely heated
+by the passage of the current, and the
+carbon points also grow hot. Since,
+however, solid matter is a better
+radiator than gaseous matter, the
+carbon points emit far more light
+than the arc itself, though they are
+not so hot. It is observed, also, that
+particles of carbon are torn away
+from the + electrode, which becomes
+hollowed out to a cup-shape, and
+some of these are deposited on the - electrode."</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 180px;">
+<img src="images/i_136a.jpg" width="180" height="592" alt="drawing" />
+<div class="caption">Fig. 207.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 349px;">
+<img src="images/i_136b.jpg" width="349" height="405" alt="drawing" />
+<div class="caption">Fig. 208.</div>
+</div></td></tr>
+</table></div>
+
+<div class="figcenter" style="width: 184px;">
+<img src="images/i_137a.jpg" width="184" height="321" alt="drawing" />
+<div class="caption">Fig. 209.</div>
+</div>
+
+<p><b><a id="par_153"></a>153. Arc Lamps.</b> As the carbons gradually wear
+away, some device is necessary to keep their ends the
+right distance apart. If they
+are too near, the arc is very
+small; and if too far apart,
+the current can not pass and
+the light goes out. The
+positive carbon gives the more
+intense light and wears away
+about twice as fast as the -
+carbon, so it is placed above
+the - carbon, to throw the
+light downwards.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 130px;">
+<img src="images/i_137b.jpg" width="130" height="248" alt="drawing" />
+<div class="caption">Fig. 210.</div>
+</div>
+</td><td align="left"><div class="figcenter" style="width: 140px;">
+<img src="images/i_137c.jpg" width="140" height="254" alt="drawing" />
+<div class="caption">Fig. 211.</div>
+</div></td></tr>
+</table></div>
+
+
+
+<p>Arc lamps contain some device by which the proper
+distance between the carbons can be kept. Most of them<span class="pagenum"><a name="Page_138" id="Page_138">[138]</a></span>
+grip the upper carbon and pull
+it far enough above the lower
+one to establish the arc. As
+soon as the distance between
+them gets too great again, the
+grip on the upper carbon is
+loosened, allowing the carbon to
+drop until it comes in contact
+with the lower one, thus starting
+the current again. These
+motions are accomplished by
+electromagnets. Fig. 207 shows
+a form of arc lamp with <i>single carbons</i> that will burn
+from 7 to 9 hours.</p>
+
+<div class="figcenter" style="width: 248px;">
+<img src="images/i_138a.jpg" width="248" height="314" alt="drawing" />
+<div class="caption">Fig. 212.</div>
+</div>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 151px;">
+<img src="images/i_138b.jpg" width="151" height="486" alt="drawing" />
+<div class="caption">Fig. 213.</div>
+</div>
+</td><td align="left"><div class="figcenter" style="width: 332px;">
+<img src="images/i_138c.jpg" width="332" height="466" alt="drawing" />
+<div class="caption">Fig. 214.</div>
+</div></td></tr>
+</table></div>
+
+<p><span class="pagenum"><a name="Page_139" id="Page_139">[139]</a></span></p>
+
+<p>Fig. 208 shows the mechanism by which the carbons
+are regulated. Fig. 209 shows a form of <i>double carbon</i>,
+or <i>all-night</i> lamp, one set of carbons being first used, the
+other set being automatically switched in at the proper
+time.</p>
+
+<div class="figcenter" style="width: 591px;">
+<img src="images/i_139.jpg" width="591" height="492" alt="drawing ship with a searchlight " />
+<div class="caption">Fig. 215.</div>
+</div>
+
+<p>Figs. 210, 211 show forms of <i>short arc lamps</i>, for use
+under low ceilings, so common in basements, etc.</p>
+
+<p>Fig. 212 shows a <i>hand-feed focussing</i> type of <i>arc lamp</i>.
+In regular street lamps, the upper carbon only is fed by
+mechanism, as it burns away about twice as fast as the
+lower one, thus bringing the arc lower and lower. When<span class="pagenum"><a name="Page_140" id="Page_140">[140]</a></span>
+it is desired to keep the arc at the focus of a reflector,
+both carbons must be fed.</p>
+
+<p>Fig. 213 shows a <i>theatre arc lamp</i>, used to throw a
+strong beam of light from the balcony to the stage.</p>
+
+<p>Fig. 214 shows the arc lamp used as a search-light.
+The reflector throws a powerful beam of light that can be
+seen for miles; in fact, the light is used for signalling at
+night. Fig. 215 shows how search-lights are used at
+night on war-vessels.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_141" id="Page_141">[141]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXIII"></a>CHAPTER XXIII.<br />
+<small>X-RAYS, AND HOW THE BONES OF THE HUMAN BODY
+ARE PHOTOGRAPHED.</small></h2>
+
+
+<div class="figleft" style="width: 95px;">
+<img src="images/i_141a.jpg" width="95" height="353" alt="drawing" />
+<div class="caption">Fig. 216.</div>
+</div>
+
+<div class="figright" style="width: 129px;">
+<img src="images/i_141b.jpg" width="129" height="399" alt="drawing" />
+<div class="caption">Fig. 217.</div>
+</div>
+
+<p><b><a id="par_154"></a>154. Disruptive Discharges.</b> We have seen, in the
+study of induction coils, that a spark can jump several
+inches between the terminals of the secondary coil. The
+attraction between the two oppositely charged terminals
+gets so great that it overcomes the resistance of the air-space
+between them, a brilliant spark passes, and they are
+discharged. This sudden discharge is said to be <i>disruptive</i>,
+and it is accompanied by a flash of light and a loud
+report. The <i>path</i> of the discharge may be nearly straight,<span class="pagenum"><a name="Page_142" id="Page_142">[142]</a></span>
+or crooked, depending upon the nature of the material in
+the gap between the terminals.</p>
+
+<div class="figcenter" style="width: 431px;">
+<img src="images/i_142a.jpg" width="431" height="356" alt="drawing" />
+<div class="caption">Fig. 218.</div>
+</div>
+
+<div class="figcenter" style="width: 284px;">
+<img src="images/i_142b.jpg" width="284" height="173" alt="drawing" />
+<div class="caption">Fig. 219.</div>
+</div>
+
+<p><b><a id="par_155"></a>155. Effect of Air Pressure on Spark.</b> The disruptive
+spark takes place in air at ordinary pressures.
+The nature of the spark is greatly changed when the pressure
+of the air decreases. Fig. 216 shows an air-tight glass
+tube so arranged that the
+air can be slowly removed
+with an air-pump. The
+upper rod shown can be
+raised or lowered to increase
+the distance between
+it and the lower rod, these
+acting as the terminals of
+an induction coil. Before exhausting any air, the spark
+will jump a small distance between the rods and act as
+in open air. As soon as a small amount of air is removed,<span class="pagenum"><a name="Page_143" id="Page_143">[143]</a></span>
+a change takes place. The spark is not so intense
+and has no definite path, there being a general
+glow throughout the tube. As the air pressure becomes
+still less, the glow becomes brighter, until the entire tube
+is full of purple light that is able to pass the entire
+length of it; that is, the discharge takes place better in
+rarefied air than it does in ordinary air.</p>
+
+<p><b><a id="par_156"></a>156. Vacuum-Tubes.</b> As electricity passes through
+rarefied gases much easier than
+through ordinary air, regular
+tubes, called <i>vacuum-tubes</i>, are
+made for such study. Fig. 217 shows a plain tube of
+this kind, platinum terminals being fused in the glass
+for connections. These tubes are often made in complicated
+forms, Fig. 218, with colored glass, and are
+called <i>Geissler tubes</i>. They are often made in such a
+way that the electrodes are in the shape of discs, etc.,
+and are called <i>Crookes tubes</i>, Fig. 219. A slight amount
+of gas is left in the tubes.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 264px;">
+<img src="images/i_143a.jpg" width="264" height="388" alt="drawing" />
+<div class="caption">Fig. 220.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 261px;">
+<img src="images/i_143b.jpg" width="261" height="290" alt="drawing" />
+<div class="caption">Fig. 220-A.</div>
+</div></td></tr>
+</table></div>
+
+<p><b><a id="par_157"></a>157. Cathode Rays.</b> The <i>cathode</i> is the electrode of<span class="pagenum"><a name="Page_144" id="Page_144">[144]</a></span>
+a vacuum-tube by which the current leaves the tube, and
+it has been known for some time that some kind of influence
+passes in straight lines from this point. Shadows,
+Fig. 219, are cast by such rays, a screen being placed in
+their path.</p>
+
+<p><b><a id="par_158"></a>158. X-Rays.</b> Professor Roentgen of Würzburg discovered
+that when the cathode rays are allowed to fall
+upon a solid body, the solid body gives out still other rays
+which differ somewhat from the original cathode rays.
+They can penetrate, more or less, through many bodies that
+are usually considered opaque. The hand, for example,
+may be used as a negative for producing a photograph of
+the bones, as the rays do not pass equally well through
+flesh and bone.</p>
+
+<div class="figcenter" style="width: 527px;">
+<img src="images/i_144.jpg" width="527" height="485" alt="drawing" />
+<div class="caption">Fig. 221.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_145" id="Page_145">[145]</a></span></p>
+
+<p>Fig. 220 shows a Crookes tube fitted with a metal plate,
+so that the cathode rays coming from C will strike it.
+The X-rays are given out from P. These rays are
+invisible and are even given out where the cathode rays
+strike the glass. Some chemical compounds are made
+luminous by these rays; so screens are made and coated
+with them in order that the shadows produced by the
+X-rays can be seen by the eye. Professor Roentgen
+named these the X-rays. Fig. 220-A shows a <i>fluoroscope</i>
+that contains a screen covered with proper chemicals.</p>
+
+<div class="figcenter" style="width: 454px;">
+<img src="images/i_145a.jpg" width="454" height="247" alt="drawing" />
+<div class="caption">Fig. 222.</div>
+</div>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/i_145b.jpg" width="480" height="239" alt="drawing" />
+<div class="caption">Fig. 223.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_146" id="Page_146">[146]</a></span></p>
+
+<p><b><a id="par_159"></a>159. X-Ray Photographs.</b> Bone does not allow the
+X-rays to pass through it as readily as flesh, so if the
+hand be placed over a sensitized photographic plate, Fig.
+221, and proper connections be made with the induction
+coil, etc., the hand acts as a photographic negative.
+Upon developing the plate, as in ordinary photography, a
+picture or shadow of the bones will be seen. Fig. 222
+shows the arrangement of battery, induction coil, focus
+tube, etc., for examining the bones of the human body.</p>
+
+<p>Fig. 223 shows the bones of a fish. Such photographs
+have been very valuable in discovering the location of
+bullets, needles, etc., that have become imbedded in the
+flesh, as well as in locating breaks in the bones.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_147" id="Page_147">[147]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXIV"></a>CHAPTER XXIV.<br />
+<small>THE ELECTRIC MOTOR, AND HOW IT DOES WORK.</small></h2>
+
+
+<p><b><a id="par_160"></a>160. Currents and Motion.</b> We have seen, Chapter
+XII., that when coils of wire are rapidly moved across a
+strong magnetic field, a current of electricity is generated.
+We have now to deal with the opposite of this; that is,
+we are to study how <i>motion</i> can be produced by allowing
+a current of electricity to pass through the armature of a
+machine.</p>
+
+<div class="figcenter" style="width: 479px;">
+<img src="images/i_147a.jpg" width="479" height="183" alt="drawing" />
+<div class="caption">Fig. 224.</div>
+</div>
+
+<div class="figcenter" style="width: 530px;">
+<img src="images/i_147b.jpg" width="530" height="284" alt="drawing" />
+<div class="caption">Fig. 225.</div>
+</div>
+
+<p><span class="pagenum"><a name="Page_148" id="Page_148">[148]</a></span></p>
+
+<p>Fig. 224 shows, by diagram, a coil H, suspended so
+that it can move easily, its ends being joined to a current
+reverser, and this, in turn, to a dry cell D C. A
+magnet, H M, will attract the core of H when no current
+passes. When the current is allowed to pass first in one
+direction and then in the opposite direction, by using the
+reverser, the core of H will jump back and forth from one
+pole of H M to the other. There are many ways by
+which motion can be produced by the current, but to
+have it practical, the motion must be a rotary one. (See
+"Study," Chapter XXVI., for numerous experiments.)</p>
+
+<div class="figcenter" style="width: 479px;">
+<img src="images/i_148.jpg" width="479" height="435" alt="drawing" />
+<div class="caption">Fig. 226.</div>
+</div>
+
+<p><b><a id="par_161"></a>161. The Electric Motor</b> is a machine for transforming
+electric energy into mechanical power. The construction
+of motors is very similar to that of dynamos. They
+have field-magnets, armature coils, commutator, etc.; in<span class="pagenum"><a name="Page_149" id="Page_149">[149]</a></span>
+fact, the armature of an ordinary direct current dynamo
+will revolve if a current be passed through it, entering by
+one brush and leaving by the other. There are many
+little differences of construction, for mechanical and electrical
+reasons, but we may say that the general construction
+of dynamos and motors is the same.</p>
+
+<p>Fig. 225 shows a coil of wire, the ends of which are
+connected to copper and zinc plates. These plates are
+floated in dilute sulphuric acid, and form a simple cell
+which sends a current through the wire, as shown by the
+arrows.</p>
+
+<div class="figcenter" style="width: 481px;">
+<a href="images/i_149-big.jpg"><img src="images/i_149.jpg" width="481" height="360" alt="drawing" /></a>
+<div class="caption">Fig. 227.</div>
+</div>
+
+<p>We have seen that a current-carrying wire has a magnetic
+field and acts like a magnet; so it will be easily seen
+that if a magnet be held near the wire it will be either
+attracted or repelled, the motion depending upon the
+poles that come near each other. As shown in the figure,
+the N pole of the magnet repels the field of the wire,<span class="pagenum"><a name="Page_150" id="Page_150">[150]</a></span>
+causing it to revolve. We see that this action is just the
+reverse to that in galvanometers, where the coil is fixed,
+and the magnet, or magnetic needle, is allowed to move.
+As soon as the part of the
+wire, marked A in Fig.
+225, gets a little distance
+from the pole, the opposite
+side of the wire, B, begins
+to be attracted by it, the
+attraction getting stronger
+and stronger, until it gets
+opposite the N pole. If
+the N pole were still held
+in place, B would vibrate
+back and forth a few times, and finally come to rest
+near the pole. If, however, as soon as B gets opposite
+N the S pole of the magnet be quickly turned toward
+B, the coil will be repelled and the rotary motion will
+continue.</p>
+
+<div class="figcenter" style="width: 311px;">
+<img src="images/i_150.jpg" width="311" height="262" alt="drawing" />
+<div class="caption">Fig. 228.</div>
+</div>
+
+<div class="figcenter" style="width: 428px;">
+<img src="images/i_150b.jpg" width="428" height="294" alt="drawing" />
+<div class="caption">Figs. 229 to 231.</div>
+</div>
+
+<div class="figcenter" style="width: 296px;">
+<img src="images/i_151a.jpg" width="296" height="217" alt="drawing" />
+<div class="caption">Fig. 232.</div>
+</div>
+
+<div class="figcenter" style="width: 310px;">
+<a href="images/i_151b-big.jpg"><img src="images/i_151b.jpg" width="310" height="330" alt="drawing" /></a>
+<div class="caption">Fig. 233.</div>
+</div>
+
+<p>Let us now see how this helps to explain electric motors.<span class="pagenum"><a name="Page_151" id="Page_151">[151]</a></span>
+We may consider the wire of Fig. 225 as one coil
+of an armature, and the plates, C and Z, as the halves of
+a commutator. In this arrangement, it must be noted,
+the current always flows through the armature coil in the
+same direction, the rotation
+being kept up by reversing
+the poles of the field-magnet.
+In ordinary simple
+motors the current is reversed
+in the armature
+coils, the field-magnets remaining
+in one position
+without changing the poles.
+This produces the same effect as the above. The
+current is reversed automatically as the brushes allow
+the current to enter first one commutator bar and then
+the opposite one as the armature revolves. The regular<span class="pagenum"><a name="Page_152" id="Page_152">[152]</a></span>
+armatures have many coils and many commutator bars,
+as will be seen by examining the illustrations shown.</p>
+
+<p>The ordinary galvanometer may be considered a form
+of motor. By properly opening
+and closing the circuit, the rotary
+motion of the needle can be kept
+up as long as current is supplied.
+Even an electric bell or telegraph
+sounder may be considered a
+motor, giving motion straight forward
+and back.</p>
+
+<p><b><a id="par_162"></a>162. The Uses of Motors</b> are
+many. It would be impossible to
+mention all the things that are
+done with the power from motors.
+A few illustrations will give an
+idea of the way motors are attached
+to machines.</p>
+
+<p>Fig. 226 shows one form of
+motor, the parts being shown in
+Fig. 227.</p>
+
+<div class="figcenter" style="width: 221px;">
+<img src="images/i_152.jpg" width="221" height="662" alt="drawing" />
+<div class="caption">Fig. 234.</div>
+</div>
+
+<p>Fig. 228 shows a fan motor run
+by a battery. They are generally
+run by the current from the street.
+Figs. 229-231 show other forms of
+fan motors. Fig. 232 shows an
+electric hat polisher. A church
+organ bellows is shown in Fig. 233, so arranged that it
+can be pumped by an electric motor. Fig. 234 shows a
+motor direct connected to a drill press.</p>
+
+<p><b><a id="par_163"></a>163. Starting Boxes.</b> If too much current were<span class="pagenum"><a name="Page_153" id="Page_153">[153]</a></span>
+suddenly allowed to pass into the armature of a motor,
+the coils would be over-heated, and perhaps destroyed,
+before it attained its full speed. A rapidly revolving
+armature will take more current, without being overheated,
+than one not in motion. A motor at full speed
+acts like a dynamo, and generates a current which tends
+to flow from the machine in a direction opposite to that
+which produces the motion. It is evident, then, that
+when the armature is at rest, all the current turned on
+passes through it without meeting with this opposing
+current.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 278px;">
+<img src="images/i_153a.jpg" width="278" height="279" alt="drawing" />
+<div class="caption">Fig. 235.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 210px;">
+<img src="images/i_153b.jpg" width="210" height="217" alt="drawing" />
+<div class="caption">Fig. 236.</div>
+</div>
+</td></tr>
+</table></div>
+
+<p>Fig. 235 shows a starting, stopping, and regulating
+box, inside of which are a number of German-silver resistance
+coils properly connected to contact-points at the
+top. By turning the knob, the field of the motor is immediately
+charged first through resistance, then direct,
+and then the current is put on the armature gradually
+through a series of coils, the amount of current depending
+upon the distance the switch is turned. Fig. 236
+shows a cross section of the same.</p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_154" id="Page_154">[154]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXV"></a>CHAPTER XXV.<br />
+<small>ELECTRIC CARS, BOATS, AND AUTOMOBILES.</small></h2>
+
+
+<p><b><a id="par_164"></a>164. Electric Cars</b>, as well as boats, automobiles, etc.,
+etc., are moved by the power that comes from electric
+motors, these receiving current from the dynamos placed
+at some "central station." We have already seen how
+the motor can do many kinds of work. By properly
+gearing it to the car wheels, motion can be given to them
+which will move the car.</p>
+
+<div class="figcenter" style="width: 582px;">
+<img src="images/i_154.jpg" width="582" height="242" alt="drawing" />
+<div class="caption">Fig. 237.</div>
+</div>
+
+<p>Fig. 237 shows two dynamos which will be supposed to
+be at a power house and which send out a current to
+propel cars. From the figure it will be seen that the
+wires over the cars, called trolley-wires, are connected to
+the positive (+) terminals of the dynamos, and that the
+negative (-) terminals are connected to the tracks. In
+case a wire were allowed to join the trolley-wire and
+track, we should have a short circuit, and current would
+not only rush back to the dynamo without doing useful<span class="pagenum"><a name="Page_155" id="Page_155">[155]</a></span>
+work, but it would probably injure the machines. When
+some of the current is allowed to pass through a car,
+motion is produced in the motors, as has been explained.
+As the number of cars increases, more current passes back
+to the dynamos, which must
+do more work to furnish
+such current.</p>
+
+<p><i>Trolley-poles</i>, fastened to
+the top of the cars and
+which end in grooved
+wheels, called <i>trolley-wheels</i>,
+are pressed by springs
+against the trolley-wires.
+The current passes down
+these through switches to
+<i>controllers</i> at each end of
+the car, one set being used
+at a time.</p>
+
+<div class="figcenter" style="width: 289px;">
+<img src="images/i_155a.jpg" width="289" height="593" alt="drawing" />
+<div class="caption">Fig. 238.</div>
+</div>
+
+<div class="figcenter" style="width: 298px;">
+<a href="images/i_155-big.jpg"><img src="images/i_155.jpg" width="298" height="156" alt="drawing" /></a>
+<div class="caption">Fig. 239.</div>
+</div>
+
+<p><b><a id="par_165"></a>165. The Controllers</b>, as the name suggests, control
+the speed of the car by allowing more or less current to
+pass through the motors. The motors, resistance coils
+and controllers are so connected with each other that
+the amount of current used can be regulated.</p>
+
+<p><span class="pagenum"><a name="Page_156" id="Page_156">[156]</a></span></p>
+
+
+<div class="figcenter" style="width: 477px;">
+<img src="images/i_156a.jpg" width="477" height="246" alt="drawing" />
+<div class="caption">Fig. 240.</div>
+</div>
+
+<p>When the motorman turns the handle of the controller
+to the first notch, the current passes through all of the
+resistance wires placed under the car, then through one
+motor after the other. The motors being joined in
+series by the proper connections at the controller, the
+greatest resistance is offered to the current and the car
+runs at the slowest speed at this first notch. As more
+resistance is cut out by turning the handle to other
+notches, the car increases
+its speed; but
+as the resistance wires
+become heated and the
+heat passes into the
+air, there is a loss of
+energy. It is not
+economical to run a
+car at such a speed
+that energy is wasted
+as heat. As soon as
+the resistance is all cut out, the current simply passes
+through the motors joined in series. This gives a fairly<span class="pagenum"><a name="Page_157" id="Page_157">[157]</a></span>
+slow speed and one that is economical because all the
+current tends to produce motion.</p>
+
+<p>By allowing the current to pass through the motors
+joined in parallel, that is, by allowing each to take a part
+of the current, the resistance is greatly reduced, and a
+higher speed attained. This is not instantly done, however,
+as too much strain would be put upon the motors.
+As soon as the next notch is reached, the motors are
+joined in parallel and the resistance also thrown in again.
+By turning the handle still more, resistance is gradually
+cut out, and the highest speed produced when the current
+passes only through the motors in parallel.</p>
+
+<div class="figcenter" style="width: 345px;">
+<img src="images/i_156b.jpg" width="345" height="280" alt="drawing" />
+<div class="caption">Fig. 241.</div>
+</div>
+
+<div class="figright" style="width: 167px;">
+<img src="images/i_157a.jpg" width="167" height="183" alt="drawing" />
+<div class="caption">Fig. 242.</div>
+</div>
+
+<p>Fig. 238 represents a controller, by diagram, showing
+the relative positions of the controller cylinder, reversing
+and cut-out cylinders, arrangements for
+blowing out the short electric arcs
+formed, etc. A ratchet and pawl is provided,
+which indicates positively the running
+notches, at the same time permitting
+the cylinder to move with ease. Fig.
+239 shows a top view of the controller.</p>
+
+<div class="figleft" style="width: 213px;">
+<img src="images/i_157b.jpg" width="213" height="181" alt="drawing" />
+<div class="caption">Fig. 243.</div>
+</div>
+
+<p><b><a id="par_166"></a>166. Overhead and Underground Systems.</b> When
+wires for furnishing current are placed over the tracks, as<span class="pagenum"><a name="Page_158" id="Page_158">[158]</a></span>
+in Fig. 237, we have the overhead system. In cities the
+underground system is largely used. The location of the
+conducting wires beneath the surface of the street removes
+all danger to the public, and protects them from all interference,
+leaving the street free from poles and wires.</p>
+
+<p>Fig. 240 shows a cross-section of an underground conduit.
+The rails, R R, are supported by cast-iron yokes, A,
+placed five feet apart, and thoroughly imbedded in concrete.
+The conduit has sewer connections every 100 feet.
+Conducting bars, C C, are placed on each side of the conduit,
+and these are divided into sections of about 500
+feet. Insulators, D D, are placed every 15 feet. They
+are attached to, and directly under, the slot-rails, the
+stem passing through the conductor bar.</p>
+
+<div class="figright" style="width: 140px;">
+<img src="images/i_157c.jpg" width="140" height="140" alt="drawing" />
+<div class="caption">Fig. 244.</div>
+</div>
+
+<p>Figs. 240 and 241 show the plow E. The contact
+plates are carried on coiled springs to allow a free motion.
+Two guide-wheels, F F, are attached to the leg of the plow.
+The conducting wires are carried up through the leg of
+the plow.</p>
+
+<p><b><a id="par_167"></a>167. Appliances.</b> A large number of articles are
+needed in the construction of electric railroads. A few,
+only, can be shown that are used for the overhead system.
+Fig. 242 shows a pole insulator. Fig. 243 shows a feeder-wire
+insulator. Fig. 244 shows a line suspension. Fig.
+245 shows a form of right-angle cross which allows the<span class="pagenum"><a name="Page_159" id="Page_159">[159]</a></span>
+trolley-wheels of crossing lines to pass. Fig. 246 shows
+a switch. In winter a part of the current is allowed to
+pass through electric heaters placed under the seats of
+electric cars.</p>
+
+<div class="figcenter" style="width: 296px;">
+<img src="images/i_158.jpg" width="296" height="98" alt="drawing" />
+<div class="caption">Fig. 245.</div>
+</div>
+
+<p><b><a id="par_168"></a>168. Electric Boats</b> are run by the current from
+storage batteries which are usually placed under the
+seats. An electric motor large enough to run a small
+boat takes up very little room and is generally placed
+under the floor. This leaves the entire boat for the use
+of passengers. The motor is connected to the shaft that
+turns the screw. Fig. 247 shows one design.</p>
+<div class="figleft" style="width: 227px;">
+<img src="images/i_159a.jpg" width="227" height="289" alt="drawing" />
+<div class="caption">Fig. 246.</div>
+</div>
+<p><b><a id="par_169"></a>169. Electric Automobiles</b> represent the highest type
+of electrical and mechanical construction. The <i>running-gear</i>
+is usually made of the best cold-drawn seamless
+steel tubing, to get the greatest strength from a given
+weight of material. The wheels are made in a variety
+of styles, but nearly all have ball bearings and pneumatic
+tires. In the lightest styles the wheels have wire spokes.</p>
+
+<p>The <i>electric motors</i>, supported by the running-gear, are<span class="pagenum"><a name="Page_160" id="Page_160">[160]</a></span>
+geared to the rear wheels. The motors are made as
+nearly dust-proof as possible.</p>
+<div class="figcenter" style="width: 533px;">
+<img src="images/i_160.jpg" width="533" height="264" alt="drawing long boat" />
+<div class="caption">Fig. 247.</div>
+</div>
+
+<p><i>Storage batteries</i> are put in a convenient place, depending
+upon the design of the carriage, and from these the
+motors receive the current. These can be charged from
+the ordinary 110-volt lighting circuits or from private
+dynamos. The proper plugs and attachments are usually
+furnished by the various makers for connecting the
+batteries with the street current, which is shut off when
+the batteries are full by an automatic switch.</p>
+
+<p><i>Controllers</i> are used, as on electric cars, the lever for
+starting, stopping, etc., being usually placed on the left-hand
+side of the seat. The <i>steering</i> is done by a lever
+that moves the front wheels. Strong brakes, and the
+ability to quickly reverse the motors, allow electric carriages
+to be stopped suddenly in case of accidents.</p>
+
+<p>Electric automobiles are largely used in cities, or where
+the current can be easily had. The batteries must be
+re-charged after they have run the motors for a certain
+time which depends upon the speed and road, as well as<span class="pagenum"><a name="Page_161" id="Page_161">[161]</a></span>
+upon the construction. Where carriages are to be run
+almost constantly, as is the case with those used for general
+passenger service in cities, duplicate batteries are necessary,
+so that one or two sets can be charged while another
+is in use. Fig. 248 shows one form of electric vehicle,
+the storage batteries being placed under and back of the
+seat.</p>
+
+<div class="figcenter" style="width: 531px;">
+<img src="images/i_161.jpg" width="531" height="425" alt="drawing" />
+<div class="caption">Fig. 248.</div>
+</div>
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_162" id="Page_162">[162]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXVI"></a>CHAPTER XXVI.<br />
+<small>A WORD ABOUT CENTRAL STATIONS.</small></h2>
+
+<div class="figleft" style="width: 143px;">
+<img src="images/i_162.jpg" width="143" height="505" alt="drawing" />
+<div class="caption">Fig. 249.</div>
+</div>
+
+<p><b><a id="par_170"></a>170. Central Stations</b>, as the word implies, are places
+where, for example, electricity is generated for the incandescent
+or arc lights used in a certain neighborhood;
+where telephone or telegraph messages
+are sent to be resent to some other station;
+where operators are kept to switch different
+lines together, so that those on
+one line can talk to those on another,
+etc., etc. There are many kinds of central
+stations, each requiring a large
+amount of special apparatus to carry on
+the work. Fig. 249 gives a hint in
+regard to the way car lines get their
+power from a central power station.
+As a large part of the apparatus required
+in ordinary central stations has already
+been described, it is not necessary to go
+into the details of such stations.</p>
+
+
+<p>In lighting stations, for example, we
+have three principal kinds of apparatus.
+Boilers produce the steam that runs the
+steam engines, and these run the dynamos
+that give the current. Besides these there are many
+other things needed. The electrical energy that goes over
+the wires to furnish light, heat, and power, really comes
+indirectly from the coal that is used to boil water and<span class="pagenum"><a name="Page_163" id="Page_163">[163]</a></span>
+convert it into steam. The various parts of the central
+station merely aid in this transformation of energy.</p>
+
+<p>The dynamos are connected to the engines by belts, or<span class="pagenum"><a name="Page_164" id="Page_164">[164]</a></span>
+they are direct connected. Figs. 250, 251, show dynamos
+connected to engines without belts.</p>
+
+<p>The current from the dynamos is led to large switchboards
+which contain switches, voltmeters, ammeters,
+lightning arresters, and various other apparatus for the
+proper control and measurement of the current. From
+the switchboard it is allowed to pass through the various
+street mains, from which it is finally led to lamps, motors,
+etc.</p>
+
+<div class="figcenter" style="width: 519px;">
+<img src="images/i_163a.jpg" width="519" height="413" alt="drawing" />
+<div class="caption">Fig. 250.</div>
+</div>
+
+<div class="figcenter" style="width: 529px;">
+<img src="images/i_163b.jpg" width="529" height="328" alt="drawing" />
+<div class="caption">Fig. 251.</div>
+</div>
+
+<p>Water-power is frequently used to drive the dynamos
+instead of steam engines. The water turns some form of
+water-wheel which is connected to the dynamos. At
+Niagara Falls, for example, immense quantities of current
+are generated for light, heat, power, and industrial purposes.</p>
+
+<div class="figcenter" style="width: 125px;">
+<img src="images/i_164.jpg" width="125" height="150" alt="decoration" />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_165" id="Page_165">[165]</a></span></p>
+
+
+
+
+<h2><a id="CHAPTER_XXVII"></a>CHAPTER XXVII.<br />
+<small>MISCELLANEOUS USES OF ELECTRICITY.</small></h2>
+
+
+<p><b><a id="par_171"></a>171. The Many Uses</b> to which the electric current is
+put are almost numberless. New uses are being found
+for it every day. Some of the common applications are
+given below.</p>
+
+<p><b><a id="par_172"></a>172. Automatic Electric Program Clocks</b>, Fig.
+252, are largely used in all sorts of establishments, schools,
+etc., for ringing bells at certain stated periods. The
+lower dial shown has many contact-points that can be
+inserted to correspond to given times. As this revolves,
+the circuits are closed, one after the other, and it may be
+so set that bells will be rung in different parts of the
+house every five minutes, if desired.</p>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 141px;">
+<img src="images/i_165a.jpg" width="141" height="339" alt="drawing" />
+<div class="caption">Fig. 252.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 287px;">
+<a href="images/i_165b-big.jpg"><img src="images/i_165.jpg" width="287" height="287" alt="drawing" /></a>
+<div class="caption">Fig. 253.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_173"></a>173. Call Boxes</b> are used to send in calls of various<span class="pagenum"><a name="Page_166" id="Page_166">[166]</a></span>
+kinds to central stations. Fig. 253 shows one form. The
+number of different calls provided includes messenger, carrier,
+coupé, express wagon, doctor, laborer, police, fire,
+together with three more, which may be made special to
+suit the convenience of the individual customer. The
+instruments are provided with apparatus for receiving a
+return signal, the object of which is to notify the subscriber
+that his call has been received and is having
+attention.</p>
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 226px;">
+<a href="images/i_166a-big.jpg"><img src="images/i_166a.jpg" width="226" height="356" alt="drawing" /></a>
+<div class="caption">Fig. 254.</div>
+</div>
+</td><td align="left"><div class="figcenter" style="width: 230px;">
+<img src="images/i_166b.jpg" width="230" height="304" alt="drawing" />
+<div class="caption">Fig. 255.</div>
+</div></td></tr>
+</table></div>
+
+<p>Fig. 254 shows another form of call box, the handle
+being moved around to the call desired. As it springs
+back to the original position, an interrupted current
+passes through the box to the central station, causing a
+bell to tap a certain number of times, giving the call and
+location of the box.</p>
+
+<p><b><a id="par_174"></a>174. Electric Gas-Lighters.</b> Fig. 255 shows a
+<i>ratchet burner</i>. The first pull of the chain turns on the
+gas through a four-way gas-cock, governed by a ratchet-wheel<span class="pagenum"><a name="Page_167" id="Page_167">[167]</a></span>
+and pawl. The issuing gas is lighted by a wipe-spark
+at the tip of the burner. Alternate pulls shut off
+the gas. As the lever brings the attached wire A, in contact
+with the wire B, a bright spark passes, which ignites
+the gas, the burner being joined with a battery and induction
+or spark coil.</p>
+
+<p><i>Automatic burners</i> are used when it is desired to light
+gas at a distance from the push-button. Fig. 256 shows
+one form. Two electromagnets are shown, one being
+generally joined to a white push-button for turning on
+the gas and lighting it, the other being joined to a black
+button which turns off the gas when it is pressed. The
+armatures of the magnets work the gas-valve. Sparks
+ignite the gas, as explained above.</p>
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="drawings">
+<tr><td align="left"><div class="figcenter" style="width: 219px;">
+<img src="images/i_167a.jpg" width="219" height="323" alt="drawing" />
+<div class="caption">Fig. 256.</div>
+</div></td><td align="left"><div class="figcenter" style="width: 273px;">
+<img src="images/i_167b.jpg" width="273" height="359" alt="drawing" />
+<div class="caption">Fig. 257.</div>
+</div></td></tr>
+</table></div>
+
+
+
+
+<p><b><a id="par_175"></a>175. Door Openers.</b> Fig. 257 shows one form.
+They contain electromagnets so arranged that when the<span class="pagenum"><a name="Page_168" id="Page_168">[168]</a></span>
+armature is attracted by the pushing of a button anywhere
+in the building, the door can be pushed open.</p>
+
+<p><b><a id="par_176"></a>176. Dental Outfits.</b> Fig. 258 shows a motor
+arranged to run dental apparatus. The motor can be
+connected to an ordinary incandescent
+light socket. In case the current
+gives out, the drills, etc., can
+be run by foot power.</p>
+
+<div class="figcenter" style="width: 214px;">
+<img src="images/i_168a.jpg" width="214" height="653" alt="drawing" />
+<div class="caption">Fig. 258.</div>
+</div>
+
+<p><b><a id="par_177"></a>177. Annunciators</b> of various
+kinds are used in hotels, factories,
+etc., to indicate a certain room
+when a bell rings at the office.
+The bell indicates that some one
+has called, and the annunciator
+shows the location of the call by
+displaying the number of the room
+or its location. Fig. 259 shows a
+small annunciator. They contain
+electromagnets which are connected to push-buttons
+located in the building, and which bring the numbers
+into place as soon as the current passes through them.</p>
+
+<div class="figcenter" style="width: 237px;">
+<img src="images/i_168.jpg" width="237" height="224" alt="drawing" />
+<div class="caption">Fig. 259.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_169" id="Page_169">[169]</a></span></p>
+
+
+
+
+<h2>INDEX.</h2>
+
+
+<p>Numbers refer to paragraphs. See <a href="#TABLE_OF_CONTENTS">Table of Contents</a> for the
+titles of the various chapters.</p>
+
+<div>
+Action of magnets upon each other, <a href="#par_32">32</a>.<br />
+<br />
+Adjuster, for lamp cords, <a href="#par_151">151</a>.<br />
+<br />
+Air pressure, effect of spark upon, <a href="#par_155">155</a>.<br />
+<br />
+Aluminum-leaf, for electroscopes, <a href="#par_5">5</a>.<br />
+<br />
+Alternating current, <a href="#par_129">129</a>, <a href="#par_130">130</a>;<br />
+<span style="margin-left: 1em;">system of wiring for, <a href="#par_144">144</a>.</span><br />
+<br />
+Amalgamation of zincs, <a href="#par_47">47</a>.<br />
+<br />
+Amber, electrification upon, <a href="#par_3">3</a>.<br />
+<br />
+Ammeter, the, <a href="#par_74">74</a>;<br />
+<span style="margin-left: 1em;">how placed in circuit, <a href="#par_77">77</a>.</span><br />
+<br />
+Ampere, the, <a href="#par_72">72</a>.<br />
+<br />
+Annunciators, <a href="#par_177">177</a>.<br />
+<br />
+Anode, <a href="#par_79">79</a>, <a href="#par_82">82</a>.<br />
+<br />
+Apparatus for electrical measurements, <a href="#CHAPTER_VI">Chap. VI</a>.<br />
+<br />
+Appliances, for distribution of currents, <a href="#par_141">141</a>;<br />
+<span style="margin-left: 1em;">for electric railways, <a href="#par_167">167</a>;</span><br />
+<span style="margin-left: 1em;">for heating by electricity, <a href="#par_147">147</a>.</span><br />
+<br />
+Arc, the electric, <a href="#par_152">152</a>.<br />
+<br />
+Arc lamp, the, <a href="#par_153">153</a>;<br />
+<span style="margin-left: 1em;">how light is produced by, <a href="#CHAPTER_XXII">Chap. XXI</a>I.;</span><br />
+<span style="margin-left: 1em;">double carbon, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">hand-feed focussing, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">for search-lights, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">short, for basements, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">single carbon, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">for theater use, <a href="#par_153">153</a>.</span><br />
+<br />
+Armature, of dynamo, <a href="#par_127">127</a>, <a href="#par_129">129</a>;<br />
+<span style="margin-left: 1em;">of electromagnets, <a href="#par_98">98</a>;</span><br />
+<span style="margin-left: 1em;">of horseshoe magnet, <a href="#par_26">26</a>;</span><br />
+<span style="margin-left: 1em;">of motors, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">uses of, <a href="#par_39">39</a>.</span><br />
+<br />
+Artificial magnets, <a href="#par_25">25</a>.<br />
+<br />
+Astatic, detectors, <a href="#par_94">94</a>;<br />
+<span style="margin-left: 1em;">galvanometer, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">needles, <a href="#par_94">94</a>.</span><br />
+<br />
+Aurora borealis, <a href="#par_23">23</a>.<br />
+<br />
+Automatic, current interrupters, <a href="#par_104">104</a>, <a href="#par_115">115</a>;<br />
+<span style="margin-left: 1em;">gas lighters, <a href="#par_174">174</a>;</span><br />
+<span style="margin-left: 1em;">program clocks, <a href="#par_172">172</a>.</span><br />
+<br />
+Automobiles, <a href="#par_169">169</a>;<br />
+<span style="margin-left: 1em;">controllers for, <a href="#par_169">169</a>;</span><br />
+<span style="margin-left: 1em;">motors for, <a href="#par_169">169</a>;</span><br />
+<span style="margin-left: 1em;">steering of, <a href="#par_169">169</a>;</span><br />
+<span style="margin-left: 1em;">storage batteries for, <a href="#par_169">169</a>.</span><br />
+<br />
+<br />
+Bamboo filaments, <a href="#par_149">149</a>.<br />
+<br />
+Bar magnets, <a href="#par_27">27</a>;<br />
+<span style="margin-left: 1em;">magnetic figures of, <a href="#par_38">38</a>.</span><br />
+<br />
+Batteries, large plunge, <a href="#par_54">54</a>;<br />
+<span style="margin-left: 1em;">plunge, <a href="#par_53">53</a>;</span><br />
+<span style="margin-left: 1em;">secondary, <a href="#par_86">86</a>;</span><br />
+<span class="pagenum"><a name="Page_170" id="Page_170">[170]</a></span><span style="margin-left: 1em;">storage, and how they work, <a href="#CHAPTER_IX">Chap. IX</a>.</span><br />
+<br />
+Bell, the electric, and some of its uses, <a href="#CHAPTER_XV">Chap. XV</a>.;<br />
+<span style="margin-left: 1em;">electric, <a href="#par_116">116</a>;</span><br />
+<span style="margin-left: 1em;">magneto testing, <a href="#par_117">117</a>;</span><br />
+<span style="margin-left: 1em;">trembling, etc., <a href="#par_116">116</a>.</span><br />
+<br />
+Bell transmitter, <a href="#par_120">120</a>.<br />
+<br />
+Belts, electricity generated by friction upon, <a href="#par_1">1</a>.<br />
+<br />
+Benjamin Franklin, <a href="#par_18">18</a>.<br />
+<br />
+Bichromate of potash cells, <a href="#par_51">51</a>, etc.<br />
+<br />
+Binding-posts, <a href="#CHAPTER_V">Chap. V</a>.;<br />
+<span style="margin-left: 1em;">common forms of, <a href="#par_63">63</a>.</span><br />
+<br />
+Blasting, by electricity, <a href="#par_147">147</a>;<br />
+<span style="margin-left: 1em;">electric machines for, <a href="#par_147">147</a>.</span><br />
+<br />
+Bluestone cell, <a href="#par_56">56</a>.<br />
+<br />
+Boats, electric, <a href="#par_168">168</a>.<br />
+<br />
+Boilers, use of in central stations, <a href="#par_170">170</a>.<br />
+<br />
+Bones, photographed by x-rays, <a href="#CHAPTER_XXIII">Chap. XXIII</a>.<br />
+<br />
+Boosters, <a href="#par_136">136</a>.<br />
+<br />
+Brushes, <a href="#par_129">129</a>.<br />
+<br />
+Bunsen cells, <a href="#par_56">56</a><i>a</i>.<br />
+<br />
+Burner, automatic, <a href="#par_174">174</a>;<br />
+<span style="margin-left: 1em;">for gas-lights, <a href="#par_174">174</a>;</span><br />
+<span style="margin-left: 1em;">ratchet, <a href="#par_174">174</a>.</span><br />
+<br />
+Buzzers, electric, <a href="#par_118">118</a>.<br />
+<br />
+<br />
+Cables and wires, <a href="#par_143">143</a>.<br />
+<br />
+Call boxes, electric, <a href="#par_173">173</a>.<br />
+<br />
+Carbon, in arc lamps, <a href="#par_152">152</a>, <a href="#par_153">153</a>;<br />
+<span style="margin-left: 1em;">filament, <a href="#par_149">149</a>;</span><br />
+<span style="margin-left: 1em;">transmitter, <a href="#par_123">123</a>.</span><br />
+<br />
+Carpet, electricity generated upon, <a href="#par_1">1</a>.<br />
+<br />
+Cars, electric, <a href="#par_164">164</a>;<br />
+<span style="margin-left: 1em;">controllers for, <a href="#par_165">165</a>;</span><br />
+<span style="margin-left: 1em;">heating by electricity, <a href="#par_167">167</a>;</span><br />
+<span style="margin-left: 1em;">overhead system for, <a href="#par_166">166</a>;</span><br />
+<span style="margin-left: 1em;">underground system for, <a href="#par_166">166</a>.</span><br />
+<br />
+Cat, electricity generated upon, <a href="#par_1">1</a>.<br />
+<br />
+Cathode, definition of, <a href="#par_79">79</a>;<br />
+<span style="margin-left: 1em;">rays, <a href="#par_157">157</a>.</span><br />
+<br />
+Cells, Bunsen, <a href="#par_56">56</a><i>a</i>;<br />
+<span style="margin-left: 1em;">bichromate of potash, <a href="#par_51">51</a>;</span><br />
+<span style="margin-left: 1em;">closed circuit, <a href="#par_50">50</a>;</span><br />
+<span style="margin-left: 1em;">dry, <a href="#par_58">58</a>;</span><br />
+<span style="margin-left: 1em;">Edison-Lelande, <a href="#par_59">59</a>;</span><br />
+<span style="margin-left: 1em;">electricity generated by, <a href="#CHAPTER_III">Chap. III</a>.;</span><br />
+<span style="margin-left: 1em;">Fuller, <a href="#par_55">55</a>;</span><br />
+<span style="margin-left: 1em;">Gonda, <a href="#par_57">57</a>;</span><br />
+<span style="margin-left: 1em;">gravity, <a href="#par_56">56</a>;</span><br />
+<span style="margin-left: 1em;">Grenet, <a href="#par_52">52</a>;</span><br />
+<span style="margin-left: 1em;">Leclanché, <a href="#par_57">57</a>;</span><br />
+<span style="margin-left: 1em;">open circuit, <a href="#par_50">50</a>;</span><br />
+<span style="margin-left: 1em;">plates and poles of, <a href="#par_45">45</a><i>a</i>;</span><br />
+<span style="margin-left: 1em;">polarization of, <a href="#par_48">48</a>;</span><br />
+<span style="margin-left: 1em;">simple, <a href="#par_45">45</a>, <a href="#par_49">49</a>;</span><br />
+<span style="margin-left: 1em;">single-fluid, <a href="#par_49">49</a>;</span><br />
+<span style="margin-left: 1em;">two-fluid, <a href="#par_49">49</a>;</span><br />
+<span style="margin-left: 1em;">various voltaic, <a href="#CHAPTER_IV">Chap. I</a>V.</span><br />
+<br />
+Central stations, <a href="#par_170">170</a>;<br />
+<span style="margin-left: 1em;">a word about, <a href="#CHAPTER_XXVI">Chap. XXVI</a>.</span><br />
+<br />
+Chain lightning, <a href="#par_19">19</a>.<br />
+<br />
+Chafing-dishes, electrical, <a href="#par_147">147</a>.<br />
+<br />
+Charging condensers, <a href="#par_15">15</a>.<br />
+<br />
+Chemical action, and electricity, <a href="#par_81">81</a>.<br />
+<br />
+Chemical effects of electric current, <a href="#CHAPTER_VII">Chap. VII</a>.<br />
+<br />
+Chemical meters, <a href="#par_78">78</a>.<br />
+<br />
+Church organs, pumped by motors, <a href="#par_162">162</a>.<br />
+<br />
+Circuits, electric, <a href="#par_50">50</a>;<br />
+<span class="pagenum"><a name="Page_171" id="Page_171">[171]</a></span><span style="margin-left: 1em;">for lamps, <a href="#par_144">144</a>.</span><br />
+<br />
+Cleats, porcelain, <a href="#par_141">141</a>;<br />
+<span style="margin-left: 1em;">wooden, <a href="#par_141">141</a>.</span><br />
+<br />
+Clocks, automatic electric, <a href="#par_172">172</a>.<br />
+<br />
+Closed circuit cells, <a href="#par_50">50</a>.<br />
+<br />
+Coils, induction, and how they work, <a href="#CHAPTER_XIII">Chap. XIII</a>.;<br />
+<span style="margin-left: 1em;">induction, construction of, <a href="#par_104">104</a>;</span><br />
+<span style="margin-left: 1em;">method of joining, <a href="#par_98">98</a>;</span><br />
+<span style="margin-left: 1em;">primary and secondary, <a href="#par_103">103</a>;</span><br />
+<span style="margin-left: 1em;">resistance, <a href="#par_69">69</a>;</span><br />
+<span style="margin-left: 1em;">rotation of, <a href="#par_95">95</a>;</span><br />
+<span style="margin-left: 1em;">of transformers, <a href="#par_135">135</a>.</span><br />
+<br />
+Collectors on dynamos, <a href="#par_129">129</a>.<br />
+<br />
+Commutators, <a href="#par_129">129</a>.<br />
+<br />
+Compasses, magnetic, <a href="#par_31">31</a>.<br />
+<br />
+Compound, magnets, <a href="#par_28">28</a>;<br />
+<span style="margin-left: 1em;">wound dynamo, <a href="#par_131">131</a>.</span><br />
+<br />
+Condensation of static electricity, <a href="#par_15">15</a>.<br />
+<br />
+Condensers, <a href="#par_15">15</a>;<br />
+<span style="margin-left: 1em;">for induction coils, <a href="#par_104">104</a>.</span><br />
+<br />
+Conductors, and insulators, <a href="#par_4">4</a>, <a href="#par_138">138</a>.<br />
+<br />
+Conduits, electric, <a href="#par_140">140</a>.<br />
+<br />
+Connections, electrical, <a href="#par_60">60</a>;<br />
+<span style="margin-left: 1em;">for telegraph lines, <a href="#par_111">111</a>.</span><br />
+<br />
+Controllers, for automobiles, <a href="#par_169">169</a>;<br />
+<span style="margin-left: 1em;">for electric cars, <a href="#par_165">165</a>.</span><br />
+<br />
+Copper sulphate, effects of current on, <a href="#par_82">82</a>;<br />
+<span style="margin-left: 1em;">formula of, <a href="#par_79">79</a>.</span><br />
+<br />
+Copper voltameters, <a href="#par_75">75</a>.<br />
+<br />
+Cords, adjustable for lamps, <a href="#par_151">151</a>.<br />
+<br />
+Coulomb, the, <a href="#par_76">76</a>.<br />
+<br />
+Crater of hot carbons, <a href="#par_152">152</a>.<br />
+<br />
+Crookes tubes, <a href="#par_156">156</a>, <a href="#par_158">158</a>.<br />
+<br />
+Current, detectors, <a href="#par_93">93</a>;<br />
+<span style="margin-left: 1em;">direction of in cell, <a href="#par_46">46</a>;</span><br />
+<span style="margin-left: 1em;">from magnet and coil, <a href="#par_100">100</a>;</span><br />
+<span style="margin-left: 1em;">from two coils, <a href="#par_102">102</a>;</span><br />
+<span style="margin-left: 1em;">induced, <a href="#par_127">127</a>;</span><br />
+<span style="margin-left: 1em;">of induction coils, <a href="#par_105">105</a>;</span><br />
+<span style="margin-left: 1em;">interrupters, automatic, <a href="#par_104">104</a>, <a href="#par_115">115</a>;</span><br />
+<span style="margin-left: 1em;">local, <a href="#par_47">47</a>;</span><br />
+<span style="margin-left: 1em;">primary and secondary, <a href="#par_102">102</a>;</span><br />
+<span style="margin-left: 1em;">transformation of, <a href="#CHAPTER_XVIII">Chap. XVIII</a>.;</span><br />
+<span style="margin-left: 1em;">transmission of, <a href="#par_134">134</a>.</span><br />
+<br />
+Currents, and motion, <a href="#par_160">160</a>;<br />
+<span style="margin-left: 1em;">how distributed for use, <a href="#CHAPTER_XIX">Chap. XIX</a>.</span><br />
+<br />
+Current strength, <a href="#par_71">71</a>;<br />
+<span style="margin-left: 1em;">measurement of, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">unit of, <a href="#par_72">72</a>.</span><br />
+<br />
+Cylinder electric machines, <a href="#par_9">9</a>.<br />
+<br />
+<br />
+Daniell cell, <a href="#par_56">56</a>.<br />
+<br />
+D'Arsonval galvanometer, <a href="#par_73">73</a>.<br />
+<br />
+Declination, <a href="#par_41">41</a>.<br />
+<br />
+Decorative incandescent lamps, <a href="#par_151">151</a>.<br />
+<br />
+Dental, lamps, <a href="#par_151">151</a>;<br />
+<span style="margin-left: 1em;">outfits, <a href="#par_176">176</a>.</span><br />
+<br />
+Detectors, astatic, <a href="#par_94">94</a>;<br />
+<span style="margin-left: 1em;">current, <a href="#par_93">93</a>.</span><br />
+<br />
+Diamagnetic bodies, <a href="#par_29">29</a>.<br />
+<br />
+Diaphragm for telephones, <a href="#par_120">120</a>.<br />
+<br />
+Dip, of magnetic needle, <a href="#par_42">42</a>.<br />
+<br />
+Direct current, <a href="#par_129">129</a>, <a href="#par_130">130</a>.<br />
+<br />
+Direction of current in cell, <a href="#par_46">46</a>.<br />
+<br />
+Discharging condensers, <a href="#par_15">15</a>.<br />
+<br />
+Disruptive discharges, <a href="#par_154">154</a>.<br />
+<br />
+<span class="pagenum"><a name="Page_172" id="Page_172">[172]</a></span>Distribution of currents for use, <a href="#CHAPTER_XIX">Chap. XIX</a>.<br />
+<br />
+Door opener, electric, <a href="#par_175">175</a>.<br />
+<br />
+Dots and dashes, <a href="#par_110">110</a>.<br />
+<br />
+Drill press, run by motor, <a href="#par_162">162</a>.<br />
+<br />
+Dry cells, <a href="#par_58">58</a>.<br />
+<br />
+Dynamo, the, <a href="#par_126">126</a>;<br />
+<span style="margin-left: 1em;">alternating current, <a href="#par_130">130</a>;</span><br />
+<span style="margin-left: 1em;">commutator of, <a href="#par_129">129</a>;</span><br />
+<span style="margin-left: 1em;">compound wound, <a href="#par_131">131</a>;</span><br />
+<span style="margin-left: 1em;">direct current, <a href="#par_130">130</a>;</span><br />
+<span style="margin-left: 1em;">lamps connected to, <a href="#par_132">132</a>;</span><br />
+<span style="margin-left: 1em;">series wound, <a href="#par_131">131</a>;</span><br />
+<span style="margin-left: 1em;">shunt wound, <a href="#par_131">131</a>;</span><br />
+<span style="margin-left: 1em;">used as motor, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">use of in central stations, <a href="#par_170">170</a>;</span><br />
+<span style="margin-left: 1em;">used with water power, <a href="#par_170">170</a>.</span><br />
+<br />
+Dynamos, electricity generated by, <a href="#CHAPTER_XVII">Chap. XVII</a>.;<br />
+<span style="margin-left: 1em;">types of, <a href="#par_130">130</a>;</span><br />
+<span style="margin-left: 1em;">various machines, <a href="#par_132">132</a>;</span><br />
+<span style="margin-left: 1em;">winding of, <a href="#par_131">131</a>.</span><br />
+<br />
+Dynamotors, <a href="#par_137">137</a>.<br />
+<br />
+<br />
+Earth, inductive influence of, <a href="#par_43">43</a>;<br />
+<span style="margin-left: 1em;">lines of force about, <a href="#par_40">40</a>, <a href="#par_42">42</a>.</span><br />
+<br />
+Ebonite, electricity by friction upon, <a href="#par_3">3</a>, <a href="#par_4">4</a>.<br />
+<br />
+Edison-Lelande cells, <a href="#par_59">59</a>.<br />
+<br />
+Electric, automobiles, <a href="#par_169">169</a>;<br />
+<span style="margin-left: 1em;">bell, and some of its uses, <a href="#CHAPTER_XV">Chap. XV</a>.;</span><br />
+<span style="margin-left: 1em;">boats, <a href="#par_168">168</a>;</span><br />
+<span style="margin-left: 1em;">buzzers, <a href="#par_118">118</a>;</span><br />
+<span style="margin-left: 1em;">cars, <a href="#par_164">164</a>;</span><br />
+<span style="margin-left: 1em;">conduits, <a href="#par_140">140</a>;</span><br />
+<span style="margin-left: 1em;">fans, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">flat-irons, <a href="#par_146">146</a>;</span><br />
+<span style="margin-left: 1em;">gas lighters, <a href="#par_174">174</a>;</span><br />
+<span style="margin-left: 1em;">griddles, <a href="#par_147">147</a>;</span><br />
+<span style="margin-left: 1em;">kitchen, <a href="#par_147">147</a>;</span><br />
+<span style="margin-left: 1em;">lights, arc, <a href="#CHAPTER_XXII">Chap. XXII</a>.;</span><br />
+<span style="margin-left: 1em;">lights, incandescent, <a href="#CHAPTER_XXII">Chap. XXI</a>.;</span><br />
+<span style="margin-left: 1em;">machines, static, <a href="#par_7">7</a> to 13;</span><br />
+<span style="margin-left: 1em;">machines, uses of, <a href="#par_14">14</a>;</span><br />
+<span style="margin-left: 1em;">motor, the, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">motor, and how it does work, <a href="#CHAPTER_XXIV">Chap. XXIV</a>.;</span><br />
+<span style="margin-left: 1em;">soldering irons, <a href="#par_146">146</a>;</span><br />
+<span style="margin-left: 1em;">telegraph, and how it sends messages, <a href="#CHAPTER_XIV">Chap. XIV</a>.;</span><br />
+<span style="margin-left: 1em;">telephone, and how it transmits speech, <a href="#CHAPTER_XVI">Chap. XVI</a>.;</span><br />
+<span style="margin-left: 1em;">welding, <a href="#par_146">146</a>.</span><br />
+<br />
+Electric current, and work, <a href="#par_133">133</a>;<br />
+<span style="margin-left: 1em;">and chemical action, <a href="#par_81">81</a>;</span><br />
+<span style="margin-left: 1em;">chemical effects of, <a href="#CHAPTER_VII">Chap. VII</a>.;</span><br />
+<span style="margin-left: 1em;">how distributed for use, <a href="#CHAPTER_XIX">Chap. XIX</a>.;</span><br />
+<span style="margin-left: 1em;">magnetic effects of, <a href="#CHAPTER_XI">Chap. XI</a>.;</span><br />
+<span style="margin-left: 1em;">how transformed, <a href="#CHAPTER_XVIII">Chap. XVIII</a>.</span><br />
+<br />
+Electrical, connections, <a href="#par_60">60</a>;<br />
+<span style="margin-left: 1em;">horse-power, <a href="#par_77">77</a>;</span><br />
+<span style="margin-left: 1em;">measurements, <a href="#CHAPTER_VI">Chap. VI</a>.;</span><br />
+<span style="margin-left: 1em;">resistance, <a href="#par_68">68</a>;</span><br />
+<span style="margin-left: 1em;">resistance, unit of, <a href="#par_69">69</a>;</span><br />
+<span style="margin-left: 1em;">units, <a href="#CHAPTER_VI">Chap. VI</a>.</span><br />
+<br />
+Electricity, about frictional, <a href="#CHAPTER_I">Chap. I</a>.;<br />
+<span style="margin-left: 1em;">and chemical action, <a href="#par_81">81</a>;</span><br />
+<span style="margin-left: 1em;">atmospheric, <a href="#par_18">18</a>;</span><br />
+<span style="margin-left: 1em;">heat produced by, <a href="#CHAPTER_XX">Chap. XX</a>.;</span><br />
+<span style="margin-left: 1em;">history of, <a href="#par_3">3</a>;</span><br />
+<span class="pagenum"><a name="Page_173" id="Page_173">[173]</a></span><span style="margin-left: 1em;">how generated upon cat, <a href="#par_1">1</a>;</span><br />
+<span style="margin-left: 1em;">how generated by dynamos, <a href="#CHAPTER_XVII">Chap. XVII</a>.;</span><br />
+<span style="margin-left: 1em;">how generated by heat, <a href="#CHAPTER_X">Chap. X</a>.;</span><br />
+<span style="margin-left: 1em;">how generated by induction, <a href="#CHAPTER_XII">Chap. XII</a>.;</span><br />
+<span style="margin-left: 1em;">how generated by voltaic cell, <a href="#CHAPTER_III">Chap. III</a>.;</span><br />
+<span style="margin-left: 1em;">origin of name, <a href="#par_2">2</a>.</span><br />
+<br />
+Electrification, kinds of, <a href="#par_6">6</a>;<br />
+<span style="margin-left: 1em;">laws of, <a href="#par_7">7</a>.</span><br />
+<br />
+Electrolysis, <a href="#par_79">79</a>.<br />
+<br />
+Electrolyte, <a href="#par_79">79</a>.<br />
+<br />
+Electromagnetic induction, <a href="#par_99">99</a>.<br />
+<br />
+Electromagnetism, <a href="#par_91">91</a>.<br />
+<br />
+Electromagnets, <a href="#par_96">96</a>;<br />
+<span style="margin-left: 1em;">forms of, <a href="#par_97">97</a>.</span><br />
+<br />
+Electro-mechanical gong, <a href="#par_116">116</a>.<br />
+<br />
+Electromotive force, defined, <a href="#par_65">65</a>, <a href="#par_71">71</a>;<br />
+<span style="margin-left: 1em;">measurement of, <a href="#par_67">67</a>;</span><br />
+<span style="margin-left: 1em;">of polarization, <a href="#par_85">85</a>;</span><br />
+<span style="margin-left: 1em;">of static electricity, <a href="#par_17">17</a>;</span><br />
+<span style="margin-left: 1em;">unit of, <a href="#par_66">66</a>.</span><br />
+<br />
+Electrophorus, the, <a href="#par_8">8</a>.<br />
+<br />
+Electroplating, <a href="#par_82">82</a>.<br />
+<br />
+Electroscopes, <a href="#par_5">5</a>.<br />
+<br />
+Electrotyping, <a href="#par_83">83</a>.<br />
+<br />
+Experiments, early, with currents, <a href="#par_44">44</a>;<br />
+<span style="margin-left: 1em;">some simple, <a href="#par_1">1</a>.</span><br />
+<br />
+External resistance, <a href="#par_68">68</a>.<br />
+<br />
+<br />
+Fan motors, <a href="#par_162">162</a>.<br />
+<br />
+Field, magnetic, <a href="#par_37">37</a>.<br />
+<br />
+Field-magnets, <a href="#par_129">129</a>.<br />
+<br />
+Figures, magnetic, <a href="#par_38">38</a>.<br />
+<br />
+Filaments, carbon, <a href="#par_149">149</a>;<br />
+<span style="margin-left: 1em;">bamboo, etc., <a href="#par_149">149</a>.</span><br />
+<br />
+Fire, St. Elmo's, <a href="#par_22">22</a>.<br />
+<br />
+Flat-irons, electric, <a href="#par_147">147</a>.<br />
+<br />
+Floor mains, <a href="#par_139">139</a>.<br />
+<br />
+Fluoroscope, <a href="#par_158">158</a>.<br />
+<br />
+Force, and induced currents, <a href="#par_101">101</a>;<br />
+<span style="margin-left: 1em;">lines of magnetic, <a href="#par_38">38</a>;</span><br />
+<span style="margin-left: 1em;">lines of about a wire, <a href="#par_92">92</a>, <a href="#par_96">96</a>;</span><br />
+<span style="margin-left: 1em;">lines of about a magnet, <a href="#par_37">37</a>, <a href="#par_38">38</a>.</span><br />
+<br />
+Frictional electricity, about, <a href="#CHAPTER_I">Chap. I</a>.;<br />
+<span style="margin-left: 1em;">location of charge of, <a href="#par_4">4</a>;</span><br />
+<span style="margin-left: 1em;">sparks from, <a href="#par_4">4</a>.</span><br />
+<br />
+Fuller cell, the, <a href="#par_55">55</a>.<br />
+<br />
+Fuse, link, <a href="#par_142">142</a>;<br />
+<span style="margin-left: 1em;">plug, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">ribbons, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">wire, <a href="#par_142">142</a>.</span><br />
+<br />
+Fusible rosettes, <a href="#par_142">142</a>.<br />
+<br />
+<br />
+Galvani, early experiments of, <a href="#par_44">44</a>.<br />
+<br />
+Galvanometers, <a href="#par_73">73</a>;<br />
+<span style="margin-left: 1em;">astatic, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">considered as motor, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">D'Arsonval, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">tangent, <a href="#par_73">73</a>.</span><br />
+<br />
+Galvanoscope, <a href="#par_73">73</a>;<br />
+<span style="margin-left: 1em;">astatic, <a href="#par_94">94</a>.</span><br />
+<br />
+Gas lighters, electric, <a href="#par_174">174</a>.<br />
+<br />
+Geissler tubes, <a href="#par_156">156</a>.<br />
+<br />
+Generators, electric, <a href="#par_126">126</a>.<br />
+<br />
+Glass, electricity generated upon, <a href="#par_4">4</a>.<br />
+<br />
+Glue pots, electric, <a href="#par_147">147</a>.<br />
+<br />
+Gold-leaf, for electroscopes, <a href="#par_5">5</a>.<br />
+<br />
+Gold plating, <a href="#par_82">82</a>.<br />
+<br />
+Gonda cell, <a href="#par_57">57</a>.<br />
+<br />
+<span class="pagenum"><a name="Page_174" id="Page_174">[174]</a></span>Gong, electro-mechanical, <a href="#par_116">116</a>.<br />
+<br />
+Gravity cell, the, <a href="#par_56">56</a>;<br />
+<span style="margin-left: 1em;">replaced by dynamotors, <a href="#par_137">137</a>.</span><br />
+<br />
+Grenet cell, <a href="#par_52">52</a>.<br />
+<br />
+Griddles, electric, <a href="#par_147">147</a>.<br />
+<br />
+Guard, for lamps, <a href="#par_151">151</a>.<br />
+<br />
+<br />
+Heat, how generated by electricity, <a href="#CHAPTER_X">Chap. X</a>.;<br />
+<span style="margin-left: 1em;">and magnetism, <a href="#par_35">35</a>;</span><br />
+<span style="margin-left: 1em;">and resistance, <a href="#par_145">145</a>.</span><br />
+<br />
+Heat lightning, <a href="#par_19">19</a>.<br />
+<br />
+Heaters, for cars, <a href="#par_167">167</a>.<br />
+<br />
+History of electricity, <a href="#par_3">3</a>.<br />
+<br />
+Horse-power, electrical, <a href="#par_77">77</a>.<br />
+<br />
+Horseshoe, permanent magnets, <a href="#par_26">26</a>;<br />
+<span style="margin-left: 1em;">electromagnets, <a href="#par_97">97</a>, <a href="#par_98">98</a>.</span><br />
+<br />
+Human body, bones of, photographed by x-rays, <a href="#CHAPTER_XXIII">Chap. XXIII</a>.<br />
+<br />
+Hydrogen, action of in cell, <a href="#par_48">48</a>;<br />
+<span style="margin-left: 1em;">attraction of for oxygen, <a href="#par_85">85</a>.</span><br />
+<br />
+Incandescence, <a href="#par_148">148</a>.<br />
+<br />
+Incandescent lamp, <a href="#par_149">149</a>;<br />
+<span style="margin-left: 1em;">candle-power of, <a href="#par_150">150</a>;</span><br />
+<span style="margin-left: 1em;">current for, <a href="#par_150">150</a>;</span><br />
+<span style="margin-left: 1em;">light produced by, <a href="#CHAPTER_XXI">Chap. XXI</a>.;</span><br />
+<span style="margin-left: 1em;">construction of, <a href="#par_149">149</a>;</span><br />
+<span style="margin-left: 1em;">uses of, <a href="#par_151">151</a>.</span><br />
+<br />
+Inclination of magnetic needle, <a href="#par_42">42</a>.<br />
+<br />
+Indicating push-button, <a href="#par_61">61</a>.<br />
+<br />
+Induced currents, <a href="#par_127">127</a>;<br />
+<span style="margin-left: 1em;">and lines of force, <a href="#par_101">101</a>;</span><br />
+<span style="margin-left: 1em;">by rotary motion, <a href="#par_128">128</a>;</span><br />
+<span style="margin-left: 1em;">of induction coils, <a href="#par_105">105</a>;</span><br />
+<span style="margin-left: 1em;">of transformers, <a href="#par_135">135</a>.</span><br />
+<br />
+Induced magnetism, <a href="#par_36">36</a>.<br />
+<br />
+Induction, electricity generated by, <a href="#CHAPTER_XII">Chap. XII</a>.;<br />
+<span style="margin-left: 1.5em;">electromagnetic, <a href="#par_99">99</a>.</span><br />
+<br />
+Induction coils, condensers for, <a href="#par_104">104</a>;<br />
+<span style="margin-left: 1em;">construction of, <a href="#par_104">104</a>;</span><br />
+<span style="margin-left: 1em;">currents of, <a href="#par_105">105</a>;</span><br />
+<span style="margin-left: 1em;">how they work, <a href="#CHAPTER_XIII">Chap. XIII</a>.;</span><br />
+<span style="margin-left: 1em;">in telephone work, <a href="#par_124">124</a>;</span><br />
+<span style="margin-left: 1em;">uses of, <a href="#par_106">106</a>.</span><br />
+<br />
+Inductive influence of earth, <a href="#par_43">43</a>.<br />
+<br />
+Influence machines for medical purposes, <a href="#par_13">13</a>.<br />
+<br />
+Ink writing registers, <a href="#par_114">114</a>.<br />
+<br />
+Insulating tubing, <a href="#par_141">141</a>.<br />
+<br />
+Insulators, <a href="#par_141">141</a>;<br />
+<span style="margin-left: 1em;">and conductors, <a href="#par_4">4</a>, <a href="#par_138">138</a>;</span><br />
+<span style="margin-left: 1em;">feeder-wire, <a href="#par_167">167</a>;</span><br />
+<span style="margin-left: 1em;">for poles, <a href="#par_167">167</a>;</span><br />
+<span style="margin-left: 1em;">porcelain, <a href="#par_141">141</a>.</span><br />
+<br />
+Internal resistance, <a href="#par_68">68</a>.<br />
+<br />
+Interrupters, automatic current, <a href="#par_104">104</a>, <a href="#par_115">115</a>.<br />
+<br />
+Ions, <a href="#par_80">80</a>.<br />
+<br />
+Iron, electricity upon, by friction, <a href="#par_4">4</a>.<br />
+<br />
+<br />
+Jar, Leyden, <a href="#par_15">15</a>.<br />
+<br />
+Jarring magnets, effects of, <a href="#par_33">33</a>.<br />
+<br />
+<br />
+Keeper of magnets, <a href="#par_26">26</a>.<br />
+<br />
+Keys, telegraph, <a href="#par_109">109</a>.<br />
+<br />
+Kinds of electrification, <a href="#par_6">6</a>.<br />
+<br />
+Kitchen, electric, <a href="#par_147">147</a>.<br />
+<br />
+Knife switch, <a href="#par_62">62</a>.<br />
+<br />
+<br />
+<span class="pagenum"><a name="Page_175" id="Page_175">[175]</a></span>Lamp, incandescent, candle-power of, <a href="#par_150">150</a>;<br />
+<span style="margin-left: 1em;">cord, adjustable, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">current for, <a href="#par_150">150</a>;</span><br />
+<span style="margin-left: 1em;">dental, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">for desks, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">for throat, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">guard for, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">incandescent, <a href="#par_149">149</a>;</span><br />
+<span style="margin-left: 1em;">socket, <a href="#par_151">151</a>;</span><br />
+<span style="margin-left: 1em;">with half shade, <a href="#par_151">151</a>.</span><br />
+<br />
+Lamp, the arc, <a href="#par_153">153</a>;<br />
+<span style="margin-left: 1em;">how light is produced by, <a href="#CHAPTER_XXII">Chap. XXII</a>.;</span><br />
+<span style="margin-left: 1em;">double carbon, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">hand-feed focussing, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">for search-lights, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">single carbon, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">short, for basements, <a href="#par_153">153</a>;</span><br />
+<span style="margin-left: 1em;">for theater use, <a href="#par_153">153</a>.</span><br />
+<br />
+Lamp circuits, alternating system, <a href="#par_144">144</a>.<br />
+<br />
+Lamps, in parallel, <a href="#par_144">144</a>;<br />
+<span style="margin-left: 1em;">lamps in series, <a href="#par_144">144</a>;</span><br />
+<span style="margin-left: 1em;">three-wire system, <a href="#par_144">144</a>;</span><br />
+<span style="margin-left: 1em;">two-wire system, <a href="#par_144">144</a>.</span><br />
+<br />
+Laws, of electrification, <a href="#par_7">7</a>;<br />
+<span style="margin-left: 1em;">of magnetic attraction, <a href="#par_32">32</a>;</span><br />
+<span style="margin-left: 1em;">of resistance, <a href="#par_70">70</a>.</span><br />
+<br />
+Leaf electroscopes, <a href="#par_5">5</a>.<br />
+<br />
+Leclanché cell, <a href="#par_57">57</a>.<br />
+<br />
+Leyden, battery, <a href="#par_16">16</a>;<br />
+<span style="margin-left: 1em;">jar, <a href="#par_15">15</a>.</span><br />
+<br />
+Light, how produced by arc lamp, <a href="#CHAPTER_XXII">Chap. XXII</a>.;<br />
+<span style="margin-left: 1em;">how produced by incandescent lamp, <a href="#CHAPTER_XXI">Chap. XXI</a>.</span><br />
+<br />
+Lightning, <a href="#par_19">19</a>;<br />
+<span style="margin-left: 1em;">rods, <a href="#par_21">21</a>.</span><br />
+<br />
+Line, telegraph, <a href="#CHAPTER_XIV">Chap. XIV</a>.;<br />
+<span style="margin-left: 1em;">connections for, <a href="#par_111">111</a>;</span><br />
+<span style="margin-left: 1em;">operation of, <a href="#par_112">112</a>.</span><br />
+<br />
+Line suspension, for trolley-wires, <a href="#par_167">167</a>.<br />
+<br />
+Line wire, <a href="#par_111">111</a>.<br />
+<br />
+Lines of force, conductors of, <a href="#par_39">39</a>, <a href="#par_96">96</a>;<br />
+<span style="margin-left: 1em;">about the earth, <a href="#par_40">40</a>, <a href="#par_42">42</a>;</span><br />
+<span style="margin-left: 1em;">and induced currents, <a href="#par_101">101</a>;</span><br />
+<span style="margin-left: 1em;">about a magnet, <a href="#par_38">38</a>;</span><br />
+<span style="margin-left: 1em;">about a wire, <a href="#par_92">92</a>.</span><br />
+<br />
+Local currents, <a href="#par_47">47</a>.<br />
+<br />
+<br />
+Magnetic, bodies, <a href="#par_29">29</a>;<br />
+<span style="margin-left: 1em;">declination, <a href="#par_41">41</a>;</span><br />
+<span style="margin-left: 1em;">effects of electric current, <a href="#CHAPTER_XI">Chap. XI</a>.;</span><br />
+<span style="margin-left: 1em;">field, <a href="#par_37">37</a>;</span><br />
+<span style="margin-left: 1em;">figure of one bar magnet, <a href="#par_38">38</a>;</span><br />
+<span style="margin-left: 1em;">figure of two bar magnets, <a href="#par_38">38</a>;</span><br />
+<span style="margin-left: 1em;">figure of horseshoe magnet, <a href="#par_38">38</a>;</span><br />
+<span style="margin-left: 1em;">needle, dip of, <a href="#par_42">42</a>;</span><br />
+<span style="margin-left: 1em;">needles and compasses, <a href="#par_31">31</a>.</span><br />
+<br />
+Magnetism, and heat, <a href="#par_35">35</a>;<br />
+<span style="margin-left: 1em;">induced, <a href="#par_36">36</a>;</span><br />
+<span style="margin-left: 1em;">laws of, <a href="#par_32">32</a>;</span><br />
+<span style="margin-left: 1em;">residual, <a href="#par_34">34</a>;</span><br />
+<span style="margin-left: 1em;">retentivity, <a href="#par_34">34</a>;</span><br />
+<span style="margin-left: 1em;">temporary, <a href="#par_36">36</a>;</span><br />
+<span style="margin-left: 1em;">terrestrial, <a href="#par_40">40</a>;</span><br />
+<span style="margin-left: 1em;">theory of, <a href="#par_33">33</a>.</span><br />
+<br />
+Magneto, signal bells, <a href="#par_117">117</a>;<br />
+<span style="margin-left: 1em;">testing bells, <a href="#par_117">117</a>;</span><br />
+<span style="margin-left: 1em;">transmitter, <a href="#par_120">120</a>.</span><br />
+<br />
+Magnets, action upon each other, <a href="#par_32">32</a>;<br />
+<span style="margin-left: 1em;">artificial, <a href="#par_25">25</a>;</span><br />
+<span style="margin-left: 1em;">bar, <a href="#par_27">27</a>;</span><br />
+<span class="pagenum"><a name="Page_176" id="Page_176">[176]</a></span><span style="margin-left: 1em;">compound, <a href="#par_28">28</a>;</span><br />
+<span style="margin-left: 1em;">effects of jarring, <a href="#par_33">33</a>;</span><br />
+<span style="margin-left: 1em;">electro, <a href="#par_96">96</a>;</span><br />
+<span style="margin-left: 1em;">electro, forms of, <a href="#par_97">97</a>;</span><br />
+<span style="margin-left: 1em;">horseshoe, <a href="#par_26">26</a>;</span><br />
+<span style="margin-left: 1em;">and magnetism, about, <a href="#CHAPTER_II">Chap. II</a>.;</span><br />
+<span style="margin-left: 1em;">making of, <a href="#par_30">30</a>;</span><br />
+<span style="margin-left: 1em;">natural, <a href="#par_24">24</a>.</span><br />
+<br />
+Mains, electric, <a href="#par_139">139</a>.<br />
+<br />
+Man-holes, in conduits, <a href="#par_140">140</a>.<br />
+<br />
+Measurements, electric, <a href="#CHAPTER_VI">Chap. VI</a>.;<br />
+<span style="margin-left: 1em;">of current strength, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">of E.M.F., <a href="#par_67">67</a>.</span><br />
+<br />
+Meters, chemical, <a href="#par_78">78</a>;<br />
+<span style="margin-left: 1em;">permanent record, <a href="#par_77">77</a>.</span><br />
+<br />
+Microphone, the, <a href="#par_122">122</a>.<br />
+<br />
+Motion and currents, <a href="#par_160">160</a>.<br />
+<br />
+Motor, acting like dynamo, <a href="#par_163">163</a>;<br />
+<span style="margin-left: 1em;">armature of, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">controlling speed of, <a href="#par_165">165</a>;</span><br />
+<span style="margin-left: 1em;">electric, <a href="#par_161">161</a>;</span><br />
+<span style="margin-left: 1em;">electric, and how it does work, <a href="#CHAPTER_XXIV">Chap. XXIV</a>.;</span><br />
+<span style="margin-left: 1em;">fans, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">for automobiles, <a href="#par_169">169</a>;</span><br />
+<span style="margin-left: 1em;">for boats, <a href="#par_168">168</a>;</span><br />
+<span style="margin-left: 1em;">for pumping bellows, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">for running drill press, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">parts of, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">starting boxes for, <a href="#par_163">163</a>;</span><br />
+<span style="margin-left: 1em;">uses of, <a href="#par_162">162</a>.</span><br />
+<br />
+Motor-dynamos, <a href="#par_136">136</a>.<br />
+<br />
+Mouldings, for wires, <a href="#par_141">141</a>.<br />
+<br />
+<br />
+Name, electricity, origin of, <a href="#par_2">2</a>.<br />
+<br />
+Natural magnets, <a href="#par_24">24</a>.<br />
+<br />
+Needles, astatic, <a href="#par_94">94</a>;<br />
+<span style="margin-left: 1em;">dipping, <a href="#par_42">42</a>;</span><br />
+<span style="margin-left: 1em;">magnetic, <a href="#par_31">31</a>.</span><br />
+<br />
+Negative electrification, <a href="#par_5">5</a>.<br />
+<br />
+Non-conductors, <a href="#par_4">4</a>.<br />
+<br />
+North pole, magnetic of earth, <a href="#par_40">40</a>;<br />
+<span style="margin-left: 1em;">of magnets, <a href="#par_26">26</a>.</span><br />
+<br />
+Northern lights, <a href="#par_23">23</a>.<br />
+<br />
+<br />
+Ohm, the, <a href="#par_69">69</a>.<br />
+<br />
+Open circuit cells, <a href="#par_50">50</a>.<br />
+<br />
+Openers, for doors, <a href="#par_175">175</a>.<br />
+<br />
+Outfits, dental, <a href="#par_175">175</a>.<br />
+<br />
+Overhead trolley system, <a href="#par_166">166</a>.<br />
+<br />
+Oxygen, attraction for hydrogen, <a href="#par_85">85</a>.<br />
+<br />
+<br />
+Parallel arrangement of lamps, <a href="#par_144">144</a>.<br />
+<br />
+Peltier effect, <a href="#par_89">89</a>.<br />
+<br />
+Pendant, electric, <a href="#par_151">151</a>.<br />
+<br />
+Pith-ball electroscope, <a href="#par_5">5</a>.<br />
+<br />
+Plate electrical machine, <a href="#par_10">10</a>.<br />
+<br />
+Plates of cells, <a href="#par_45">45</a><i>a</i>.<br />
+<br />
+Plunge batteries, <a href="#par_53">53</a>;<br />
+<span style="margin-left: 1em;">large, <a href="#par_54">54</a>.</span><br />
+<br />
+Polarity of coils, <a href="#par_95">95</a>.<br />
+<br />
+Polarization, <a href="#par_84">84</a>;<br />
+<span style="margin-left: 1em;">electromotive force of, <a href="#par_85">85</a>;</span><br />
+<span style="margin-left: 1em;">of cells, <a href="#par_48">48</a>.</span><br />
+<br />
+Pole-changing switch, <a href="#par_62">62</a>.<br />
+<br />
+Poles, of cells, <a href="#par_45">45</a><i>a</i>;<br />
+<span style="margin-left: 1em;">of horseshoe magnet, <a href="#par_26">26</a>.</span><br />
+<br />
+Positive electrification, <a href="#par_6">6</a>.<br />
+<br />
+Potential, defined, <a href="#par_65">65</a>.<br />
+<br />
+Push-buttons, <a href="#CHAPTER_V">Chap. V</a>.;<br />
+<span style="margin-left: 1em;">indicating, <a href="#par_61">61</a>;</span><br />
+<span style="margin-left: 1em;">modifications of, <a href="#par_61">61</a>;</span><br />
+<span style="margin-left: 1em;">table clamp, <a href="#par_61">61</a>.</span><br />
+<br />
+<br />
+Quantity of electricity, <a href="#par_76">76</a>;<br />
+<span class="pagenum"><a name="Page_177" id="Page_177">[177]</a></span><span style="margin-left: 1em;">unit of, <a href="#par_76">76</a>.</span><br />
+<br />
+Rays, cathode, <a href="#par_157">157</a>;<br />
+<span style="margin-left: 1em;">x-rays, <a href="#par_158">158</a>.</span><br />
+<br />
+Receiver, telephone, <a href="#par_121">121</a>.<br />
+<br />
+Reflectors, for lamps, <a href="#par_151">151</a>.<br />
+<br />
+Registers, ink writing, <a href="#par_114">114</a>.<br />
+<br />
+Relay, the, <a href="#par_113">113</a>.<br />
+<br />
+Residual magnetism, <a href="#par_34">34</a>.<br />
+<br />
+Resistance, coils and boxes, <a href="#par_69">69</a>;<br />
+<span style="margin-left: 1em;">electrical, <a href="#par_68">68</a>;</span><br />
+<span style="margin-left: 1em;">external, <a href="#par_68">68</a>;</span><br />
+<span style="margin-left: 1em;">and heat, <a href="#par_145">145</a>;</span><br />
+<span style="margin-left: 1em;">internal, <a href="#par_68">68</a>;</span><br />
+<span style="margin-left: 1em;">laws of, <a href="#par_70">70</a>;</span><br />
+<span style="margin-left: 1em;">unit of, <a href="#par_69">69</a>.</span><br />
+<br />
+Retentivity, <a href="#par_34">34</a>.<br />
+<br />
+Risers, in buildings, <a href="#par_139">139</a>.<br />
+<br />
+Rods, lightning, <a href="#par_21">21</a>.<br />
+<br />
+Roentgen, Prof., <a href="#par_158">158</a>.<br />
+<br />
+Rosette, fusible, <a href="#par_142">142</a>.<br />
+<br />
+Running-gear, of automobiles, <a href="#par_169">169</a>.<br />
+<br />
+<br />
+Safety, devices, <a href="#par_142">142</a>;<br />
+<span style="margin-left: 1em;">fuse, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">fuse link, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">fuse plug, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">fuse ribbon, <a href="#par_142">142</a>;</span><br />
+<span style="margin-left: 1em;">fuse wire, <a href="#par_142">142</a>.</span><br />
+<br />
+Search-lights, <a href="#par_153">153</a>;<br />
+<span style="margin-left: 1em;">signals sent by, <a href="#par_153">153</a>.</span><br />
+<br />
+Secondary batteries, <a href="#par_86">86</a>;<br />
+<span style="margin-left: 1em;">uses of, <a href="#par_87">87</a>.</span><br />
+<br />
+Series arrangement of lamps, <a href="#par_144">144</a>.<br />
+<br />
+Series wound dynamo, <a href="#par_131">131</a>.<br />
+<br />
+Service wires, <a href="#par_139">139</a>.<br />
+<br />
+Shunt-wound dynamo, <a href="#par_131">131</a>.<br />
+<br />
+Signal bells, magneto, <a href="#par_117">117</a>.<br />
+<br />
+Simple cell, the, <a href="#par_45">45</a>, <a href="#par_49">49</a>.<br />
+<br />
+Single-fluid cells, <a href="#par_49">49</a>.<br />
+<br />
+Single-point switch, <a href="#par_62">62</a>.<br />
+<br />
+Single-stroke bell, <a href="#par_116">116</a>.<br />
+<br />
+Socket, for incandescent lamps, <a href="#par_151">151</a>.<br />
+<br />
+Soldering irons, electric, <a href="#par_147">147</a>.<br />
+<br />
+Sounders, telegraph, <a href="#par_110">110</a>;<br />
+<span style="margin-left: 1em;">home-made, <a href="#par_110">110</a>.</span><br />
+<br />
+Spark, effect of air pressure on, <a href="#par_155">155</a>.<br />
+<br />
+Sparks, from cells, <a href="#par_17">17</a>;<br />
+<span style="margin-left: 1em;">from frictional electricity, <a href="#par_4">4</a>.</span><br />
+<br />
+St. Elmo's fire, <a href="#par_22">22</a>.<br />
+<br />
+Starting boxes, for motors, <a href="#par_163">163</a>.<br />
+<br />
+Static electric machines, <a href="#par_8">8</a>.<br />
+<br />
+Static electricity, condensation of, <a href="#par_15">15</a>;<br />
+<span style="margin-left: 1em;">electromotive force of, <a href="#par_17">17</a>;</span><br />
+<span style="margin-left: 1em;">to test presence of, <a href="#par_5">5</a>;</span><br />
+<span style="margin-left: 1em;">uses of, <a href="#par_14">14</a>.</span><br />
+<br />
+Steam engines, in central stations, <a href="#par_170">170</a>.<br />
+<br />
+Steel, inductive influence of earth upon, <a href="#par_43">43</a>;<br />
+<span style="margin-left: 1em;">retentivity of, <a href="#par_26">26</a>.</span><br />
+<br />
+Storage batteries, the, and how they work, <a href="#CHAPTER_IX">Chap. IX</a>.;<br />
+<span style="margin-left: 1em;">for automobiles, <a href="#par_169">169</a>;</span><br />
+<span style="margin-left: 1em;">for boats, <a href="#par_168">168</a>;</span><br />
+<span style="margin-left: 1em;">for natural sources of power, <a href="#par_87">87</a>.</span><br />
+<br />
+Stoves, electric, <a href="#par_147">147</a>.<br />
+<br />
+Strength of current, <a href="#par_71">71</a>;<br />
+<span style="margin-left: 1em;">measurement of, <a href="#par_73">73</a>;</span><br />
+<span style="margin-left: 1em;">unit of, <a href="#par_72">72</a>.</span><br />
+<br />
+Switchboards, <a href="#par_62">62</a>.<br />
+<br />
+Switches, <a href="#CHAPTER_V">Chap. V</a>.;<br />
+<span style="margin-left: 1em;">knife, <a href="#par_62">62</a>;</span><br />
+<span style="margin-left: 1em;">pole-changing, <a href="#par_62">62</a>;</span><br />
+<span style="margin-left: 1em;">single point, <a href="#par_62">62</a>;</span><br />
+<span class="pagenum"><a name="Page_178" id="Page_178">[178]</a></span><span style="margin-left: 1em;">for trolley lines, <a href="#par_167">167</a>.</span><br />
+<br />
+Table clamp-push, <a href="#par_61">61</a>.<br />
+<br />
+Tangent galvanometer, <a href="#par_73">73</a>.<br />
+<br />
+Teakettles, electric, <a href="#par_147">147</a>.<br />
+<br />
+Telegraph, electric, and how it sends messages, <a href="#CHAPTER_XIV">Chap. XIV</a>.;<br />
+<span style="margin-left: 1em;">ink writing registers, <a href="#par_114">114</a>;</span><br />
+<span style="margin-left: 1em;">keys, <a href="#par_109">109</a>;</span><br />
+<span style="margin-left: 1em;">relay, <a href="#par_113">113</a>;</span><br />
+<span style="margin-left: 1em;">sounders, <a href="#par_110">110</a>.</span><br />
+<br />
+Telegraph line, <a href="#par_107">107</a>, <a href="#par_108">108</a>;<br />
+<span style="margin-left: 1em;">operation of, <a href="#par_112">112</a>;</span><br />
+<span style="margin-left: 1em;">simple connections of, <a href="#par_111">111</a>.</span><br />
+<br />
+Telephone, the, and how it transmits speech, <a href="#CHAPTER_XVI">Chap. XVI</a>.;<br />
+<span style="margin-left: 1em;">receiver, <a href="#par_121">121</a>;</span><br />
+<span style="margin-left: 1em;">transmitter, <a href="#par_120">120</a>;</span><br />
+<span style="margin-left: 1em;">use of induction coil with, <a href="#par_124">124</a>;</span><br />
+<span style="margin-left: 1em;">various forms of, <a href="#par_125">125</a>.</span><br />
+<br />
+Temporary magnetism, <a href="#par_36">36</a>.<br />
+<br />
+Terrestrial magnetism, <a href="#par_40">40</a>.<br />
+<br />
+Theory of magnetism, <a href="#par_33">33</a>.<br />
+<br />
+Thermoelectricity, <a href="#par_88">88</a>.<br />
+<br />
+Thermopiles, <a href="#par_90">90</a>.<br />
+<br />
+Three-wire system, <a href="#par_144">144</a>.<br />
+<br />
+Throat, lamp for, <a href="#par_151">151</a>.<br />
+<br />
+Thunder, <a href="#par_20">20</a>.<br />
+<br />
+Toepler-Holtz machines, <a href="#par_11">11</a>.<br />
+<br />
+Transformers, <a href="#par_135">135</a>.<br />
+<br />
+Transforming electric current, <a href="#CHAPTER_XVIII">Chap. XVIII</a>.;<br />
+<span style="margin-left: 1em;">for electric welding, <a href="#par_146">146</a>.</span><br />
+<br />
+Transmission of currents, <a href="#par_134">134</a>.<br />
+<br />
+Transmitter, Bell, <a href="#par_120">120</a>;<br />
+<span style="margin-left: 1em;">carbon, <a href="#par_123">123</a>.</span><br />
+<br />
+Trembling bell, <a href="#par_116">116</a>.<br />
+<br />
+Trolley-wires, <a href="#par_164">164</a>;<br />
+<span style="margin-left: 1em;">-poles, <a href="#par_164">164</a>;</span><br />
+<span style="margin-left: 1em;">-wheels, <a href="#par_164">164</a>.</span><br />
+<br />
+Tubes, Crookes, <a href="#par_156">156</a>, <a href="#par_158">158</a>;<br />
+<span style="margin-left: 1em;">Geissler, <a href="#par_156">156</a>;</span><br />
+<span style="margin-left: 1em;">vacuum, <a href="#par_156">156</a>.</span><br />
+<br />
+Two-fluid cells, <a href="#par_49">49</a>.<br />
+<br />
+Two-wire system, <a href="#par_144">144</a>.<br />
+<br />
+<br />
+Underground trolley system 166;<br />
+<span style="margin-left: 1em;">conduits for, <a href="#par_166">166</a>.</span><br />
+<br />
+Unit, of current strength, <a href="#par_72">72</a>;<br />
+<span style="margin-left: 1em;">of electromotive force, <a href="#par_66">66</a>;</span><br />
+<span style="margin-left: 1em;">of quantity, <a href="#par_76">76</a>;</span><br />
+<span style="margin-left: 1em;">of resistance, <a href="#par_69">69</a>.</span><br />
+<br />
+Units, electrical, <a href="#CHAPTER_VI">Chap. VI</a>.<br />
+<br />
+Uses, of armatures, <a href="#par_39">39</a>;<br />
+<span style="margin-left: 1em;">of electricity, miscellaneous, <a href="#CHAPTER_XXVII">Chap. XXVII</a>.;</span><br />
+<span style="margin-left: 1em;">of induction coils, <a href="#par_106">106</a>;</span><br />
+<span style="margin-left: 1em;">of motors, <a href="#par_162">162</a>;</span><br />
+<span style="margin-left: 1em;">of storage batteries, <a href="#par_87">87</a>.</span><br />
+<br />
+<br />
+Vacuum-tubes, <a href="#par_156">156</a>.<br />
+<br />
+Variation, angle of, <a href="#par_41">41</a>.<br />
+<br />
+Volt, the, <a href="#par_66">66</a>.<br />
+<br />
+Volta, <a href="#par_66">66</a>;<br />
+<span style="margin-left: 1em;">early experiments of, <a href="#par_44">44</a>.</span><br />
+<br />
+Voltaic cell, electricity generated by, <a href="#CHAPTER_III">Chap. III</a>.<br />
+<br />
+Voltaic pile, <a href="#par_44">44</a>.<br />
+<br />
+Voltameters, <a href="#par_75">75</a>;<br />
+<span style="margin-left: 1em;">copper, <a href="#par_75">75</a>;</span><br />
+<span style="margin-left: 1em;">water, <a href="#par_75">75</a>.</span><br />
+<br />
+Voltmeters, <a href="#par_67">67</a>, <a href="#par_77">77</a>.<br />
+<br />
+<br />
+Water, decomposition of, <a href="#par_79">79</a>;<br />
+<span style="margin-left: 1em;">power, source of energy, <a href="#par_170">170</a>;</span><br />
+<span style="margin-left: 1em;">voltameters, <a href="#par_73">73</a>.</span><br />
+<br />
+Watt, the, <a href="#par_77">77</a>.<br />
+<br />
+Wattmeters, <a href="#par_77">77</a>.<br />
+<br />
+<span class="pagenum"><a name="Page_179" id="Page_179">[179]</a></span>Welding, electric, <a href="#par_146">146</a>.<br />
+<br />
+Wimshurst electric machine, <a href="#par_12">12</a>.<br />
+<br />
+Wires and cables, <a href="#par_143">143</a>.<br />
+<br />
+Wiring, for alternating system, <a href="#par_144">144</a>;<br />
+<span style="margin-left: 1em;">three-wire system, <a href="#par_144">144</a>;</span><br />
+<span style="margin-left: 1em;">two-wire system, <a href="#par_144">144</a>.</span><br />
+<br />
+Work, and electric current, <a href="#par_133">133</a>.<br />
+<br />
+<br />
+X-ray photographs, <a href="#par_159">159</a>.<br />
+<br />
+X-rays, <a href="#par_156">156</a>;<br />
+<span style="margin-left: 1em;">and how the bones of the human body are photographed, <a href="#CHAPTER_XXIII">Chap. XXIII</a>.</span><br />
+<br />
+<br />
+Yokes, <a href="#par_97">97</a>, <a href="#par_98">98</a>.<br />
+<br />
+<br />
+Zincs, amalgamation of, <a href="#par_47">47</a>.<br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_180" id="Page_180">[180]</a></span></p>
+
+
+
+
+<div class='adtitle2'>THINGS A BOY SHOULD KNOW<br />
+ABOUT ELECTRICITY.</div>
+
+
+<div class='center'><br /><b>By THOMAS M. ST. JOHN, Met. E.</b><br />
+<br />
+
+The book contains 180 pages, and 260 illustrations; it measures<br />
+5 x 7½ in., and is bound in cloth.<br />
+<br />
+<b>PRICE, POST-PAID, $1.00.</b><br />
+</div>
+
+<blockquote>
+
+<p><b>CONTENTS:</b> <i>Chapter</i> I. About Frictional Electricity.&mdash;II. About Magnets
+and Magnetism.&mdash;III. How Electricity is Generated by the Voltaic Cell.&mdash;IV.
+Various Voltaic Cells.&mdash;V. About Push-Buttons, Switches and Binding-Posts.&mdash;VI.
+Units and Apparatus for Electrical Measurements.&mdash;VII. Chemical Effects
+of the Electric Current.&mdash;VIII. How Electroplating and Electrotyping are
+Done.&mdash;IX. The Storage Battery and How it Works.&mdash;X. How Electricity is
+Generated by Heat.&mdash;XI. Magnetic Effects of the Electric Current.&mdash;XII. How
+Electricity is Generated by Induction.&mdash;XIII. How the Induction Coil Works.&mdash;XIV.
+The Electric Telegraph, and How it Sends Messages.&mdash;XV. The Electric
+Bell and Some of its Uses.&mdash;XVI. The Telephone, and How it Transmits Speech.&mdash;XVII.
+How Electricity is Generated by Dynamos.&mdash;XVIII. How the Electric
+Current is Transformed.&mdash;XIX. How Electric Currents are Distributed for
+Use.&mdash;XX. How Heat is Produced by the Electric Current.&mdash;XXI. How Light
+is Produced by the Incandescent Lamp.&mdash;XXII. How Light is Produced by the
+Arc Lamp.&mdash;XXIII. X-Rays, and How the Bones of the Human Body are Photographed.&mdash;XXIV.
+The Electric Motor and How it Does Work.&mdash;XXV. Electric
+Cars, Boats and Automobiles.&mdash;XXVI. A Word About Central Stations.&mdash;XXVII.
+Miscellaneous Uses of Electricity.</p></blockquote>
+
+<p>This book explains, in simple, straightforward language, many
+things about electricity; things in which the American boy is intensely
+interested; things he wants to know; things he should
+know.</p>
+
+<p>It is free from technical language and rhetorical frills, but it
+tells how things work, and why they work.</p>
+
+<p>It is brimful of illustrations&mdash;the best that can be had&mdash;illustrations
+that are taken directly from apparatus and machinery,
+and that show what they are intended to show.</p>
+
+<p>This book does not contain experiments, or tell how to make
+apparatus; our other books do that. After explaining the simple
+principles of electricity, it shows how these principles are used
+and combined to make electricity do every-day work.</p>
+
+<div class='center'>
+<i>Everyone Should Know About Electricity.</i><br />
+<br />
+<b>A VERY APPROPRIATE PRESENT</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_181" id="Page_181">[181]</a></span></p>
+
+
+
+
+<div class='adtitle2'><span class="smcap">Third Edition</span></div>
+<div class='adtitle2'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />How Two Boys Made Their Own<br />
+Electrical Apparatus.</div>
+
+
+<blockquote>
+
+<p>Containing complete directions for making all kinds of simple
+electrical apparatus for the study of elementary electricity. By
+<span class="smcap">Professor Thomas M. St. John</span>, New York City.</p>
+
+<p>The book measures 5 × 7½ in., and is beautifully bound in
+cloth. It contains 141 pages and 125 illustrations. Complete
+directions are given for making 152 different pieces of Apparatus
+for the practical use of students, teachers, and others who wish
+to experiment.</p></blockquote>
+
+<div class='center'>
+<b><big>PRICE, POST-PAID, $1.00.</big></b><br />
+</div>
+
+<p>The shocking coils, telegraph instruments, batteries, electromagnets,
+motors, etc., etc., are so simple in construction that any
+boy of average ability can make them; in fact, the illustrations
+have been made directly from apparatus constructed by young boys.</p>
+
+<p>The author has been working along this line for several years, and
+he has been able, <i>with the help of boys</i>, to devise a complete line of
+simple electrical apparatus.</p>
+
+
+<blockquote>
+
+<p><b><i>THE APPARATUS IS SIMPLE because the designs and
+methods of construction have been worked out practically
+in the school-room, absolutely no machine-work
+being required.</i></b></p>
+
+<p><b><i>THE APPARATUS IS PRACTICAL because it has been
+designed for real use in the experimental study of
+elementary electricity.</i></b></p>
+
+<p><b><i>THE APPARATUS IS CHEAP because most of the parts
+can be made of old tin cans and cracker boxes, bolts,
+screws, wires and wood.</i></b></p></blockquote>
+
+
+<div class='center'>
+<b>Address, THOMAS M. ST. JOHN,</b><br />
+<span style="margin-left: 6em;"><b>407 West 51st Street,</b></span><br />
+<span style="margin-left: 15em;"><b>New York.</b></span><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_182" id="Page_182">[182]</a></span></p>
+
+
+
+
+<div class='adtitle2'>How Two Boys Made Their Own<br />
+Electrical Apparatus.</div>
+
+
+<p><b>CONTENTS:</b> <i>Chapter</i> I. Cells and Batteries.&mdash;II. Battery Fluids and Solutions.&mdash;III.
+Miscellaneous Apparatus and Methods of Construction.&mdash;IV.
+Switches and Cut-Outs.&mdash;V. Binding-Posts and Connectors.&mdash;VI. Permanent
+Magnets,&mdash;VII. Magnetic Needles and Compasses.&mdash;VIII. Yokes and Armatures.&mdash;IX.
+Electro-Magnets.&mdash;X. Wire-Winding Apparatus.&mdash;XI. Induction
+Coils and Their Attachments.&mdash;XII. Contact Breakers and Current Interrupters.&mdash;XIII.
+Current Detectors and Galvanometers.&mdash;XIV. Telegraph Keys
+and Sounders.&mdash;XV. Electric Bells and Buzzers.&mdash;XVI. Commutators and Current
+Reversers.&mdash;XVII. Resistance Coils.&mdash;XVIII. Apparatus for Static Electricity.&mdash;XIX.
+Electric Motors.&mdash;XX. Odds and Ends.&mdash;XXI. Tools and Materials.</p>
+
+<p>"The author of this book is a teacher and wirier of great ingenuity,
+and we imagine that the effect of such a book as this falling into juvenile
+hands must be highly stimulating and beneficial. It is full of explicit
+details and instructions in regard to a great variety of apparatus, and the
+materials required are all within the compass of very modest pocket-money.
+Moreover, it is systematic and entirely without rhetorical frills,
+so that the student can go right along without being diverted from good
+helpful work that will lead him to build useful apparatus and make him
+understand what he is about. The drawings are plain and excellent. We
+heartily commend the book."&mdash;<i>Electrical Engineer.</i></p>
+
+
+<p>"Those who visited the electrical exhibition last May cannot have
+failed to notice on the south gallery a very interesting exhibit, consisting,
+as it did, of electrical apparatus made by boys. The various devices there
+shown, comprising electro-magnets, telegraph keys and sounders, resistance
+coils, etc., were turned out by boys following the instructions given
+in the book with the above title, which is unquestionably one of the most
+practical little works yet written that treat of similar subjects, for with
+but a limited amount of mechanical knowledge, and by closely following
+the instructions given, almost any electrical device may be made at very
+small expense. That such a book fills a long-felt want may be inferred
+from the number of inquiries we are constantly receiving from persons
+desiring to make their own induction coils and other apparatus."&mdash;<i>Electricity.</i></p>
+
+
+<p>"At the electrical show in New York last May one of the most interesting
+exhibits was that of simple electrical apparatus made by the boys
+in one of the private schools in the city. This apparatus, made by boys of
+thirteen to fifteen years of age, was from designs by the author of this
+clever little book, and it was remarkable to see what an ingenious use had
+been made of old tin tomato-cans, cracker-boxes, bolts, screws, wire, and
+wood. With these simple materials telegraph instruments, coils, buzzers,
+current detectors, motors, switches, armatures, and an almost endless
+variety of apparatus were made, In this book Mr. St. John has given
+directions in simple language for making and using these devices, and has
+illustrated these directions with admirable diagrams and cuts. The little
+volume is unique, and will prove exceedingly helpful to those of our
+young readers who are fortunate enough to possess themselves of a copy.
+For schools where a course of elementary science is taught, no better text-book
+in the first-steps in electricity is obtainable."&mdash;<i>The Great Round
+World.</i></p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_183" id="Page_183">[183]</a></span></p>
+
+
+
+
+<div class='adtitle2'>Exhibit of Experimental Electrical Apparatus</div>
+<div class='center'><b>AT THE ELECTRICAL SHOW, MADISON SQUARE GARDEN, NEW YORK.</b></div>
+
+
+<p>While only 40 pieces of simple apparatus were shown in this exhibit, it gave visitors something of an idea
+of what young boys can do if given proper designs.</p>
+
+<div class="figcenter" style="width: 328px;">
+<a href="images/i_183-big.jpg"><img src="images/i_183.jpg" width="328" height="119" alt="Photograph" /></a>
+<div class="caption">"HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS"<br />Gives Proper Designs&mdash;Designs for over 150 Things.</div>
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_184" id="Page_184">[184]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Photography</span></div>
+<div class='center'><b>BOOK AND COMPLETE OUTFIT.</b></div>
+
+
+<div class="figcenter" style="width: 592px;">
+<img src="images/i_184.jpg" width="592" height="231" alt="photograph" />
+</div>
+
+<p><b>PHOTOGRAPHY is now an educational amusement, and to
+many it is the most fascinating of all amusements. The magic of
+sunshine, the wonders of nature, and the beauties of art are tools
+in the hand of the amateur photographer.</b></p>
+
+<p><b>A great many things can be done with this outfit, and it will give
+an insight into this most popular pastime.</b></p>
+
+
+<blockquote>
+
+<p><b>THE OUTFIT</b> contains everything necessary for making ordinary prints&mdash;together
+with other articles to be used in various ways. The following things
+are included: One Illustrated Book of Instructions, called "Fun With Photography;"
+1 Package of Sensitized Paper; 1 Printing Frame, including Glass,
+Back, and Spring; 1 Set of Masks for Printing Frame; 1 Set of Patterns for
+Fancy Shapes; 1 Book of Negatives (Patent Pending) Ready for Use; 6 Sheets
+of Blank Negative Paper; 1 Alphabet Sheet; 1 Package of Card Mounts; 1
+Package of Folding Mounts; 1 Package of "Fixo."</p>
+
+<p><b>CONTENTS OF BOOK:</b>&mdash;<b>Chapter I. Introduction.</b>&mdash;Photography.&mdash;Magic
+Sunshine.&mdash;The Outfit.&mdash;<b>II. General Instructions.</b>&mdash;The Sensitized Paper.&mdash;How
+the Effects are Produced.&mdash;Negatives.&mdash;Prints.&mdash;Printing Frames.&mdash;Our
+Printing Frame.&mdash;Putting Negatives in Printing Frame.&mdash;Printing.&mdash;Developing.&mdash;Fixing.&mdash;Drying.&mdash;Trimming.&mdash;Fancy
+Shapes.&mdash;Mounting.&mdash;<b>III. Negatives
+and How to Make Them.</b>&mdash;The Paper.&mdash;Making Transparent Paper.&mdash;Making
+the Negatives.&mdash;Printed Negatives.&mdash;Perforated Negatives.&mdash;Negatives
+Made from Magazine Pictures.&mdash;Ground Glass Negatives.&mdash;<b>IV. Nature Photography.</b>&mdash;Aids
+to Nature Study.&mdash;Ferns and Leaves.&mdash;Photographing Leaves.&mdash;Perforating
+Leaves.&mdash;Drying Leaves, Ferns, etc., for Negatives.&mdash;Flowers.&mdash;<b>V.
+Miscellaneous Photographs.</b>&mdash;Magnetic Photographs.&mdash;Combination Pictures.&mdash;Initial
+Pictures.&mdash;Name Plates.&mdash;Christmas, Easter and Birthday Cards.</p></blockquote>
+
+<div class='center'>
+<b><i>The Book and Complete Outfit will be sent, by mail or<br />
+express, Charges Prepaid, upon receipt of 65 Cents, by</i></b><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 W. 51st St., New York.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_185" id="Page_185">[185]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Magnetism.</span></div>
+<div class='center'><b>BOOK AND COMPLETE OUTFIT FOR SIXTY-ONE<br />
+EXPERIMENTS IN MAGNETISM.&nbsp;.&nbsp;.</b></div>
+
+
+<div class="figcenter" style="width: 419px;">
+<a href="images/i_185-big.jpg"><img src="images/i_185.jpg" width="419" height="235" alt="drawing" /></a>
+</div>
+
+<p>Children like to do experiments; and in this way, better than in
+any other, <i>a practical knowledge of the elements of magnetism</i> may be
+obtained.</p>
+
+<p>These experiments, although arranged to <i>amuse</i> boys and girls,
+have been found to be very <i>useful in the class-room</i> to supplement
+the ordinary exercises given in text-books of science.</p>
+
+<p>To secure the <i>best possible quality of apparatus</i>, the horseshoe
+magnets were made at Sheffield, England, especially for these sets.
+They are new and strong. Other parts of the apparatus have also
+been selected and made with great care, to adapt them particularly
+to these experiments.&mdash;<i>From the author's preface.</i></p>
+
+
+<blockquote>
+
+<p><b>CONTENTS.</b>&mdash;Experiments With Horseshoe Magnet.&mdash;Experiments
+With Magnetized Needles.&mdash;Experiments With Needles, Corks, Wires, Nails,
+etc.&mdash;Experiments With Bar Magnets.&mdash;Experiments With Floating Magnets.&mdash;Miscellaneous
+Experiments.&mdash;Miscellaneous Illustrations showing what very
+small children can do with the Apparatus.&mdash;Diagrams showing how Magnetized
+Needles may be used by little children to make hundreds of pretty designs
+upon paper.</p>
+
+
+<p><b>AMUSING EXPERIMENTS.</b>&mdash;Something for Nervous People to
+Try.&mdash;The Jersey Mosquito.&mdash;The Stampede.&mdash;The Runaway.&mdash;The Dog-fight.&mdash;The
+Whirligig.&mdash;The Naval Battle.&mdash;A String of Fish.&mdash;A Magnetic Gun.&mdash;A
+Top Upsidedown.&mdash;A Magnetic Windmill.&mdash;A Compass Upsidedown.&mdash;The
+Magnetic Acrobat.&mdash;The Busy Ant-hill.&mdash;The Magnetic Bridge.&mdash;The Merry-go-Round.&mdash;The
+Tight-rope Walker.&mdash;A Magnetic Motor Using Attractions and
+Repulsions.</p></blockquote>
+
+<div class='center'>
+<b><i>The Book and Complete Outfit will be sent, Post-paid,<br />
+upon receipt of 35 Cents, by</i></b><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 W. 51st St., New York.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_186" id="Page_186">[186]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">FUN WITH SHADOWS</span></div>
+<div class='center'><b>BOOK AND COMPLETE OUTFIT FOR SHADOW PICTURES,<br />
+PANTOMIMES, ENTERTAINMENTS, Etc., Etc.</b></div>
+
+
+<div class="figcenter" style="width: 584px;">
+<img src="images/i_186.jpg" width="584" height="185" alt="keystone cops outline" />
+</div>
+
+<p><b>Shadow Making</b> has been a very popular amusement
+for several centuries. There is a great deal of <i>fun</i>
+and instruction in it, and its long life is due to the fact
+that it has always been a source of keen delight to grown
+people as well as to children.</p>
+
+<p>In getting material together for this little book, the
+author has been greatly aided by English, French and
+American authors, some of whom are professional shadowists.
+It has been the author's special effort to get the
+subject and apparatus into a practical, cheap form for
+boys and girls.</p>
+
+
+<blockquote>
+
+<p><b>THE OUTFIT</b> contains everything necessary for all ordinary shadow
+pictures, shadow entertainments, shadow plays, etc. The following articles
+are included:</p>
+
+<p>One book of Instructions called "Fun with Shadows"; 1 Shadow Screen;
+2 Sheets of Tracing Paper; 1 Coil of Wire for Movable Figures; 1 Cardboard
+Frame for Circular Screen; 1 Cardboard House for Stage Scenery; 1 Jointed
+Wire Fish-pole and Line; 2 Bent Wire Scenery Holders; 4 Clamps for Screen;
+1 Wire Figure Support; 1 Wire for Oar; 2 Spring Wire Table Clamps; 1 Wire
+Candlestick Holder; 5 Cardboard Plates containing the following printed figures
+that should be cut out with shears: 12 Character Hats; 1 Boat; 1 Oar-blade; 1
+Fish; 1 Candlestick; 1 Cardboard Plate containing printed parts for making
+movable figures.</p>
+
+<p><b>CONTENTS OF BOOK:</b> One Hundred Illustrations and Diagrams, including
+Ten Full-page Book Plates, together with Six Full-page Plates on Cardboard.</p>
+
+<p><i>Chapter</i> I. Introduction.&mdash;II. General Instructions.&mdash;III. Hand Shadows of
+Animals.&mdash;IV. Hand Shadows of Heads, Character Faces, etc.&mdash;V. Moving
+Shadow Figures and How to Make Them.&mdash;VI. Shadow Pantomimes.&mdash;VII.
+Miscellaneous Shadows.</p></blockquote>
+
+<div class='center'>
+<i>The Book and Complete Outfit will be sent, <b>POST-PAID</b>,<br />
+upon receipt of 35 cents, by</i><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 West 51st St., New York City.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_187" id="Page_187">[187]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Electricity.</span></div>
+<div class='center'><b>BOOK AND COMPLETE OUTFIT FOR SIXTY<br />
+EXPERIMENTS IN ELECTRICITY.&nbsp;.&nbsp;.&nbsp;.</b></div>
+
+
+<div class="figcenter" style="width: 376px;">
+<a href="images/i_187-big.jpg"><img src="images/i_187.jpg" width="376" height="258" alt="drawing" /></a>
+</div>
+
+<p>Enough of the principles of electricity are brought out to make
+the book instructive as well as amusing. The experiments are
+systematically arranged, and make a fascinating science course. No
+chemicals, no danger.</p>
+
+<p>The book is conversational and not at all "schooly," Harry and
+Ned being two boys who perform the experiments and talk over the
+results as they go along.</p>
+
+<p>"The book reads like a story."&mdash;"An appropriate present for a
+boy or girl."&mdash;"Intelligent parents will appreciate 'Fun With Electricity.'"&mdash;"Very
+complete, because it contains both book and
+apparatus."&mdash;"There is no end to the fun which a boy or girl can
+have with this fascinating amusement."</p>
+
+
+<blockquote>
+
+<p><b>THERE IS FUN IN THESE EXPERIMENTS.</b>&mdash;Chain Lightning.&mdash;An
+Electric Whirligig.&mdash;The Baby Thunderstorm.&mdash;A Race with Electricity.&mdash;An
+Electric Frog Pond.&mdash;An Electric Ding-Dong.&mdash;The Magic Finger.&mdash;Daddy
+Long-Legs.&mdash;Jumping Sally.&mdash;An Electric Kite.&mdash;Very Shocking.&mdash;Condensed
+Lightning.&mdash;An Electric Fly-Trap.&mdash;The Merry Pendulum.&mdash;An
+Electric Ferry-Boat.&mdash;A Funny Piece of Paper.&mdash;A Joke on the Family Cat.&mdash;Electricity
+Plays Leap-Frog.&mdash;Lightning Goes Over a Bridge.&mdash;Electricity
+Carries a Lantern.&mdash;And <i><b>40 Others</b></i>.</p>
+
+<p>The <b><i>OUTFIT</i></b> contains 20 different articles. The <b><i>BOOK OF INSTRUCTION</i></b>
+measures 5 x 7½ inches, and has 38 illustrations, 55 pages, good paper
+and clear type.</p></blockquote>
+
+<div class='center'>
+<b><i>The Book, and Complete Outfit will be sent, by mail or<br />
+express, Charges Prepaid, upon receipt of 65 Cents, by</i></b><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 W. 51st St., New York.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_188" id="Page_188">[188]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Puzzles.</span></div>
+<div class='center'><b>BOOK, KEY, AND COMPLETE OUTFIT FOR<br />
+FOUR HUNDRED PUZZLES.&nbsp;.&nbsp;.</b></div>
+
+
+<p>The BOOK measures 5 × 7½ inches. It is well printed, nicely
+bound, and contains 15 chapters, 80 pages, and 128 illustrations.
+The KEY is illustrated. It is bound with the book, and contains
+the solution of every puzzle. The COMPLETE OUTFIT
+is placed in a neat box with the book. It consists of numbers,
+counters, figures, pictures, etc., for doing the puzzles.</p>
+
+<blockquote>
+
+<p><b>CONTENTS:</b> <i>Chapter</i> (1) Secret Writing. (2) Magic Triangles, Squares,
+Rectangles, Hexagons, Crosses, Circles, etc. (3) Dropped Letter and Dropped
+Word Puzzles. (4) Mixed Proverbs, Prose and Rhyme. (5) Word Diamonds,
+Squares, Triangles, and Rhomboids. (6) Numerical Enigmas. (7) Jumbled
+Writing and Magic Proverbs. (8) Dissected Puzzles. (9) Hidden and Concealed
+Words. (10) Divided Cakes, Pies, Gardens, Farms, etc. (11) Bicycle and Boat
+Puzzles. (12) Various Word and Letter Puzzles. (13) Puzzles with Counters.
+(14) Combination Puzzles. (15) Mazes and Labyrinths.</p></blockquote>
+
+<p>"Fun With Puzzles" is a book that every boy and girl should
+have. It is amusing, instructive,&mdash;educational. It is just the thing
+to wake up boys and girls and make them think. They like it,
+because it is real fun. This sort of educational play should be given
+in every school-room and in every home.</p>
+
+<p>"Fun With Puzzles" will puzzle your friends, as well as yourself;
+it contains some real brain-splitters. Over 300 new and original
+puzzles are given, besides many that are hundreds of years old.</p>
+
+<p><b>Secret Writing.</b> Among the many things that "F. W. P." contains,
+is the key to <i>secret writing</i>. It shows you a very simple way
+to write letters to your friends, and it is simply impossible for others
+to read what you have written, unless they know the secret. This,
+alone is a valuable thing for any boy or girl who wants to have
+some fun.</p>
+
+<div class='center'>
+<b><i>The Book, Key, and Complete Outfit will be sent, postpaid,<br />
+upon receipt of 35 cents, by</i></b><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 West 51st St., New York City.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_189" id="Page_189">[189]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Soap-Bubbles.</span></div>
+<div class='center'><b>BOOK AND COMPLETE OUTFIT FOR FANCY<br />
+BUBBLES AND FILMS.&nbsp;.&nbsp;.&nbsp;.</b></div>
+
+
+<div class="figcenter" style="width: 379px;">
+<a href="images/i_189-big.jpg"><img src="images/i_189.jpg" width="379" height="280" alt="drawing" /></a>
+</div>
+
+<p><b>THE OUTFIT</b> contains everything necessary for thousands of beautiful
+bubbles and films. All highly colored articles have been carefully avoided, as
+cheap paints and dyes are positively dangerous in children's mouths. The
+outfit contains the following articles:</p>
+
+<p>One Book of Instructions, called "Fun With Soap-Bubbles," 1 Metal Base for
+Bubble Stand, 1 Wooden Rod for Bubble Stand, 3 Large Wire Rings for
+Bubble Stand, 1 Small Wire Ring, 3 Straws, 1 Package of Prepared Soap, 1
+Bubble Pipe, 1 Water-proof Bubble Horn. The complete outfit is placed in
+a neat box with the book. (Extra Horns, Soap, etc., furnished at slight cost.)</p>
+
+<blockquote>
+
+<p><b>CONTENTS OF BOOK.</b>&mdash;Twenty-one Illustrations.&mdash;Introduction.&mdash;The
+Colors of Soap-bubbles.&mdash;The Outfit.&mdash;Soap Mixture.&mdash;Useful Hints.&mdash;Bubbles
+Blown With Pipes.&mdash;Bubbles Blown With Straws.&mdash;Bubbles Blown With
+the Horn.&mdash;Floating Bubbles.&mdash;Baby Bubbles.&mdash;Smoke Bubbles.&mdash;Bombshell
+Bubbles.&mdash;Dancing Bubbles.&mdash;Bubble Games.&mdash;Supported Bubbles.&mdash;Bubble
+Cluster.&mdash;Suspended Bubbles.&mdash;Bubble Lamp Chimney.&mdash;Bubble Lenses.&mdash;Bubble
+Basket.&mdash;Bubble Bellows.&mdash;To Draw a Bubble Through a Ring.&mdash;Bubble
+Acorn.&mdash;Bubble Bottle.&mdash;A Bubble Within a Bubble.&mdash;Another
+Way.&mdash;Bubble Shade.&mdash;Bubble Hammock.&mdash;Wrestling Bubbles.&mdash;A Smoking
+Bubble.&mdash;Soap Films.&mdash;The Tennis Racket Film.&mdash;Fish-net Film.&mdash;Pan-shaped
+Film.&mdash;Bow and Arrow Film.&mdash;Bubble Dome.&mdash;Double Bubble Dome.&mdash;Pyramid
+Bubbles.&mdash;Turtle-back Bubbles.&mdash;Soap-bubbles and Frictional Electricity.</p></blockquote>
+
+<div class='center'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br /></div>
+<p>"There is nothing more beautiful than the airy-fairy soap-bubble with its
+everchanging colors."</p>
+
+<div class='center'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+<i>THE BEST POSSIBLE AMUSEMENT FOR OLD<br />
+AND YOUNG.</i><br />
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+<i>The Book and Complete Outfit will be sent, <b>POST-PAID</b>,<br />
+upon receipt of 35 cents, by</i><br />
+<br />
+<b>THOMAS M. ST. JOHN, 407 West 51st St., New York City.</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_190" id="Page_190">[190]</a></span></p>
+
+
+
+
+<div class='adtitle2'>The Study of Elementary Electricity and<br />
+Magnetism by Experiment.</div>
+
+
+<div class='center'><br />
+<b>By THOMAS M. ST. JOHN, Met. E.</b><br />
+<br />
+The book contains 220 pages and 168 illustrations;<br />
+it measures 5 × 7½ in. and is bound in green cloth.<br />
+<br />
+<b>PRICE, POST-PAID, $1.25.</b><br />
+</div>
+
+<p>This book is designed as a text-book for amateurs,
+students, and others who wish to take up a systematic
+course of elementary electrical experiments at home or in
+school. Full directions are given for&nbsp;.&nbsp;.&nbsp;.&nbsp;.&nbsp;.&nbsp;.&nbsp;.</p>
+
+<div class='center'>
+<b><big><i>Two Hundred Simple Experiments.</i></big></b><br />
+</div>
+<p>The experiments are discussed by the author, after the
+student has been led to form his own opinion about the
+results obtained and the points learned.</p>
+
+<p>In selecting the apparatus for the experiments in this
+book, the author has kept constantly in mind the fact
+that the average student will not buy the expensive
+pieces usually described in text-books.</p>
+
+<blockquote>
+
+<p>The two hundred experiments given can be performed with
+simple apparatus; in fact, the student should make at least a part
+of his own apparatus, and for the benefit of those who wish to do
+this, the author has given, throughout the work, explanations
+that will aid in the construction of certain pieces especially
+adapted to these experiments. For those who have the author's
+"How Two Boys Made Their Own Electrical Apparatus," constant
+references have been made to it as the "Apparatus Book,"
+as this contains full details for making almost all kinds of simple
+apparatus needed in "The Study of Elementary Electricity and
+Magnetism by Experiment."</p></blockquote>
+
+<p><b><i>If you wish to take up a systematic course of
+experiments&mdash;experiments that may be performed
+with simple, inexpensive apparatus,&mdash;this
+book will serve as a valuable guide.</i></b></p>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_191" id="Page_191">[191]</a></span></p>
+
+
+
+
+<div class='adtitle2'>Condensed List of Apparatus</div>
+<div class='center'><b>FOR</b></div>
+
+<div class='adtitle2'>"The Study of Elementary Electricity<br />
+and Magnetism by Experiment."</div>
+
+<div class='center'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br /></div>
+
+<p><i>Number</i> 1. Steel Needles; package of twenty-five.&mdash;2. Flat
+Cork.&mdash;3. Candle.&mdash;4-15. Annealed Iron Wires; assorted lengths.&mdash;16.
+Horseshoe Magnet; best quality; English.&mdash;17. Iron Filings.&mdash;18.
+Parts for Compass.&mdash;19, 20. Wire Nails; soft steel.&mdash;21,
+22. Spring Steel; for bar magnets.&mdash;23. Iron Ring.&mdash;24.
+Sifter; for iron filings.&mdash;25. Spring Steel; for flexible magnet.&mdash;26,
+27. Ebonite Sheets; with special surface.&mdash;28. Ebonite Rod.&mdash;29.
+Ebonite Rod; short.&mdash;30. Flannel Cloth.&mdash;31. Tissue Paper.&mdash;32.
+Cotton Thread.&mdash;33. Silk Thread.&mdash;34. Support Base.&mdash;35.
+Support Rod.&mdash;36. Support Wire.&mdash;37. Wire Swing.&mdash;38.
+Sheet of Glass.&mdash;39. Hairpin.&mdash;40. Circular Conductor.&mdash;41.
+Circular Conductor.&mdash;42. Electrophorus Cover.&mdash;43. Insulating
+Table.&mdash;44. Insulated Copper Wire.&mdash;45. Rubber Band.&mdash;46.
+Bent Wire Clamps.&mdash;47. Cylindrical Conductor.&mdash;48. Discharger;
+for condenser.&mdash;49. Aluminum-Leaf.&mdash;50. Wires.</p>
+
+<p>51. Dry Cell.&mdash;52. Mercury.&mdash;53. Insulated Copper Wire; for
+connections.&mdash;54. Spring Connectors; two dozen.&mdash;55. Parts for
+Key.&mdash;56. Metal Connecting Plates.&mdash;57. Parts for Current
+Reverser.&mdash;58. Parts for Galvanoscope.&mdash;59. Parts for Astatic
+Galvanoscope.&mdash;60-63. Zinc Strips.&mdash;64. Carbon Rod.&mdash;65, 66.
+Glass Tumblers.&mdash;67, 68. Copper Strips.&mdash;69. Galvanized Iron
+Nail.&mdash;70, 71. Wooden Cross-Pieces.&mdash;72. Brass Screws; one
+dozen.&mdash;73. Porous Cup.&mdash;74. Zinc Rod.&mdash;75. Copper Plate.&mdash;76.
+Iron Strip.&mdash;77, 78. Lead Strips.&mdash;79. Parts for Resistance
+Coil.&mdash;80. Parts for Wheatstone's Bridge.&mdash;81. German-Silver
+Wire; Size No. 30.&mdash;82. German-Silver Wire; No. 28.&mdash;83&mdash;85.
+Plate Binding-Posts.&mdash;86. Copper Sulphate.&mdash;87. Copper Burs;
+one dozen.&mdash;88. Combination Rule.&mdash;89. Coil of Wire; on spool
+for electromagnet.&mdash;90. Coil of Wire; on spool for electromagnet.&mdash;91.
+Carbon Rod.&mdash;92, 93. Soft Iron Cores with Screws.&mdash;94.
+Combined Base and Yoke.&mdash;95. Combination Connecting Plates.&mdash;96.
+Long Iron Core.&mdash;97. Round Bar Magnet, 5 × 3/8 in.&mdash;98.
+Thin Electromagnet.&mdash;99. Degree-Card; for galvanoscope.&mdash;100.
+Scale for Bridge.&mdash;101, 102. Soft Iron Cores with Heads.&mdash;103,
+104. Flat Bar Magnets; these are 6 × ½ × ¼ in.; highly polished
+steel; poles marked.&mdash;105. Compass.</p>
+
+<div class='center'>
+<b><i>Illustrated Price Catalogue upon Application.</i></b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_192" id="Page_192">[192]</a></span></p>
+
+
+
+
+<div class='adtitle2'><span class="u">Electrical Apparatus For Sale</span><br />
+<small>A COMPLETE ELECTRIC AND MAGNETIC<br />
+CABINET FOR STUDENTS, SCHOOLS AND<br />
+AMATEURS. SIX EXTRAORDINARY OFFERS</small></div>
+
+
+<p><b>This Cabinet of Electrical Experiments</b> contains three main
+parts: (<i>A</i>) Apparatus; (<i>B</i>) Text-Book; (<i>C</i>) Apparatus List.</p>
+
+<p>(<i>A</i>) <b>The Apparatus</b> furnished consists of one hundred and five
+pieces. Over three hundred separate articles are used in making
+up this set. Most of it is ready for use when received. Seven
+pieces, however, are not assembled; but the parts can be readily
+finished and put together. (Sold, also, <i>all</i> pieces assembled.)</p>
+
+<p>(<i>B</i>) <b>The Text-Book</b>&mdash;called "The Study of Elementary Electricity
+and Magnetism by Experiment"&mdash;gives full directions for
+two hundred experiments. (See table of contents, etc.) Price,
+post-paid, $1.25.</p>
+
+<p>(<i>C</i>) <b>The Apparatus List</b> is an illustrated book devoted entirely
+to this special set of apparatus. Not given with first offer.</p>
+
+<blockquote>
+
+<p><i>THE APPARATUS IS SIMPLE because the designs and
+methods of construction have been worked out with
+great care.</i></p>
+
+<p><i>THE APPARATUS IS PRACTICAL because it has been
+designed for real use in "The Study of Elementary
+Electricity and Magnetism by Experiment."</i></p>
+
+<p><i>THE APPARATUS IS CHEAP because the various parts
+are so designed that they can be turned out in quantity
+by machinery.</i></p></blockquote>
+
+
+
+<div class="center">
+<table border="0" cellpadding="4" cellspacing="0" summary="pricing table">
+<tr><td align="left"><b>1st Offer:</b> Pieces 1 to 50</td><td align="right">$1.00</td></tr>
+<tr><td align="left"><b>2d Offer:</b> Pieces 51 to 105, with part (<i>C</i>)</td><td align="right">3.50</td></tr>
+<tr><td align="left"><b>3d Offer:</b> Pieces 1 to 105, with part (<i>C</i>)</td><td align="right">4.00</td></tr>
+<tr><td align="left"><b>4th Offer:</b> Complete Cabinet, parts (<i>A</i>), (<i>B</i>), (<i>C</i>)</td><td align="right">5.00</td></tr>
+<tr><td align="left"><b>5th Offer:</b> Apparatus only, all pieces assembled</td><td align="right">4.60</td></tr>
+<tr><td align="left"><b>6th Offer:</b> Complete Cabinet, all pieces assembled</td><td align="right">5.60</td></tr>
+</table></div>
+
+<div class='center'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+<b><i>Express charges must be paid by you. Estimates given.</i></b><br />
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br /></div>
+
+<p>A "Special Catalogue," pertaining to the above, with complete
+price-list, will be mailed upon application.</p>
+
+<div class='center'>
+<b>THOMAS M. ST. JOHN, 407 West 51st St., New York City</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_193" id="Page_193">[193]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Fun With Telegraphy</span></div>
+<div class='center'><b><big>BOOK AND COMPLETE OUTFIT.</big></b></div>
+
+
+<div class="figcenter" style="width: 190px;">
+<img src="images/i_193.jpg" width="190" height="83" alt="drawing boy working on telegraph with wire to house next door" />
+</div>
+
+<p><b>TELEGRAPHY</b> is of the greatest importance to all civilized nations, and
+upon it depend some of the world's most important enterprises. Every boy
+and girl can make practical use of telegraphy in one way or another, and the
+time it takes to learn it will be well spent.</p>
+
+
+<p><b>THE OUTFIT.</b>&mdash;Mr. St. John has worked for a number of years to produce a
+telegraph outfit that would be simple, cheap, and practical for those who wish
+to make a study of telegraphy. After making and experimenting with nearly
+one hundred models, many of which were good, he has at last perfected an
+instrument so simple, original, and effective that it is now being made in large
+quantities.</p>
+
+<p>The sounders are so designed that they will work properly with any dry cell
+of ordinary strength, and this is a great advantage for practice lines. Dry
+batteries are cheap and clean, and there are no dangers from acids.</p>
+
+<p>The outfit consists of the following articles, placed in a neat box: One Book
+of Instruction, called "Fun With Telegraphy"; one Telegraph "Key"; one
+Telegraph "Sounder"; Insulated Copper Wires for connections. The "key"
+and "sounder" are mounted, with proper "binding-posts," upon a base of
+peculiar construction, which aids in giving a large volume of sound.</p>
+
+
+<p><b>CONTENTS OF BOOK.</b>&mdash;Telegraphy.&mdash;The Outfit.&mdash;A Complete Telegraph
+Line.&mdash;Connections.&mdash;The Telegraph Key.&mdash;The Sounder.&mdash;The Battery.&mdash;A
+Practice Line.&mdash;A Two-instrument Line.&mdash;Operation of Line.&mdash;The Morse Telegraph
+Alphabet.&mdash;Aids to Learning Alphabet.&mdash;Cautions.&mdash;Office Calls.&mdash;Receiving
+Messages.&mdash;Remember.&mdash;Extra Parts.</p>
+
+
+<p><b>ABOUT BATTERIES.</b>&mdash;For those who cannot easily secure batteries, we
+will furnish small dry cells, post-paid, at 15 cents each, in order to deliver the
+outfits complete to our customers. This price barely covers the total cost to
+us, postage alone being 6 cents.</p>
+
+<div class='center'>
+<i><b>FUN WITH TELEGRAPHY, including Book, Key, Sounder,<br />
+and Wire (no battery), post-paid, 50 cents, by</b></i><br />
+<br />
+<b>THOMAS M. ST. JOHN, 848 Ninth Ave., New York</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_194" id="Page_194">[194]</a></span></p>
+
+
+
+
+<div class='adtitle3'><span class="u">Tool Sets for Students</span></div>
+
+
+<p>The following tool sets have been arranged especially for those who wish to
+make use of the designs contained in "How Two Boys Made Their Own Electrical
+Apparatus," "Real Electric Toy-Making for Boys," "Electric Instrument-Making,"
+etc. It is very poor economy to waste valuable time and
+energy in order to save the cost of a few extra tools.</p>
+
+<p><b>NOTE.</b>&mdash;Save money by buying your tools in sets. We do not pay express
+or freight charges at the special prices below.</p>
+
+<p><b>FOR $1.00.</b>&mdash;One <i>Steel Punch</i>; round, knurled head.&mdash;One light <i>Hammer</i>;
+polished, nickel-plated, varnished handle.&mdash;One <i>Iron Clamp</i>; japanned, 2¼ in.&mdash;One
+<i>Screw-Driver</i>; tempered and polished blade, cherry stained hardwood
+handle, nickel ferrule.&mdash;One <i>Wrench</i>; retinned skeleton frame, gilt adjusting
+wheel.&mdash;One <i>Awl</i>; tempered steel point, turned and stained wood handle, with
+ferrule.&mdash;One <i>Vise</i>; full malleable, nicely retinned, 1-3/8 in. jaws, full malleable
+screw with spring.&mdash;One pair <i>Steel Pliers</i>; 4 in. long, polished tool steel, unbreakable,
+best grooved jaw.&mdash;One pair of <i>Shears</i>; carbonized steel blades,
+hardened edge, nickel-plated, heavy brass nut and bolt.&mdash;One <i>File</i>; triangular,
+good steel.&mdash;One <i>File Handle</i>; good wood, brass ferrule.&mdash;One <i>Foot Rule</i>;
+varnished wood, has English and metric system.&mdash;One <i>Soldering Set</i>; contains
+soldering iron, solder, resin, sal ammoniac, and directions. One <i>Center-Punch</i>;
+finely tempered steel.</p>
+
+<p><b>FOR $2.00.</b>&mdash;All that is contained in the $1.00 set of tools, together with the
+following: One pair of <i>Tinner's Shears</i>; cut, 2¾ in., cast iron, hardened, suitable
+for cutting thin metal.&mdash;One <i>Hollow Handle Tool Set</i>; very useful; polished
+handle holds 10 tools, gimlet, brad-awls, chisel, etc.&mdash;One <i>Try Square</i>;
+6-in. blue steel blade, marked in 1/8s, strongly riveted.&mdash;One 1-lb. <i>Hammer</i>;
+full size, polished head, wedged varnished hardwood handle.&mdash;One <i>Hack Saw</i>;
+steel frame, 9½-in. polished steel blade, black enamel handle; very useful.</p>
+
+<p><b>FOR $3.50.</b>&mdash;Two <i>Steel Punches</i>; different sizes, one solid round, knurled
+head, polished; the other, point and head brightly polished, full nickel, center
+part knurled.&mdash;One <i>Light Hammer</i>; polished and nickel plated, varnished
+handle.&mdash;One regular <i>Machinist's Hammer</i>; ball peen, solid cast steel, with
+varnished hardwood handle; a superior article.&mdash;Two <i>Iron Clamps</i>; one opens
+2¼ in., the other 3 in., japanned.&mdash;One <i>Screw-Driver</i>; tempered and polished
+blade, firmly set in cherry stained hardwood handle with nickel ferrule.&mdash;One
+<i>Wrench</i>; retinned, skeleton frame, gilt adjusting wheel.&mdash;One <i>Awl</i>; tempered
+steel blade, ground to point, firmly set in turned and stained handle with ferrule.&mdash;One
+<i>Steel Vise</i>; 2¼-in., jaws, steel screw, bright polished jaws and handle;
+a good strong vise.&mdash;One pair of <i>Steel Pliers</i>; 6 in. long, bright steel, flat
+nose, 2 wire-cutters, practically unbreakable.&mdash;One pair of <i>Shears</i>; carbonized
+steel blades, hardened edges, nickel plated, heavy brass nut and bolt.&mdash;One
+<i>File</i>; triangular and of good steel.&mdash;One <i>File Handle</i>; good wood, with brass
+ferrule.&mdash;One <i>Foot Rule</i>; varnished wood, has both the English and metric systems.&mdash;One
+<i>Soldering Set</i>; contains soldering iron, solder, resin, sal ammoniac,
+and directions; a very handy article.&mdash;One <i>Center-Punch</i>; finely tempered
+steel.&mdash;One pair of <i>Tinner's Shears</i>; these are best grade, inlaid steel cutting
+edges, polished and tempered, japanned handles; thoroughly reliable.&mdash;One
+<i>Hollow Handle Tool Set</i>; very useful; the polished handle holds 10 tools, gimlet,
+chisel, brad-awl, etc.&mdash;One <i>Try Square</i>; 6-in. blue steel blade, marked both
+sides in 1/8s, strongly riveted with brass rivets.&mdash;One <i>Hack Saw</i>; steel frame,
+9½-in. polished steel blade, black enamel handle; very useful for sawing small
+pieces of wood.</p>
+
+<p><b>FOR $5.00</b> will be included everything in the $3.50 offer, and the following:
+One <i>Glue-Pot</i>; medium size, with brush and best wood glue; inside pot
+has hinge cover.&mdash;One <i>Ratchet Screw-Driver</i>; great improvement over ordinary
+screw-drivers; well made and useful.&mdash;One <i>Hand Drill</i>; frame malleable iron;
+hollow screw top holding 6 drills; bores from 1-16 to 3-16-in. holes; solid gear
+teeth; 3-jawed nickel plated chuck; a superior tool, and almost a necessity.</p>
+
+<div class='center'>
+<b><big>GIVE THE BOY A SET OF TOOLS</big></b><br />
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br /><br />
+<b>THOMAS M. ST. JOHN, 848 Ninth Ave., New York</b><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_195" id="Page_195">[195]</a></span></p>
+
+
+
+
+<div class='adtitle2'>REAL ELECTRIC TOY-MAKING<br />
+FOR BOYS</div>
+
+
+<div class='center'><br />
+<b><big><i>By</i> THOMAS M. ST. JOHN, Met. E.</big></b><br />
+<br />
+<br />
+This book contains 140 pages and over one hundred<br />
+original drawings, diagrams, and full-page plates.<br />
+It measures 5 x 7½ in., and is bound in cloth.<br />
+<br />
+<b>Price, post-paid, $1.00</b><br />
+</div>
+
+
+<p><b>CONTENTS:</b> <i>Chapter</i> I. Toys Operated by Permanent
+Magnets.&mdash;II. Toys Operated by Static Electricity.&mdash;III. Making
+Electromagnets for Toys.&mdash;IV. Electric Batteries.&mdash;V. Circuits
+and Connections.&mdash;VI. Toys Operated by Electromagnets.
+VII. Making Solenoids for Toys.&mdash;VIII. Toys Operated by
+Solenoids.&mdash;IX. Electric Motors.&mdash;X. Power, Speed, and Gearing.&mdash;XI.
+Shafting and Bearings.&mdash;XII. Pulleys and Winding-Drums.&mdash;XIII.
+Belts and Cables.&mdash;XIV. Toys Operated by
+Electric Motors.&mdash;XV. Miscellaneous Electric Toys.&mdash;XVI. Tools.&mdash;XVII.
+Materials.&mdash;XVIII. Various Aids to Construction.</p>
+
+<p>While planning this book, Mr. St. John definitely decided that
+he would not fill it with descriptions of complicated, machine-made
+instruments and apparatus, under the name of "Toy-Making,"
+for it is just as impossible for most boys to get the
+parts for such things as it is for them to do the required machine
+work even after they have the raw materials.</p>
+
+<p>Great care has been taken in designing the toys which are
+described in this book, in order to make them so simple that
+any boy of average ability can construct them out of ordinary
+materials. The author can personally guarantee the designs,
+for there is no guesswork about them. Every toy was made,
+changed, and experimented with until it was as simple as possible;
+the drawings were then made from the perfected models.</p>
+
+<p>As the result of the enormous amount of work and experimenting
+which were required to originate and perfect so many new
+models, the author feels that this book may be truly called
+"Real Electric Toy-Making for Boys."</p>
+
+<div class='center'>
+<big><b>Every Boy Should Make Electrical Toys.</b></big><br />
+</div>
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_196" id="Page_196">[196]</a></span></p>
+
+
+
+
+<div class='adtitle2'><span class="u">The Electric Shooting Game</span></div>
+<div class='center'><b>A MOST ORIGINAL AND FASCINATING GAME<br />
+PATENT APPLIED FOR AND COPYRIGHTED</b><br /><br /></div>
+
+
+<div class="figcenter" style="width: 386px;">
+<img src="images/i_196.jpg" width="386" height="229" alt="Bison" />
+</div>
+
+<div class='center'><big><i><b>SHOOTING BY ELECTRICITY</b></i></big><br />
+&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;<br />
+</div>
+
+<p><b>The Electric Shooting Game</b> is an entirely new
+idea, and one that brings into use that most mysterious
+something&mdash;<i>electricity</i>. The game is so simple that small
+children can play it, and as there are no batteries, acids,
+or liquids of any kind, there is absolutely no danger.
+The electricity is of such a nature that it is perfectly
+harmless&mdash;but very active.</p>
+
+<p>The "<i>game-preserve</i>" is neat and attractive, being
+printed in colors, and the birds and animals are well
+worth hunting. Each has a fixed value&mdash;and some of
+them must not be shot at all&mdash;so there is ample opportunity
+for a display of skill in bringing down those which
+count most.</p>
+
+<p>"<i>Electric bullets</i>" are actually shot from the "<i>electric
+gun</i>" by electricity. This instructive game will furnish
+a vast amount of amusement to all.</p>
+
+
+<div class='hang1'><i><b>The "Game-Preserve,"&mdash;the "Electric Gun,"&mdash;the "Shooting-Box,"&mdash;the
+"Electric Bullets,"&mdash;in fact, the entire
+electrical outfit, together with complete illustrated directions,
+will be sent in a neat box, Post-Paid, upon receipt
+of 50 cents, by</b></i></div>
+
+<div class='center'><br />
+<b>THOMAS M. ST. JOHN, 848 Ninth Ave., New York</b><br />
+</div>
+
+
+<hr class="chap" />
+<p>&nbsp;</p>
+<div class='tnote'>
+<div class='center'><b>Transcriber's Note:</b></div>
+
+<p>Obvious punctuation errors were corrected.</p>
+
+<p>Page 46, "turnnd" changed to "turned" (be turned to 1)</p>
+
+<p>Page 66, word "a" added to text (in a glass jar)</p>
+</div>
+
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<hr class="full" />
+<p>***END OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY***</p>
+<p>******* This file should be named 44665-h.txt or 44665-h.zip *******</p>
+<p>This and all associated files of various formats will be found in:<br />
+<a href="http://www.gutenberg.org/dirs/4/4/6/6/44665">http://www.gutenberg.org/4/4/6/6/44665</a></p>
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+The Project Gutenberg eBook, Things a Boy Should Know About Electricity,
+by Thomas M. (Thomas Matthew) St. John
+
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+
+
+
+Title: Things a Boy Should Know About Electricity
+       Second Edition
+
+
+Author: Thomas M. (Thomas Matthew) St. John
+
+
+
+Release Date: January 14, 2014  [eBook #44665]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT
+ELECTRICITY***
+
+
+E-text prepared by Chris Curnow, Emmy, and the Online Distributed
+Proofreading Team (http://www.pgdp.net) from page images generously made
+available by Internet Archive (https://archive.org)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 44665-h.htm or 44665-h.zip:
+      (http://www.gutenberg.org/files/44665/44665-h/44665-h.htm)
+      or
+      (http://www.gutenberg.org/files/44665/44665-h.zip)
+
+
+      Images of the original pages are available through
+      Internet Archive. See
+      https://archive.org/details/thingsboyshouldk00stjo
+
+
+Transcriber's note:
+
+      Text enclosed by underscores is in italics (_italics_).
+
+      Text enclosed by equal signs is in bold face (=bold=).
+
+      Characters enclosed by curly brackets after an underscore
+      are subscripts (example: CuSO_{4} [the chemical formula
+      of copper sulfate]).
+
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+
+[Illustration]
+
+
+       *       *       *       *       *
+
+_BY THE SAME AUTHOR._
+
+
+  =FUN WITH MAGNETISM.= A book and complete outfit of apparatus
+     for _Sixty-One Experiments_.
+
+  =FUN WITH ELECTRICITY.= A book and complete outfit of
+     apparatus for _Sixty Experiments_.
+
+  =FUN WITH PUZZLES.= A book, key and complete outfit for _Four
+     Hundred Puzzles_.
+
+  =FUN WITH SOAP-BUBBLES.= A book and complete outfit of
+     apparatus for _Fancy Bubbles and Films_.
+
+  =FUN WITH SHADOWS.= Including book of instructions with one
+     hundred illustrations and a complete outfit of apparatus
+     for _Shadow Pictures, Pantomimes, Entertainments, etc.,
+     etc._
+
+  =HUSTLE-BALL.= An American game. Played by means of magic
+     wands and polished balls of steel.
+
+  =JINGO.= The great war game, including JINGO JUNIOR.
+
+  =HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS.= A book
+     containing complete directions for making all kinds of
+     simple apparatus for the study of elementary electricity.
+
+  =THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY
+     EXPERIMENT.= This book is designed as a text-book for
+     amateurs, students, and others who wish to take up a
+     systematic course of simple experiments at home or in
+     school.
+
+  =THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.= This book
+     explains, in simple, straightforward language, many things
+     about electricity; things in which the American boy is
+     intensely interested; things he wants to know; things he
+     should know.
+
+  =ANS., OR ACCURACY, NEATNESS AND SPEED.= For teachers and
+     pupils. Containing study-charts, practice devices and
+     special methods for accurate, rapid work with figures.
+
+    _Ask Your Bookseller, Stationer, or Toy Dealer for our
+    Books, Games, Puzzles, Educational Amusements, Etc._
+
+
+    CATALOGUE UPON APPLICATION
+
+    Thomas M. St. John, 407 West 51st St., New York.
+
+       *       *       *       *       *
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+by
+
+THOMAS M. ST. JOHN, Met. E.
+
+Author of "Fun With Magnetism," "Fun With Electricity,"
+"How Two Boys Made Their Own Electrical Apparatus,"
+"The Study of Elementary Electricity
+and Magnetism by Experiment," etc.
+
+SECOND EDITION
+
+
+
+
+
+
+
+[Illustration]
+
+New York
+Thomas M. St. John
+407 West 51st Street
+1903
+
+Copyright, 1900.
+By Thomas M. St. John.
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
+
+
+
+
+TABLE OF CONTENTS
+
+
+  CHAPTER                                                   PAGE
+      I. About Frictional Electricty                           7
+     II. About Magnets and Magnetism                          21
+    III. How Electricity is Generated by the Voltaic Cell,    32
+     IV. Various Voltaic Cells,                               36
+      V. About Push-Buttons, Switches and Binding-Posts,      43
+     VI. Units and Apparatus for Electrical Measurements,     48
+    VII. Chemical Effects of the Electric Current,            58
+   VIII. How Electroplating and Electrotyping are Done,       60
+     IX. The Storage Battery, and How it Works,               63
+      X. How Electricity is Generated by Heat,                68
+     XI. Magnetic Effects of the Electric Current,            71
+    XII. How Electricity is Generated by Induction,           77
+   XIII. How the Induction Coil Works,                        80
+    XIV. The Electric Telegraph, and How it Sends Messages,   84
+     XV. The Electric Bell and Some of its Uses,              91
+    XVI. The Telephone and How it Transmits Speech,           95
+   XVII. How Electricity is Generated by Dynamos,            101
+  XVIII. How the Electric Current is Transformed,            109
+    XIX. How Electric Currents are Distributed for Use,      114
+     XX. How Heat is Produced by the Electric Current,       124
+    XXI. How Light is Produced by the Incandescent Lamp,     129
+   XXII. How Light is Produced by the Arc Lamp,              135
+  XXIII. X-Rays, and How the Bones of the Human Body are
+             Photographed,                                   141
+   XXIV. The Electric Motor, and How it Does Work,           147
+    XXV. Electric Cars, Boats and Automobiles,               154
+   XXVI. A Word About Central Stations,                      162
+  XXVII. Miscellaneous Uses of Electricity,                  165
+
+
+
+
+TO THE READER
+
+
+For the benefit of those who wish to make their own electrical
+apparatus for experimental purposes, references have been made
+throughout this work to the "Apparatus Book;" by this is meant the
+author's "How Two Boys Made Their Own Electrical Apparatus."
+
+For those who wish to take up a course of elementary electrical
+experiments that can be performed with simple, home-made apparatus,
+references have been made to "Study;" by this is meant "The Study of
+Elementary Electricity and Magnetism by Experiment."
+
+                                                       THE AUTHOR.
+
+
+
+
+Things A Boy Should Know About Electricity
+
+
+
+
+CHAPTER I.
+
+ABOUT FRICTIONAL ELECTRICITY.
+
+
+=1. Some Simple Experiments.= Have you ever shuffled your feet along
+over the carpet on a winter's evening and then quickly touched your
+finger to the nose of an unsuspecting friend? Did he jump when a bright
+spark leaped from your finger and struck him fairly on the very tip of
+his sensitive nasal organ?
+
+[Illustration: Fig. 1.]
+
+Did you ever succeed in proving to the pussy-cat, Fig. 1, that
+something unusual occurs when you thoroughly rub his warm fur with your
+hand? Did you notice the bright sparks that passed to your hand when it
+was held just above the cat's back? You should be able to see, hear,
+and feel these sparks, especially when the air is dry and you are in a
+dark room.
+
+Did you ever heat a piece of paper before the fire until it was real
+hot, then lay it upon the table and rub it from end to end with your
+hand, and finally see it cling to the wall?
+
+Were you ever in a factory where there were large belts running rapidly
+over pulleys or wheels, and where large sparks would jump to your hands
+when held near the belts?
+
+If you have never performed any of the four experiments mentioned, you
+should try them the first time a chance occurs. There are dozens of
+simple, fascinating experiments that may be performed with this kind of
+electricity.
+
+=2. Name.= As this variety of electricity is made, or generated, by
+the friction of substances upon each other, it is called _frictional_
+electricity. It is also called _static_ electricity, because it
+generally stands still upon the surface of bodies and does not "flow in
+currents" as easily as some of the other varieties. Static electricity
+may be produced by induction as well as by friction.
+
+[Illustration: Fig. 2.]
+
+=3. History.= It has been known for over 2,000 years that certain
+substances act queerly when rubbed. Amber was the first substance upon
+which electricity was produced by friction, and as the Greek name for
+amber is _elektron_, bodies so affected were said to be _electrified_.
+When a body, like ebonite, is rubbed with a flannel cloth, we say that
+it becomes _charged with electricity_. Just what happens to the ebonite
+is not clearly understood. We know, however, that it will attract
+light bodies, and then quickly repel them if they be conductors. Fig.
+2 shows a piece of tissue-paper jumping toward a sheet of ebonite that
+has been electrified with a flannel cloth.
+
+=4. Conductors and Non-Conductors.= Electricity can be produced upon
+glass and ebonite because they do not carry or conduct it away. If a
+piece of iron be rubbed, the electricity passes from the iron into the
+earth as fast as it is generated, because the iron is a _conductor_ of
+electricity. Glass is an _insulator_ or _non-conductor_. Frictional
+electricity resides upon the outside, only, of conductors. A hollow
+tin box will hold as great a charge as a solid piece of metal having
+the same outside size and shape. When frictional electricity passes
+from one place to another, sparks are produced. Lightning is caused
+by the passage of static electricity from a cloud to the earth, or
+from one cloud to another. In this case air forms the conductor. (For
+experiments, see "Study," Chapter VII.)
+
+[Illustration: Fig. 3.]
+
+=5. Electroscopes.= A piece of carbon, pith, or even a small piece of
+damp tissue-paper will serve as an electroscope to test the presence of
+static electricity. The pith is usually tied to a piece of silk thread
+which is a non-conductor. Fig. 3 shows the ordinary form of _pith-ball
+electroscope_.
+
+The _leaf electroscope_ is a very delicate apparatus. Gold-leaf is
+generally used, but aluminum-leaf will stand handling and will do for
+all ordinary purposes. Fig. 4 shows a common form, the glass being
+used to keep currents of air from the leaves and at the same time to
+insulate them from the earth.
+
+Electroscopes are used to show the presence, relative amount, or kind
+of static electricity on a body. (See "Study," Chapter XI.)
+
+[Illustration: Fig. 4.]
+
+=6. Two Kinds of Electrification.= It can be shown that the
+electrification produced on all bodies by friction is not the same;
+for example, that generated with glass and silk is not the same as
+that made with ebonite and flannel. It has been agreed to call that
+produced by glass and silk _positive_, and that by ebonite and flannel
+_negative_. The signs + and - are used for positive and negative.
+
+=7. Laws of Electrification.= (1) Charges of the same kind repel each
+other; (2) charges of unlike kinds attract each other; (3) either kind
+of a charge attracts and is attracted by a neutral body.
+
+=8. Static Electric Machines.= In order to produce static electricity
+in quantities for experiments, some device is necessary.
+
+The _electrophorus_ (e-lec-troph'-o-rus) is about the simplest form
+of machine. Fig. 5 shows a simple electrophorus in which are two
+insulators and one conductor. The ebonite sheet E S is used with a
+flannel cloth to generate the electricity. The metal cover E C is
+lifted by the insulating handle E R. The cover E C is placed upon the
+thoroughly charged sheet E S, and then it is touched for an instant
+with the finger, before lifting it by E R. The charge upon E C can then
+be removed by bringing the hand near it. The bright spark that passes
+from E C to the hand indicates that E C has discharged itself into the
+earth. The action of the electrophorus depends upon induction. (For
+experiments, details of action, induced electrification, etc., see "The
+Study of Elementary Electricity and Magnetism by Experiment," Chapters
+VIII. and IX.)
+
+[Illustration: Fig. 5.]
+
+_The first electric machine_ consisted of a ball of sulphur fastened to
+a spindle which could be turned by a crank. By holding the hands or a
+pad of silk upon the revolving ball, electricity was produced.
+
+[Illustration: Fig. 6.]
+
+[Illustration: Fig. 7.]
+
+=9. The Cylinder Electric Machine= consists, as shown in Fig. 6, of a
+glass cylinder so mounted that it can be turned by a crank. Friction
+is produced by a pad of leather C, which presses against the cylinder
+as it turns. Electric sparks can be taken from the large "conductors"
+which are insulated from the earth. The opposite electricities unite
+with sparks across D and E. If use is to be made of the electricity,
+either the rubber or the prime conductor must be connected with the
+ground. In the former case positive electricity is obtained; in the
+latter, negative.
+
+=10. The Plate Electrical Machine.= Fig. 7 also shows an old form of
+machine. Such machines are made of circular plates of glass or ebonite,
+two rubbing pads being usually employed, one on each side of the plate.
+One operator is seen on an insulated stool (Fig. 7), the electricity
+passing through him before entering the earth by way of the body of the
+man at the right.
+
+[Illustration: Fig. 8.]
+
+=11. The Toepler-Holtz Machine=, in one form, is shown in Fig. 8. The
+electricity is produced by the principle of induction, and not by mere
+friction. This machine, used in connection with condensers, produces
+large sparks.
+
+=12. The Wimshurst Machine= is of recent date, and not being easily
+affected by atmospheric changes, is very useful for ordinary laboratory
+work. Fig. 9 shows one form of this machine.
+
+=13. Influence Machines for Medical Purposes= are made in a large
+variety of forms. A Wimshurst machine is generally used as an exciter
+to charge the plates of the large machine when they lose their charge
+on account of excessive moisture in the atmosphere. Fig. 10 shows a
+large machine.
+
+[Illustration: Fig. 9.]
+
+=14. Uses of Electrical Machines.= Static electricity has been used for
+many years in the laboratory for experimental purposes, for charging
+condensers, for medical purposes, etc. It is now being used for X-ray
+work, and considerable advancement has been made within a few years in
+the construction and efficiency of the machines.
+
+[Illustration: Fig. 10.]
+
+With the modern machines large sparks are produced by merely turning
+a crank, enough electricity being produced to imitate a small
+thunderstorm. The sparks of home-made lightning will jump several
+inches.
+
+Do not think that electricity is generated in a commercial way by
+static electric machines. The practical uses of static electricity are
+very few when compared with those of current electricity from batteries
+and dynamos.
+
+=15. Condensation of Static Electricity.= By means of apparatus called
+_condensers_, a terrific charge of static electricity may be stored.
+Fig. 11 shows the most common form of condenser, known as the _Leyden
+jar_. It consists of a glass jar with an inside and outside coating of
+tin-foil.
+
+[Illustration: Fig. 11.]
+
+[Illustration: Fig. 12.]
+
+_To charge_ the jar it is held in the hand so that the outside coating
+shall be connected with the earth, the sparks from an electric machine
+being passed to the knob at the top, which is connected by a chain to
+the inside coating.
+
+_To discharge_ the jar, Fig. 12, a conductor with an insulating handle
+is placed against the outside coat; when the other end of the conductor
+is swung over towards the knob, a bright spark passes between them.
+This device is called a discharger. Fig. 13 shows a discharge through
+ether which the spark ignites.
+
+[Illustration: Fig. 13.]
+
+=16. The Leyden Battery=, Fig. 14, consists of several jars connected
+in such a way that the area of the inner and outer coatings is greatly
+increased. The battery has a larger capacity than one of its jars. (For
+Experiments in Condensation, see "Study," Chapter X.)
+
+[Illustration: Fig. 14.]
+
+=17. Electromotive Force of Static Electricity.= Although the sparks
+of static electricity are large, the _quantity_ of electricity is very
+small. It would take thousands of galvanic cells to produce a spark
+an inch long. While the quantity of static electricity is small, its
+potential, or electromotive force (E. M. F.), is very high. We say that
+an ordinary gravity cell has an E. M. F. of a little over one volt.
+Five such cells joined in the proper way would have an E. M. F. of a
+little over five volts. You will understand, then, what is meant when
+we say that the E. M. F. of a lightning flash is millions of volts.
+
+=18. Atmospheric Electricity.= The air is usually electrified, even
+in clear weather, although its cause is not thoroughly understood. In
+1752 it was proved by Benjamin Franklin (Fig. 15), with his famous
+kite experiment, that atmospheric and frictional electricities are
+of the same nature. By means of a kite, the string being wet by the
+rain, he succeeded, during a thunderstorm, in drawing sparks, charging
+condensers, etc.
+
+[Illustration: Fig. 15.]
+
+[Illustration: Fig. 16.]
+
+=19. Lightning= may be produced by the passage of electricity between
+clouds, or between a cloud and the earth (Fig. 16), which, with the
+intervening air, have the effect of a condenser. When the attraction
+between the two electrifications gets great enough, a spark passes.
+When the spark has a zigzag motion it is called _chain lightning_.
+In hot weather flashes are often seen which light whole clouds, no
+thunder being heard. This is called _heat lightning_, and is generally
+considered to be due to distant discharges, the light of which is
+reflected by the clouds. The lightning flash represents billions of
+volts.
+
+[Illustration: Fig. 17.]
+
+=20. Thunder= is caused by the violent disturbances produced in the
+air by lightning. Clouds, hills, etc., produce echoes, which, with the
+original sound, make the rolling effect.
+
+=21. Lightning-Rods=, when well constructed, often prevent violent
+discharges. Their pointed prongs at the top allow the negative
+electricity of the earth to pass quietly into the air to neutralize
+the positive in the cloud above. In case of a discharge, or stroke of
+lightning, the rods aid in conducting the electricity to the earth. The
+ends of the rods are placed deep in the earth, Fig. 17.
+
+=22. St. Elmo's Fire.= Electrification from the earth is often drawn up
+from the earth through the masts of ships, Fig. 18, to neutralize that
+in the clouds, and, as it escapes from the points of the masts, light
+is produced.
+
+[Illustration: Fig. 18.]
+
+=23. Aurora Borealis=, also called Northern Lights, are luminous
+effects, Fig. 19, often seen in the north. They often occur at the
+same time with magnetic storms, when telegraph and telephone work may
+be disturbed. The exact cause of this light is not known, but it is
+thought by many to be due to disturbances in the earth's magnetism
+caused by the action of the sun.
+
+[Illustration: Fig. 19.]
+
+
+
+
+CHAPTER II.
+
+
+ABOUT MAGNETS AND MAGNETISM.
+
+=24. Natural Magnets.= Hundreds of years ago it was discovered that
+a certain ore of iron, called lodestone, had the power of picking up
+small pieces of iron. It was used to indicate the north and south
+line, and it was discovered later that small pieces of steel could be
+permanently magnetized by rubbing them upon the lodestone.
+
+=25. Artificial Magnets.= Pieces of steel, when magnetized, are called
+artificial magnets. They are made in many forms. The electromagnet is
+also an artificial magnet; this will be treated separately.
+
+[Illustration: Fig. 20]
+
+=26. The Horseshoe Magnet=, Fig. 20, is, however, the one with which we
+are the most familiar. They are always painted red, but the red paint
+has nothing to do with the magnetism.
+
+The little end-piece is called the keeper, or armature; it should
+always be kept in place when the magnet is not in use. The magnet
+itself is made of steel, while the armature is made of soft iron. Steel
+retains magnetism for a long time, while soft iron loses it almost
+instantly. The ends of the magnet are called its _poles_, and nearly
+all the strength of the magnet seems to reside at the poles, the curved
+part having no attraction for outside bodies. One of the poles of the
+magnet is marked with a line, or with the letter N. This is called the
+north pole of the magnet, the other being its south pole.
+
+[Illustration: Fig. 21.]
+
+=27. Bar Magnets= are straight magnets. Fig. 21 shows a round bar
+magnet. The screw in the end is for use in the telephone, described
+later.
+
+=28. Compound Magnets.= When several thin steel magnets are riveted
+together, a compound magnet is formed. These can be made with
+considerable strength. Fig. 22 shows a compound horseshoe magnet. Fig.
+23 shows a form of compound bar magnet used in telephones. The use of
+the coil of wire will be explained later. A thick piece of steel can
+not be magnetized through and through. In the compound magnet we have
+the effect of a thick magnet practically magnetized through and through.
+
+[Illustration: Fig. 22.]
+
+[Illustration: Fig. 23.]
+
+=29. Magnetic and Diamagnetic Bodies.= Iron, and substances containing
+iron, are the ones most readily attracted by a magnet. Iron is said to
+be _magnetic_. Some substances, like nickel, for example, are visibly
+attracted by very strong magnets only. Strange as it may seem, some
+substances are actually repelled by strong magnets; these are called
+_diamagnetic_ bodies. Brass, copper, zinc, etc., are not visibly
+affected by a magnet. Magnetism will act through paper, glass, copper,
+lead, etc.
+
+[Illustration: Fig. 24.]
+
+=30. Making Magnets.= One of the strangest properties that a magnet
+has is its power to give magnetism to another piece of steel. If
+a sewing-needle be properly rubbed upon one of the poles of a
+magnet, it will become strongly magnetized and will retain its
+magnetism for years. Strong permanent magnets are made with the aid
+of electromagnets. Any number of little magnets may be made from a
+horseshoe magnet without injuring it.
+
+[Illustration: Fig. 25.]
+
+31. Magnetic Needles and Compasses. If a bar magnet be suspended
+by a string, or floated upon a cork, which can easily be done with
+the magnet made from a sewing-needle, Fig. 24, it will swing around
+until its poles point north and south. Such an arrangement is called
+a _magnetic needle_. In the regular _compass_, a magnetic needle is
+supported upon a pivot. Compasses have been used for many centuries
+by mariners and others. Fig. 25 shows an ordinary pocket compass, and
+Fig. 26 a form of mariner's compass, in which the small bar magnets are
+fastened to a card which floats, the whole being so mounted that it
+keeps a horizontal position, even though the vessel rocks.
+
+[Illustration: Fig. 26.]
+
+32. Action of Magnets Upon Each Other. By making two small
+sewing-needle magnets, you can easily study the laws of attraction and
+repulsion. By bringing the two north poles, or the two south poles,
+near each other, a repulsion will be noticed. Unlike poles attract each
+other. The attraction between a magnet and iron is mutual; that is,
+each attracts the other. Either pole of a magnet attracts soft iron.
+
+In magnetizing a needle, either end may be made a north pole at will;
+in fact, the poles of a weak magnet can easily be reversed by properly
+rubbing it upon a stronger magnet.
+
+=33. Theory of Magnetism.= Each little particle of a piece of steel or
+iron is supposed to be a magnet, even before it touches a magnet. When
+these little magnets are thoroughly mixed up in the steel, they pull in
+all sorts of directions upon each other and tend to keep the steel from
+attracting outside bodies. When a magnet is properly rubbed upon a bar
+of steel, the north poles of the little molecular magnets of the steel
+are all made to point in the same direction. As the north poles help
+each other, the whole bar can attract outside bodies.
+
+By jarring a magnet its molecules are thoroughly shaken up; in fact,
+most of the magnetism can be knocked out of a weak magnet by hammering
+it.
+
+=34. Retentivity.= The power that a piece of steel has to hold
+magnetism is called _retentivity_. Different kinds of steel have
+different retentivities. A sewing-needle of good steel will retain
+magnetism for years, and it is almost impossible to knock the magnetism
+out by hammering it. Soft steel has very little retentivity, because
+it does not contain much carbon. Soft iron, which contains less
+carbon than steel, holds magnetism very poorly; so it is not used for
+permanent magnets. A little magnetism, however, will remain in the
+soft iron after it is removed from a magnet. This is called _residual
+magnetism_.
+
+=35. Heat and Magnetism.= Steel will completely lose its magnetism
+when heated to redness, and a magnet will not attract red-hot iron.
+The molecules of a piece of red-hot iron are in such a state of rapid
+vibration that they refuse to be brought into line by the magnet.
+
+=36. Induced Magnetism.= A piece of soft iron may be induced to become
+a magnet by holding it near a magnet, absolute contact not being
+necessary. When the soft iron is removed, again, from the influence of
+the magnet, its magnetism nearly all disappears. It is said to have
+_temporary_ magnetism; it had _induced_ magnetism. If a piece of soft
+iron be held near the north pole of a magnet, as in Fig. 27, poles will
+be produced in the soft iron, the one nearest the magnet being the
+south pole, and the other the north pole.
+
+[Illustration: Fig. 27.]
+
+[Illustration: Fig. 28.]
+
+=37. Magnetic Field.= If a bar magnet be laid upon the table, and a
+compass be moved about it, the compass-needle will be attracted by the
+magnet, and it will point in a different direction for every position
+given to the compass. This strange power, called magnetism, reaches out
+on all sides of a magnet. The magnet may be said to act by induction
+upon the compass-needle. The space around the magnet, in which this
+inductive action takes place, is called the _magnetic field_. Fig. 28
+shows some of the positions taken by a compass-needle when moved about
+on one side of a bar magnet.
+
+[Illustration: Fig. 29.]
+
+[Illustration: Fig. 30.]
+
+=38. Magnetic Figures= can be made by sprinkling iron filings upon a
+sheet of paper under which is placed a magnet. Fig. 29 shows a magnetic
+figure made with an ordinary bar magnet. The magnet was placed upon the
+table and over this was laid a piece of smooth paper. Fine iron filings
+were sifted upon the paper, which was gently tapped so that the filings
+could arrange themselves. As each particle of iron became a little
+magnet, by induction, its poles were attracted and repelled by the
+magnet; and when the paper was tapped they swung around to their final
+positions. Notice that the filings have arranged themselves in lines.
+These lines show the positions of some of the _lines of magnetic force_
+which surrounded the magnet.
+
+These lines of force pass from the north pole of a magnet through the
+air on all sides to its south pole.
+
+[Illustration: Fig. 31.]
+
+Fig. 30 shows a magnetic figure made from two bar magnets placed side
+by side, their unlike poles being next to each other. Fig. 31 shows
+the magnetic figure of a horseshoe magnet with round poles, the poles
+being uppermost.
+
+=39. The Use of Armatures.= A magnet attracts iron most strongly at its
+poles, because it is at the poles that the greatest number of lines
+of force pass into the air. Lines of force pass easily through soft
+iron, which is said to be a good conductor of them. Air is not a good
+conductor of the lines of force; in order, then, for the lines of force
+to pass from the north pole of a magnet to its south pole, they must
+overcome this resistance of the air, unless the armature is in place. A
+magnet will gradually grow weaker when its armature is left off.
+
+=40. Terrestrial Magnetism.= As the compass-needle points to the north
+and south, the earth must act like a magnet. There is a place very far
+north, about a thousand miles from the north pole of the earth, which
+is called the earth's north magnetic pole. Compass-needles point to
+this place, and not to the earth's real north pole. You can see, then,
+that if a compass be taken north of this magnetic pole, its north pole
+will point south. Lines of force pass from the earth's north magnetic
+pole through the air on all sides of the earth and enter the earth's
+south magnetic pole. The compass-needle, in pointing toward the north
+magnetic pole, merely takes the direction of the earth's lines of
+force, just as the particles of iron filings arrange themselves in the
+magnetic figures.
+
+=41. Declination.= As the magnetic needle does not point exactly to the
+north, an angle is formed between the true north and south line and the
+line of the needle. In Fig. 32 the line marked N S is the true north
+and south line. The _angle of variation_, or the declination, is the
+angle A between the line N S and the compass-needle.
+
+[Illustration: Fig. 32.]
+
+[Illustration: Fig. 33.]
+
+=42. Dip or Inclination.= If a piece of steel be carefully balanced
+upon a support, and then magnetized, it will be found that it will no
+longer balance. The north pole will _dip_ or point downward. Fig. 33
+shows what happens to a needle when it is held in different positions
+over a bar magnet. It simply takes the directions of the lines of
+force as they pass from the north to the south pole of the magnet.
+As the earth's lines of force pass in curves from the north to the
+south magnetic pole, you can see why the magnetic needle dips, unless
+its south pole is made heavier than its north. Magnetic needles are
+balanced after they are magnetized.
+
+[Illustration: Fig. 34.]
+
+Fig. 34 shows a simple form of dipping needle. These are often used
+by geologists and miners. In the hands of the prospector, the
+miner's compass, or dipping needle, proves a serviceable guide to the
+discovery and location of magnetic iron ore. In this instrument the
+magnetic needle is carefully balanced upon a horizontal axis within a
+graduated circle, and in which the needle will be found to assume a
+position inclined to the horizon. This angle of deviation is called the
+_inclination_ or _dip_, and varies in different latitudes, and even at
+different times in the same place.
+
+=43. The Earth's Inductive Influence.= The earth's magnetism acts
+inductively upon pieces of steel or iron upon its surface. If a piece
+of steel or iron, like a stove poker, for example, be held in a north
+and south line with its north end dipping considerably, it will be
+in the best position for the magnetism of the earth to act upon it;
+that is, it will lie in the direction taken by the earth's lines of
+force. If the poker be struck two or three times with a hammer to
+shake up its molecules, we shall find, upon testing it, that it has
+become magnetized. By this method we can pound magnetism right out of
+the air with a hammer. If the magnetized poker be held level, in an
+east and west direction, it will no longer be acted upon to advantage
+by the inductive influence of the earth, and we can easily hammer the
+magnetism out of it again. (For experiments on magnets and magnetism
+see "Study," Part I.)
+
+
+
+
+CHAPTER III.
+
+HOW ELECTRICITY IS GENERATED BY THE VOLTAIC CELL.
+
+
+=44. Early Experiments.= In 1786 Galvani, an Italian physician, made
+experiments to study the effect of static electricity upon the nervous
+excitability of animals, and especially upon the frog. He found that
+electric machines were not necessary to produce muscular contractions
+or kicks of the frog's legs, and that they could be produced when two
+different metals, Fig. 35, like iron and copper, for example, were
+placed in proper contact with a nerve and a muscle and then made to
+touch each other. Galvani first thought that the frog generated the
+electricity instead of the metals.
+
+[Illustration: Fig. 35.]
+
+Volta proved that the electricity was caused by the contact of the
+metals. He used the condensing electroscope as one means of proving
+that two dissimilar metals become charged differently when in contact.
+Volta also carried out his belief by constructing what is called a
+_Voltaic Pile_. He thought that by making several pairs of metals so
+arranged that all the little currents would help each other, a strong
+current could be generated. Fig. 36 shows a _pile_, it being made by
+placing a pair of zinc and copper discs in contact with one another,
+then laying on the copper disc a piece of flannel soaked in brine, then
+on top of this another pair, etc., etc. By connecting the first zinc
+and the last copper, quite a little current was produced. This was a
+start from which has been built our present knowledge of electricity.
+Strictly speaking, electricity is not generated by combinations of
+metals or by cells; they really keep up a difference of potential, as
+will be seen.
+
+[Illustration: Fig. 36.]
+
+[Illustration: Fig. 37.]
+
+[Illustration: Fig. 38.]
+
+=45. The Simple Cell.= It has been stated that two different kinds of
+electrifications may be produced by friction; one positive, the other
+negative. Either can be produced, at will, by using proper materials.
+Fig. 37 shows a section of a _simple cell_; Fig. 38 shows another view.
+Cu is a piece of copper, and Zn a piece of zinc. When they are placed
+in dilute sulphuric acid, it can be shown by delicate apparatus that
+they become charged differently, because the acid acts differently
+upon the plates. They become charged by chemical action, and not by
+friction. The zinc is gradually dissolved, and it is this chemical
+burning of the zinc that furnishes energy for the electric current in
+the simple cell. The electrification, or charge, on the plates tends to
+flow from the place of higher to the place of lower potential, just as
+water tends to flow down hill. If a wire be joined to the two metals, a
+constant current of electricity will flow through it, because the acid
+continues to act upon the plates. The simple cell is a _single-fluid_
+cell, as but one liquid is used in its construction.
+
+=45a. Plates and Poles.= The metal strips used in voltaic cells are
+called _plates_ or _elements_. The one most acted upon by the acid
+is called the positive (+) plate. In the simple cell the zinc is the
++ plate, and the copper the negative (-) plate. The end of a wire
+attached to the - plate is called the + pole, or electrode. Fig. 37
+shows the negative (-) electrode as the end of the wire attached to the
++ plate.
+
+=46. Direction of Current.= In the cell the current passes from the
+zinc to the copper; that is, from the positive to the negative plate,
+where bubbles of hydrogen gas are deposited. In the wire connecting the
+plates, the current passes from the copper to the zinc plate. In most
+cells, carbon takes the place of copper. (See "Study," Sec. 268.)
+
+=47. Local Currents; Amalgamation.= Ordinary zinc contains impurities
+such as carbon, iron, etc., and when the acid comes in contact with
+these, they form with the zinc a small cell. This tends to eat away the
+zinc without producing useful currents. The little currents in the cell
+from this cause are called _local currents_. (See "Study," Exp. 111, Sec.
+273.) This is largely overcome by coating the zinc with mercury. This
+process is called _amalgamation_. It makes the zinc act like pure zinc,
+which is not acted upon by dilute sulphuric acid when the current does
+not pass. (See "Study," Secs. 257, 274.)
+
+=48. Polarization of Cells.= Bubbles of hydrogen gas are formed when
+zinc is dissolved by an acid. In the ordinary simple cell these bubbles
+collect on the copper plate, and not on the zinc plate, as might be
+expected. The hydrogen is not a conductor of electricity, so this film
+of gas holds the current back. The hydrogen acts like a metal and sets
+up a current that opposes the zinc to the copper current. Several
+methods are employed to get rid of the hydrogen. (See "Study," Secs.
+278, 279, 280.)
+
+
+
+
+CHAPTER IV.
+
+VARIOUS VOLTAIC CELLS.
+
+
+=49. Single-Fluid and Two-Fluid Cells.= The simple cell (Sec. 45) is a
+single-fluid cell. The liquid is called the _electrolyte_, and this
+must act upon one of the plates; that is, chemical action must take
+place in order to produce a current. The simple cell polarizes rapidly,
+so something must be used with the dilute sulphuric acid to destroy the
+hydrogen bubbles. This is done in the _bichromate of potash cell_.
+
+In order to get complete depolarization--that is, to keep the carbon
+plate almost perfectly free from hydrogen, it is necessary to use
+_two-fluid cells_, or those to which some solid depolarizer is added to
+the one fluid.
+
+=50. Open and Closed Circuit Cells.= If we consider a voltaic cell, the
+wires attached to it, and perhaps some instrument through which the
+current passes, we have an _electric circuit_. When the current passes,
+the circuit is _closed_, but when the wire is cut, or in any way
+disconnected so that the current can not pass, the circuit is _open_ or
+_broken_. (See "Study," Sec. 266.)
+
+_Open Circuit Cells_ are those which can give momentary currents at
+intervals, such as are needed for bells, telephones, etc. These must
+have plenty of time to rest, as they polarize when the circuit is
+closed for a long time. The _Leclanche_ and _dry_ cells are the most
+common open circuit cells.
+
+_Closed Circuit Cells._ For telegraph lines, motors, etc., where a
+current is needed for some time, the cell must be of such a nature
+that it will not polarize quickly; it must give a strong and constant
+current. The _bichromate_ and _gravity cells_ are examples of this
+variety. (See "Study," Sec. 286.)
+
+=51. Bichromate of Potash Cells= are very useful for general laboratory
+work. They are especially useful for operating induction coils, small
+motors, small incandescent lamps, for heating platinum wires, etc.
+These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is
+used as the exciting fluid, and in this is dissolved the bichromate of
+potash which keeps the hydrogen bubbles from the carbon plate. (See
+"Apparatus Book," Sec. 26.) Zinc and carbon are used for the plates, the +
+pole being the wire attached to the carbon.
+
+[Illustration: Fig. 39.]
+
+Fig. 39 shows one form of bichromate cell. It furnishes a large
+quantity of current, and as the zinc can be raised from the fluid, it
+may be kept charged ready for use for many months, and can be set in
+action any time when required by lowering the zinc into the liquid. Two
+of these cells will burn a one candle-power miniature incandescent lamp
+several hours. The carbon is indestructible.
+
+    =Note.= For various forms of home-made cells, see "Apparatus
+    Book," Chapter I., and for battery fluids see Chapter II.
+
+=52. The Grenet Cell.= Fig. 40 is another form of bichromate cell. The
+carbon plates are left in the fluid constantly. The zinc plate should
+be raised when the cell is not in use, to keep it from being uselessly
+dissolved.
+
+[Illustration: Fig. 40.]
+
+[Illustration: Fig. 41.]
+
+=53. Plunge Batteries.= Two or more cells are often arranged so that
+their elements can be quickly lowered into the acid solution. Such a
+combination, Fig. 41, is called a _plunge battery_. The binding-posts
+are so arranged that currents of different strengths can be taken from
+the combination. The two binding-posts on the right of the battery
+will give the current of one cell; the two binding-posts on the left
+of the battery will give the current of two cells, and the two end
+binding-posts will give the current of all three cells. When not in
+use the elements must always be hung on the hooks and kept out of the
+solution.
+
+=54. Large Plunge Batteries=. Fig. 42, are arranged with a winch and
+a bar above the cells; these afford a ready and convenient means of
+lifting or lowering the elements and avoiding waste. In the battery
+shown, Fig. 42, the zincs are 4x6 inches; the carbons have the same
+dimensions, but there are two carbon plates to each zinc, thus giving
+double the carbon surface.
+
+[Illustration: Fig. 42.]
+
+=55. The Fuller Cell=, Fig. 43, is another type of bichromate cell,
+used largely for long-distance telephone service, for telephone
+exchange and switch service, for running small motors, etc. It consists
+of a glass jar, a carbon plate, with proper connections, a clay porous
+cup, containing the zinc, which is made in the form of a cone. A little
+mercury is placed in the porous cup to keep the zinc well amalgamated.
+Either bichromate of potash or bichromate of soda can be used as a
+depolarizer.
+
+[Illustration: Fig. 43.]
+
+[Illustration: Fig. 44.]
+
+=56. The Gravity Cell=, sometimes called the _bluestone_ or _crowfoot_
+cell, is used largely for telegraph, police, and fire-alarm signal
+service, laboratory and experimental work, or whenever a closed circuit
+cell is required. The E.M.F. is about one volt. This is a modified form
+of the Daniell cell. Fig. 44 shows a home-made gravity cell.
+
+A copper plate is placed at the bottom of the glass jar, and upon
+this rests a solution of copper sulphate (bluestone). The zinc plate
+is supported about four inches above the copper, and is surrounded
+by a solution of zinc sulphate which floats upon the top of the blue
+solution. An insulated wire reaches from the copper to the top of the
+cell and forms the positive pole. (See "Apparatus Book," Secs. 11 to 15,
+for home-made gravity cell, its regulation, etc. For experiments with
+two-fluid Daniell cell, see "Study," Exp. 113, Secs. 281 to 286.)
+
+[Illustration: Fig. 45.]
+
+=56a. Bunsen Cells,= Fig. 45, are used for motors, small incandescent
+lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of
+which is a cylinder of zinc that stands in dilute sulphuric acid, the
+carbon being in nitric acid.
+
+=57. The Leclanche Cell= is an open circuit cell. Sal ammoniac is used
+as the exciting fluid, carbon and zinc being used for plates. Manganese
+dioxide is used as the depolarizer; this surrounds the carbon plate,
+the two being either packed together in a porous cup or held together
+in the form of cakes. The porous cup, or pressed cake, stands in the
+exciting fluid. The E. M. F. is about 1.5 volts.
+
+[Illustration: Fig. 46.]
+
+[Illustration: Fig. 47.]
+
+[Illustration: Fig. 48.]
+
+[Illustration: Fig. 49.]
+
+Fig. 46 shows a form with porous cup. The binding-post at the top of
+the carbon plate forms the + electrode, the current leaving the cell at
+this point.
+
+_The Gonda Prism Cell_ (Fig. 47), is a form of Leclanche in which the
+depolarizer is in the form of a cake.
+
+=58. Dry Cells= are open circuit cells, and can be carried about,
+although they are moist inside. The + pole is the end of the carbon
+plate. Zinc is used as the outside case and + plate. Fig. 48 shows the
+ordinary forms.
+
+Fig. 49 shows a number of dry cells arranged in a box with switch in
+front, so that the current can be regulated at will.
+
+[Illustration: Fig. 50.]
+
+=59. The Edison-Lelande Cells=, Fig. 50, are made in several sizes and
+types. Zinc and copper oxide, which is pressed into plates, form the
+elements. The exciting fluid consists of a 25 per cent. solution of
+caustic potash in water. They are designed for both open and closed
+circuit work.
+
+
+
+
+CHAPTER V.
+
+ABOUT PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.
+
+
+=60. Electrical Connections.= In experimental work, as well as in
+the everyday work of the electrician, electrical connections must
+constantly be made. One wire must be joined to another, just for a
+moment, perhaps, or one piece of apparatus must be put in an electric
+circuit with other apparatus, or the current must be turned on or off
+from motors, lamps, etc. In order to conveniently and quickly make such
+connections, apparatus called push-buttons, switches and binding-posts
+are used.
+
+[Illustration: Fig. 51.]
+
+[Illustration: Fig. 52.]
+
+=61. Push-Buttons.= The simple act of pressing your finger upon a
+movable button, or knob, may ring a bell a mile away, or do some other
+equally wonderful thing. Fig. 51 shows a simple push-button, somewhat
+like a simple key in construction. If we cut a wire, through which a
+current is passing, then join one of the free ends to the screw A and
+the other end to screw C, we shall be able to let the current pass at
+any instant by pressing the spring B firmly upon A.
+
+Push-buttons are made in all sorts of shapes and sizes. Fig. 52 gives
+an idea of the general internal construction. The current enters A by
+one wire, and leaves by another wire as soon as the button is pushed
+and B is forced down to A. The bottom of the little button rests upon
+the top of B.
+
+Fig. 53 shows a _Table Clamp-Push_ for use on dining-tables,
+card-tables, chairs, desks, and other movable furniture. Fig. 54 shows
+a combination of push-button, speaking-tube, and letter-box used in
+city apartment houses. Fig. 55 shows an _Indicating Push_. The buzzer
+indicates, by the sound, whether the call has been heard; that is, the
+person called answers back.
+
+[Illustration: Fig. 53.]
+
+[Illustration: Fig. 54.]
+
+_Modifications_ of ordinary push-buttons are used for floor
+push-buttons, on doors, windows, etc., for burglar-alarms, for turning
+off or on lights, etc., etc. (See "Apparatus Book," Chapter III., for
+home-made push-buttons.)
+
+[Illustration: Fig. 55.]
+
+=62. Switches= have a movable bar or plug of metal, moving on a pivot,
+to make or break a circuit, or transfer a current from one conductor to
+another.
+
+Fig. 56 shows a _single point switch_. The current entering the pivoted
+arm can go no farther when the switch is open, as shown. To close
+the circuit, the arm is pushed over until it presses down upon the
+contact-point. For neatness, both wires are joined to the under side of
+the switch or to binding-posts.
+
+[Illustration: Fig. 56.]
+
+Fig. 57 shows a _knife switch_. Copper blades are pressed down between
+copper spring clips to close the circuit. The handle is made of
+insulating material.
+
+_Pole-changing switches_, Fig. 58, are used for changing or reversing
+the poles of batteries, etc.
+
+Fig. 59 shows a home-made switch, useful in connection with resistance
+coils. By joining the ends of the coils A, B, C, D, with the
+contact-points 1, 2, 3, etc., more or less resistance can be easily
+thrown in by simply swinging the lever E around to the left or right.
+If E be turned to 1, the current will be obliged to pass through all
+the coils A, B, etc., before it can pass out at Y. If E be moved to
+3, coils A and B will be cut out of the circuit, thus decreasing the
+resistance to the current on its way from X to Y. Current regulators
+are made upon this principle. (See "Apparatus Book," Chapter IV., for
+home-made switches.)
+
+[Illustration: Fig. 57.]
+
+[Illustration: Fig. 58.]
+
+[Illustration: Fig. 59.]
+
+_Switchboards_ are made containing from two or three to hundreds of
+switches, and are used in telegraph and telephone work, in electric
+light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60
+shows a switch used for incandescent lighting currents.
+
+[Illustration: Fig. 60.]
+
+[Illustration: Fig. 61.]
+
+=63. Binding-Posts= are used to make connections between two pieces of
+apparatus, between two or more wires, between a wire and any apparatus,
+etc., etc. They allow the wires to be quickly fastened or unfastened
+to the apparatus. A large part of the apparatus shown in this book has
+binding-posts attached. Fig. 61 shows a few of the common forms used.
+(See "Apparatus Book," Chapter V., for home-made binding-posts.)
+
+
+
+
+CHAPTER VI.
+
+UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.
+
+
+=64. Electrical Units.= In order to measure electricity for
+experimental or commercial purposes, standards or units are just as
+necessary as the inch or foot for measuring distances.
+
+=65. Potential; Electromotive Force.= If water in a tall tank be
+allowed to squirt from two holes, one near the bottom, the other near
+the top, it is evident that the force of the water that comes from the
+hole at the bottom will be the greater. The pressure at the bottom is
+greater than that near the top, because the "head" is greater.
+
+When a spark of static electricity jumps a long distance, we say that
+the charge has a high _potential_; that is, it has a high electrical
+pressure. Potential, for electricity, means the same as pressure, for
+water. The greater the potential, or _electromotive force_ (E.M.F.) of
+a cell, the greater its power to push a current through wires. (See
+"Study," Secs. 296 to 305, with experiments.)
+
+=66. Unit of E.M.F.; the Volt.=--In speaking of water, we say that its
+pressure is so many pounds to the square inch, or that it has a fall,
+or head, of so many feet. We speak of a current as having so many
+volts; for example, we say that a wire is carrying a 110-volt current.
+The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F.
+of about one volt. This name was given in honor of Volta.
+
+=67. Measurement of Electromotive Force.= There are several ways by
+which the E.M.F. of a cell, for example, can be measured. It is usually
+measured _relatively_, by comparison with the E. M. F. of some standard
+cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by
+comparison with the two-fluid cell.)
+
+[Illustration: Fig. 62.]
+
+_Voltmeters_ are instruments by means of which E. M. F. can be read on
+a printed scale. They are a variety of galvanometer, and are made with
+coils of such high resistance, compared with the resistance of a cell
+or dynamo, that the E. M. F. can be read direct. The reason for this
+will be seen by referring to Ohm's law ("Study," Sec. 356); the resistance
+is so great that the strength of the current depends entirely upon the
+E. M. F.
+
+[Illustration: Fig. 63.]
+
+Voltmeters measure electrical pressure just as steam gauges measure
+the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63
+shows a voltmeter with illuminated dial. An electrical bulb behind the
+instrument furnishes light so that the readings can be easily taken.
+
+=68. Electrical Resistance.= Did you ever ride down hill on a
+hand-sled? How easily the sled glides over the snow! What happens,
+though, when you strike a bare place, or a place where some evil-minded
+person has sprinkled ashes? Does the sled pass easily over bare ground
+or ashes? Snow offers very little _resistance_ to the sled, while ashes
+offer a great resistance.
+
+[Illustration: Fig. 64.]
+
+All substances do not allow the electric current to pass through
+them with the same ease. Even the liquid in a cell tends to hold the
+current back and offers _internal resistance_. The various wires and
+instruments connected to a cell offer _external resistance_. (See
+"Study," Chapter XVIII., for experiments, etc.)
+
+=69. Unit of Resistance.= =The Ohm= is the name given to the unit of
+resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in.
+of No. 24 copper wire, will make a fairly accurate ohm.
+
+_Resistance coils_, having carefully measured resistances, are made
+for standards. (See "Apparatus Book," Chapter XVII., for home-made
+resistance coils.) Fig. 64 shows a commercial form of a standard
+resistance coil. The coil is inclosed in a case and has large wires
+leading from its ends for connections. Fig. 65 gives an idea of
+the way in which coils are wound and used with plugs to build up
+_resistance boxes_, Fig. 66.
+
+=70. Laws of Resistance.= 1. The resistance of a wire is directly
+proportional to its length, provided its cross-section, material, etc.,
+are uniform.
+
+2. The resistance of a wire is inversely proportional to its area of
+cross-section; or, in other words, inversely proportional to the square
+of its diameter, other things being equal.
+
+[Illustration: Fig. 65.]
+
+3. The resistance of a wire depends upon its material, as well as upon
+its length, size, etc.
+
+4. The resistance of a wire increases as its temperature rises. (See
+"Study," Chapters XVIII. and XIX., for experiments on resistance, its
+measurement, etc.)
+
+[Illustration: Fig. 66.]
+
+=71. Current Strength.= The strength of a current at the end of a
+circuit depends not only upon the _electrical pressure_, or E. M. F.,
+which drives the current, but also upon the _resistance_ which has to
+be overcome. The greater the resistance the weaker the current at the
+end of its journey.
+
+=72. Unit of Current Strength; The Ampere.= A current having an E. M.
+F. of _one volt_, pushing its way through a resistance of _one ohm_,
+would have a unit of strength, called _one ampere_. This current, one
+ampere strong, would deposit, under proper conditions, .0003277 gramme
+of copper in _one second_ from a solution of copper sulphate.
+
+=73. Measurement of Current Strength.= A magnetic needle is deflected
+when a current passes around it, as in instruments like the
+galvanometer. The _galvanoscope_ merely indicates the presence of a
+current. _Galvanometers_ measure the strength of a current, and they
+are made in many forms, depending upon the nature and strength of the
+currents to be measured. Galvanometers are standardized, or calibrated,
+by special measurements, or by comparison with some standard
+instrument, so that when the deflection is a certain number of degrees,
+the current passing through it is known to be of a certain strength.
+
+[Illustration: Fig. 67.]
+
+Fig. 67 shows an _astatic galvanometer_. Fig. 68 shows a _tangent
+galvanometer_, in which the strength of the current is proportional
+to the tangent of the angle of deflection. Fig. 69 shows a _D'Arsonval
+galvanometer_, in which a coil of wire is suspended between the poles
+of a permanent horseshoe magnet. The lines of force are concentrated
+by the iron core of the coil. The two thin suspending wires convey the
+current to the coil. A ray of light is reflected from the small mirror
+and acts as a pointer as in other forms of reflecting galvanometers.
+
+[Illustration: Fig. 68.]
+
+=74. The Ammeter=, Fig. 70, is a form of galvanometer in which the
+strength of a current, in amperes, can be read. In these the strength
+of current is proportional to the angular deflections. The coils are
+made with a small resistance, so that the current will not be greatly
+reduced in strength in passing through them.
+
+[Illustration: Fig. 69.]
+
+=75. Voltameters= measure the strength of a current by chemical means,
+the quantity of metal deposited or gas generated being proportional
+to the time that the current flows and to its strength. In the _water
+voltameter_, Fig. 71, the hydrogen and oxygen produced in a given time
+are measured. (See "Study," Chapter XXI.)
+
+[Illustration: Fig. 70.]
+
+The _copper voltameter_ measures the amount of copper deposited in a
+given time by the current. Fig. 72 shows one form. The copper cathode
+is weighed before and after the current flows. The weight of copper
+deposited and the time taken are used to calculate the current strength.
+
+[Illustration: Fig. 71.]
+
+=76. Unit of Quantity=; =The Coulomb= is the quantity of electricity
+given, in _one second_, by a current having a strength of one ampere.
+Time is an important element in considering the work a current can do.
+
+[Illustration: Fig. 72.]
+
+=77. Electrical Horse-power=; =The Watt= is the unit of electrical
+power. A current having the strength of one ampere, and an E. M.
+F. of one volt has a unit of power. 746 watts make one electrical
+horse-power. Watts = amperes x volts. Fig. 73 shows a direct reading
+wattmeter based on the international volt and ampere. They save taking
+simultaneous ammeter and voltmeter readings, which are otherwise
+necessary to get the product of volts and amperes, and are also used on
+alternating current measurements.
+
+[Illustration: Fig. 73.]
+
+There are also forms of wattmeters, Fig. 74, in which the watts are
+read from dials like those on an ordinary gas-meter, the records being
+permanent.
+
+Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit
+that readings can be taken. D represents a dynamo. A is placed so that
+the whole current passes through it, while V is placed between the main
+wires to measure the difference in potential. The product of the two
+readings in volts and amperes gives the number of watts.
+
+[Illustration: Fig. 74.]
+
+=78. Chemical Meters= also measure the quantity of current that is
+used; for example, one may be placed in the cellar to measure the
+quantity of current used to light the house.
+
+[Illustration: Fig. 75.]
+
+Fig. 76 shows a chemical meter, a part of the current passing through
+a jar containing zinc plates and a solution of zinc sulphate. Metallic
+zinc is dissolved from one plate and deposited upon the other. The
+increase in weight shows the amount of chemical action which is
+proportional to the ampere hours. Knowing the relation between the
+quantity of current that can pass through the solution to that which
+can pass through the meter by another conductor, a calculation can be
+made which will give the current used. A lamp is so arranged that it
+automatically lights before the meter gets to the freezing-point; this
+warms it up to the proper temperature, at which point the light goes
+out again.
+
+[Illustration: Fig. 76.]
+
+
+
+
+CHAPTER VII.
+
+CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.
+
+
+=79. Electrolysis.= It has been seen that in the voltaic cell
+electricity is generated by chemical action. Sulphuric acid acts upon
+zinc and dissolves it in the cell, hydrogen is produced, etc. When
+this process is reversed, that is, when the electric current is passed
+through some solutions, they are decomposed, or broken up into their
+constituents. This process is called _electrolysis_, and the compound
+decomposed is the _electrolyte_. (See "Study," Sec. 369, etc., with
+experiments.)
+
+[Illustration: Fig. 77.]
+
+Fig. 77 shows how water can be decomposed into its two constituents,
+hydrogen and oxygen, there being twice as much hydrogen formed as
+oxygen.
+
+Fig. 78 shows a glass jar in which are placed two metal strips, A and
+C, these being connected with two cells. In this jar may be placed
+various conducting solutions to be tested. If, for example, we use
+a solution of copper sulphate, its chemical formula being CuSO_{4},
+the current will break it up into Cu (copper) and SO_{4}. The Cu will
+be deposited upon C as the current passes from A to C through the
+solution. A is called the _anode_, and C the _cathode_.
+
+[Illustration: Fig. 78.]
+
+Fig. 79 shows another form of jar used to study the decomposition of
+solutions by the electric current.
+
+[Illustration: Fig 79.]
+
+=80. Ions.= When a solution is decomposed into parts by a current, the
+parts are called the _Ions_. When copper sulphate (Cu SO_{4}) is used,
+the ions are Cu, which is a metal, and SO_{4}, called an acid radical.
+When silver nitrate (Ag NO_{3}) is used, Ag and NO_{3} are the ions.
+The metal part of the compound goes to the cathode.
+
+
+
+
+CHAPTER VIII.
+
+HOW ELECTROPLATING AND ELECTROTYPING ARE DONE.
+
+
+=81. Electricity and Chemical Action.= We have just seen, Chapter VII.,
+that the electric current has the power to decompose certain compounds
+when they are in solution. By choosing the right solutions, then, we
+shall be able to get copper, silver, and other metals set free by
+electrolysis.
+
+=82. Electroplating= consists in coating substances with metal with
+the aid of the electric current. If we wish to electroplate a piece
+of metal with copper, for example, we can use the arrangement shown
+in Fig. 78, in which C is the cathode plate to be covered, and A is
+a copper plate. The two are in a solution of copper sulphate, and,
+as explained in Sec. 79, the solution will be decomposed. Copper will
+be deposited upon C, and the SO_{4} part of the solution will go to
+the anode A, which it will attack and gradually dissolve. The SO_{4},
+acting upon the copper anode, makes CuSO_{4} again, and this keeps the
+solution at a uniform strength. The amount of copper dissolved from the
+copper anode equals, nearly, the amount deposited upon the cathode. The
+metal is carried in the direction of the current.
+
+If we wish to plate something with silver or gold, it will be necessary
+to use a solution of silver or gold for the electrolyte, a plate of
+metallic silver or gold being used for the anode, as the case may be.
+
+Great care is used in cleaning substances to be plated, all dirt and
+grease being carefully removed.
+
+Fig. 80 shows a plating bath in which several articles can be plated
+at the same time by hanging them upon a metal bar which really forms a
+part of the cathode. If, for example, we wish to plate knives, spoons,
+etc., with silver, they would be hung from the bar shown, each being a
+part of the cathode. The vat would contain a solution of silver, and
+from the other bar would be hung a silver plate having a surface about
+equal to that of the combined knives, etc.
+
+[Illustration: Fig. 80.]
+
+Most metals are coated with copper before they are plated with silver
+or gold. When plating is done on a large scale, a current from a dynamo
+is used. For experimental purposes a Gravity cell will do very well.
+(See "Study," Secs. 374 to 380 with experiments.)
+
+=83. Electrotyping.= It was observed by De La Rue in 1836 that in the
+Daniell cell an even coating of copper was deposited upon the copper
+plate. From this was developed the process of electrotyping, which
+consists in making a copy in metal of a wood-cut, page of type, etc.
+A mould or impression of the type or coin is first made in wax, or
+other suitable material. These moulds are, of course, the reverse
+of the original, and as they do not conduct electricity, have to be
+coated with graphite. This thin coating lines the mould with conducting
+material so that the current can get to every part of the mould.
+These are then hung upon the cathode in a bath of copper sulphate
+as described in Sec. 82. The electric current which passes through the
+vat deposits a thin layer of metallic copper next to the graphite.
+When this copper gets thick enough, the wax is melted away from it,
+leaving a thin shell of copper, the side next to the graphite being
+exactly alike in shape to the type, but made of copper. These thin
+copper sheets are too thin to stand the pressure necessary on printing
+presses, so they are strengthened by backing them with soft metal which
+fills every crevice, making solid plates about 1/4 in. thick. These
+plates or _electrotypes_ are used to print from, the original type
+being used to set up another page.
+
+
+
+
+CHAPTER IX.
+
+THE STORAGE BATTERY, AND HOW IT WORKS.
+
+
+=84. Polarization.= It has been stated that a simple cell polarizes
+rapidly on account of hydrogen bubbles that form upon the copper plate.
+They tend to send a current in the opposite direction to that of the
+main current, which is thereby weakened.
+
+[Illustration: Fig. 81.]
+
+=85. Electromotive Force of Polarization.= It has been shown, Fig. 71,
+that water can be decomposed by the electric current. Hydrogen and
+oxygen have a strong attraction or chemical affinity for each other, or
+they would not unite to form water. This attraction has to be overcome
+before the water can be decomposed. As soon as the decomposing current
+ceases to flow, the gases formed try to rush together again; in fact,
+if the water voltameter be disconnected from the cells and connected
+with a galvanoscope, the presence of a current will be shown. This
+voltameter will give a current with an E. M. F. of nearly 1.5 volts; so
+it is evident that we must have a current with a higher voltage than
+this to decompose water. This E. M. F., due to polarization, is called
+the E. M. F. of polarization.
+
+=86. Secondary or Storage Batteries=, also called _accumulators_, do
+not really store electricity. They must be charged by a current before
+they can give out any electricity. Chemical changes are produced in the
+storage cells by the charging current just as they are in voltameters,
+electroplating solutions, etc.; so it is potential chemical energy
+that is really stored. When the new products are allowed to go back to
+their original state, by joining the electrodes of the charged cell, a
+current is produced.
+
+Fig. 81 shows two lead plates, A and B, immersed in dilute sulphuric
+acid, and connected with two ordinary cells. A strong current will pass
+through the liquid between A and B at first, but it will quickly become
+weaker, as chemical changes take place in the liquid. This may be shown
+by a galvanometer put in the circuit before beginning the experiment.
+By disconnecting the wires from the cells and joining them to the
+galvanometer, it will be shown that a current comes from the lead
+plates. This arrangement may be called a simple storage cell. Regular
+storage cells are charged with the current from a dynamo. (See "Study,"
+Exp. 151.)
+
+[Illustration: Fig. 82.]
+
+The first storage cells were made of plain lead plates, rolled up in
+such a way that they were close to each other, but did not touch. These
+were placed in dilute sulphuric acid. They were charged in alternate
+directions several times, until the lead became properly acted upon, at
+which time the cell would furnish a current.
+
+A great improvement was made in 1881, by Faure, who coated the plates
+with red lead.
+
+[Illustration: Fig. 83.]
+
+The method now generally practiced is to cast a frame of lead, with
+raised right-angled ribs on each side, thus forming little depressed
+squares, or to punch a lead plate full of holes, which squares or holes
+are then filled with a pasty mixture of red oxide of lead in positive
+plates, and with litharge in negatives. In a form called the chloride
+battery, instead of cementing lead oxide paste into or against a lead
+framing in order to obtain the necessary active material, the latter is
+obtained by a strictly chemical process.
+
+Fig. 82 shows a storage cell with plates, etc., contained in a glass
+jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood
+tank. Fig. 84 shows three cells joined in series. Many storage cells
+are used in central electric light stations to help the dynamos during
+the "rush" hours at night. They are charged during the day when the
+load on the dynamos is not heavy.
+
+Fig. 85 shows another form of storage cell containing a number of
+plates.
+
+[Illustration: Fig. 84.]
+
+=87. The Uses of Storage Batteries= are almost numberless. The current
+can be used for nearly everything for which a constant current is
+adapted, the following being some of its applications: Carriage
+propulsion; electric launch propulsion; train lighting; yacht lighting;
+carriage lighting; bicycle lighting; miners' lamps; dental, medical,
+surgical, and laboratory work; phonographs; kinetoscopes; automaton
+pianos; sewing-machine motors; fan motors; telegraph; telephone;
+electric bell; electric fire-alarm; heat regulating; railroad switch
+and signal apparatus.
+
+By the installing of a storage plant many natural but small sources
+of power may be utilized in furnishing light and power; sources which
+otherwise are not available, because not large enough to supply maximum
+demands. The force of the tides, of small water powers from irrigating
+ditches, and even of the wind, come under this heading.
+
+[Illustration: Fig. 85.]
+
+As a regulator of pressure, in case of fluctuations in the load, the
+value of a storage plant is inestimable. These fluctuations of load are
+particularly noticeable in electric railway plants, where the demand is
+constantly rising and falling, sometimes jumping from almost nothing to
+the maximum, and _vice versa_, in a few seconds. If for no other reason
+than the prevention of severe strain on the engines and generators,
+caused by these fluctuations of demand, a storage plant will be
+valuable.
+
+
+
+
+CHAPTER X.
+
+HOW ELECTRICITY IS GENERATED BY HEAT.
+
+
+=88. Thermoelectricity= is the name given to electricity that is
+generated by heat. If a strip of iron, I, be connected between two
+strips of copper, C C, these being joined by a copper wire, C W, we
+shall have an arrangement that will generate a current when heated at
+either of the junctions between C and I. When it is heated at A the
+current will flow as shown by arrows, from C to I. If we heat at B,
+the current will flow in the opposite direction through the metals,
+although it will still go from C to I as before. Such currents are
+called _thermoelectric currents_.
+
+[Illustration: Fig. 86.]
+
+Different pairs of metals produce different results. Antimony and
+bismuth are generally used, because the greatest effect is produced
+by them. If the end of a strip of bismuth be soldered to the end of
+a similar strip of antimony, and the free ends be connected to a
+galvanometer of low resistance, the presence of a current will be shown
+when the point of contact becomes hotter than the rest of the circuit.
+The current will flow from bismuth to antimony across the joint. By
+cooling the juncture below the temperature of the rest of the circuit,
+a current will be produced in the opposite direction to the above. The
+energy of the current is kept up by the heat absorbed, just as it is
+kept up by chemical action in the voltaic cell.
+
+=89. Peltier Effect.= If an electric current be passed through pairs of
+metals, the parts at the junction become slightly warmer or cooler than
+before, depending upon the direction of the current. This action is
+really the reverse of that in which currents are produced by heat.
+
+[Illustration: Fig. 87.]
+
+=90. Thermopiles.= As the E.M.F. of the current produced by a single
+pair of metals is very small, several pairs are usually joined in
+series, so that the different currents will help each other by flowing
+in the same direction. Such combinations are called thermoelectric
+piles, or simply _thermopiles_.
+
+Fig. 87 shows such an arrangement, in which a large number of elements
+are placed in a small space. The junctures are so arranged that the
+alternate ones come together at one side.
+
+Fig. 88 shows a thermopile connected with a galvanometer. The heat of
+a match, or the cold of a piece of ice, will produce a current, even if
+held at some distance from the thermopile. The galvanometer should be
+a short-coil astatic one. (See "Study," Chapter XXIV., for experiments
+and home-made thermopile.)
+
+[Illustration: Fig. 88.]
+
+
+
+
+CHAPTER XI.
+
+MAGNETIC EFFECTS OF THE ELECTRIC CURRENT.
+
+
+=91. Electromagnetism= is the name given to magnetism that is developed
+by electricity. We have seen that if a magnetic needle be placed in the
+field of a magnet, its N pole will point in the direction taken by the
+lines of force as they pass from the N to the S pole of the magnet.
+
+[Illustration: Fig. 89.]
+
+=92. Lines of Force about a Wire.= When a current passes through a
+wire, the magnetic needle placed over or under it tends to take a
+position at right angles to the wire. Fig. 89 shows such a wire and
+needle, and how the needle is deflected; it twists right around from
+its N and S position as soon as the current begins to flow. This shows
+that the lines of force pass _around_ the wire and not in the direction
+of its length. The needle does not swing entirely perpendicular to the
+wire, that is, to the E and W line, because the earth is at the same
+time pulling its N pole toward the N.
+
+Fig. 90 shows a bent wire through which a current passes from C to Z.
+If you look along the wire from C toward the points A and B, you will
+see that _under_ the wire the lines of force pass to the left. Looking
+along the wire from Z toward D you will see that the lines of force
+pass opposite to the above, as the current comes _toward_ you. This is
+learned by experiment. (See "Study," Exp. 152, Sec. 385, etc.)
+
+[Illustration: Fig. 90.]
+
+[Illustration: Fig. 91.]
+
+_Rule._ Hold the right hand with the thumb extended (Fig. 89) and with
+the fingers pointing in the direction of the current, the palm being
+toward the needle and on the opposite side of the wire from the needle.
+The north-seeking pole will then be deflected in the direction in which
+the thumb points.
+
+=93. Current Detectors.= As there is a magnetic field about a wire when
+a current passes through it, and as the magnetic needle is affected, we
+have a means of detecting the presence of a current. When the current
+is strong it is simply necessary to let it pass once over or under a
+needle; when it is weak, the wire must pass several times above and
+below the needle, Fig. 91, to give the needle motion. (See "Apparatus
+Book," Chapter XIII., for home-made detectors.)
+
+[Illustration: Fig. 92.]
+
+=94. Astatic Needles and Detectors.= By arranging two magnetized
+needles with their poles opposite each other, Fig. 92, an _astatic
+needle_ is formed. The pointing-power is almost nothing, although
+their magnetic fields are retained. This combination is used to detect
+feeble currents. In the ordinary detector, the tendency of the needle
+to point to the N and S has to be overcome by the magnetic field about
+the coil before the needle can be moved; but in the _astatic detector_
+and _galvanoscope_ this pointing-power is done away with. Fig. 93 shows
+a simple _astatic galvanoscope_. Fig. 67 shows an astatic galvanometer
+for measuring weak currents.
+
+[Illustration: Fig. 93.]
+
+=95. Polarity of Coils.= When a current of electricity passes through
+a coil of wire, the coil acts very much like a magnet, although no
+iron enters into its construction. The coil becomes magnetized by the
+electric current, lines of force pass from it into the air, etc. Fig.
+94 shows a coil connected to copper and zinc plates, so arranged with
+cork that the whole can float in a dish of dilute sulphuric acid. The
+current passes as shown by the arrows, and when the N pole of a magnet
+is brought near the right-hand end, there is a repulsion, showing that
+that end of the coil has a N pole.
+
+_Rule._ When you face the right-hand end of the coil, the current is
+seen to pass around it in an anti-clockwise direction; this produces a
+N pole. When the current passes in a clockwise direction a S pole is
+produced.
+
+[Illustration: Fig. 94.]
+
+=96. Electromagnets.= A coil of wire has a stronger field than a
+straight wire carrying the same current, because each turn adds its
+field to the fields of the other turns. By having the central part of
+the coil made of iron, or by having the coil of insulated wire wound
+upon an iron _core_, the strength of the magnetic field of the coil is
+greatly increased.
+
+Lines of force do not pass as readily through air as through iron;
+in fact, lines of force will go out of their way to go through iron.
+With a coil of wire the lines of force pass from its N pole through
+the air on all sides of the coil to its S pole; they then pass through
+the inside of the coil and through the air back to the N pole. When
+the resistance to their passage through the coil is decreased by the
+core, the magnetic field is greatly strengthened, and we have an
+_electromagnet_.
+
+The coil of wire temporarily magnetizes the iron core; it can
+permanently magnetize a piece of steel used as a core. (See "Study,"
+Chapter XXII., for experiments.)
+
+[Illustration: Fig. 95.]
+
+=97. Forms of Electromagnets.= Fig. 95 shows a _straight, or
+bar electromagnet_. Fig. 96 shows a simple form of _horseshoe
+electromagnet_. As this form is not easily wound, the coils are
+generally wound on two separate cores which are then joined by a
+_yoke_. The yoke merely takes the place of the curved part shown
+in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe
+electromagnet used for all sorts of electrical instruments. (See
+"Apparatus Book," Chapter IX., for home-made electromagnets.)
+
+=98. Yokes and Armatures.= In the horseshoe magnet there are two poles
+to attract and two to induce. The lines of force pass through the yoke
+on their way from one core to the other, instead of going through
+the air. This reduces the resistance to them. If we had no yoke we
+should simply have two straight electromagnets, and the resistance to
+the lines of force would be so great that the total strength would
+be much reduced. Yokes are made of soft iron, as well as the cores
+and armature. The _armature_, as with permanent horseshoe magnets, is
+strongly drawn toward the poles. As soon as the current ceases to flow,
+the attraction also ceases.
+
+[Illustration: Fig. 96.]
+
+[Illustration: Fig. 97.]
+
+[Illustration: Fig. 98.]
+
+Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98
+shows that the coils must be joined so that the current can pass around
+the cores in opposite directions to make unlike poles. (See "Study,"
+Exp. 164 to 173.)
+
+
+
+
+CHAPTER XII.
+
+HOW ELECTRICITY IS GENERATED BY INDUCTION.
+
+
+=99. Electromagnetic Induction.= We have seen that a magnet has the
+power to act through space and induce another piece of iron or steel
+to become a magnet. A charge of static electricity can induce a
+charge upon another conductor. We have now to see how a _current_ of
+electricity in one conductor can induce a current in another conductor,
+not in any way connected with the first, and how a magnet and a coil
+can generate a current.
+
+[Illustration: Fig. 99.]
+
+[Illustration: Fig. 100.]
+
+=100. Current from Magnet and Coil.= If a bar magnet, Fig. 99, be
+suddenly thrust into a hollow coil of wire, a momentary current of
+electricity will be generated in the coil. No current passes when the
+magnet and coil are still; at least one of them must be in motion. Such
+a current is said to be _induced_, and is an _inverse_ one when the
+magnet is inserted, and a _direct_ one when the magnet is withdrawn
+from the coil.
+
+=101. Induced Currents and Lines of Force.= Permanent magnets are
+constantly sending out thousands of lines of force. Fig. 100 shows
+a bar magnet entering a coil of wire; the number of lines of force
+is increasing, and the induced current passes in an anti-clockwise
+direction when looking down into the coil along the lines of force.
+This produces an indirect current. If an iron core be used in the coil,
+the induced current will be greatly strengthened.
+
+[Illustration: Fig. 101.]
+
+It takes force to move a magnet through the center of a coil, and it
+is this work that is the source of the induced current. We have, in
+this simple experiment, the key to the action of the dynamo and other
+electrical machines.
+
+=102. Current from two Coils.= Fig. 101 shows two coils of wire, the
+smaller being connected to a cell, the larger to a galvanometer.
+By moving the small coil up and down inside of the large one,
+induced currents are generated, first in one direction and then in
+the opposite. We have here two entirely separate circuits, in no
+way connected. The _primary_ current comes from the cell, while the
+_secondary_ current is an induced one. By placing a core in the small
+coil of Fig. 101, the induced current will be greatly strengthened.
+
+It is not necessary to have the two coils so that one or both of them
+can move. They may be wound on the same core, or otherwise arranged as
+in the induction coil. (See "Study," Chapter XXV., for experiments on
+induced currents.)
+
+
+
+
+CHAPTER XIII.
+
+HOW THE INDUCTION COIL WORKS.
+
+
+=103. The Coils.= We saw, Sec. 102, that an induced current was generated
+when a current-carrying coil, Fig. 101, was thrust into another coil
+connected with a galvanometer. The galvanometer was used merely to show
+the presence of the current. The _primary coil_ is the one connected
+with the cell; the other one is called the _secondary coil_.
+
+[Illustration: Fig. 102.]
+
+When a current suddenly begins to flow through a coil, the effect upon
+a neighboring coil is the same as that produced by suddenly bringing
+a magnet near it; and when the current stops, the opposite effect is
+produced. It is evident, then, that we can keep the small coil of
+Fig. 101 with its core inside of the large coil, and generate induced
+currents by merely making and breaking the primary circuit.
+
+We may consider that when the primary circuit is closed, the lines of
+force shoot out through the turns of the secondary coil just as they
+do when a magnet or a current-carrying coil is thrust into it. Upon
+opening the circuit, the lines of force cease to exist; that is, we may
+imagine them drawn in again.
+
+=104. Construction.= Fig. 102 shows one form of home-made induction
+coil, given here merely to explain the action and connections. Nearly
+all induction coils have some form of automatic current interrupter,
+placed in the primary circuit, to rapidly turn the current off and on.
+
+_Details of Figs. 102 and 103._ Wires 5 and 6 are the ends of the
+primary coil, while wires 7 and 8 are the terminals of the secondary
+coil. The primary coil is wound on a bolt which serves as the core, and
+on this coil is wound the secondary which consists of many turns of
+fine wire. The wires from a battery should be joined to binding-posts W
+and X, and the handles, from which the shock is felt, to Y and Z. Fig.
+103 shows the details of the interrupter.
+
+[Illustration: Fig. 103.]
+
+If the current from a cell enters at W, it will pass through the
+primary coil and out at X, after going through 5, R, F, S I, B, E and
+C. The instant the current passes, the bolt becomes magnetized; this
+attracts A, which pulls B away from the end of S I, thus automatically
+opening the circuit. B at once springs back to its former position
+against SI, as A is no longer attracted; the circuit being closed, the
+operation is rapidly repeated.
+
+A _condenser_ is usually connected to commercial forms. It is placed
+under the wood-work and decreases sparking at the interrupter. (See
+"Apparatus Book," Chapter XI., for home-made induction coils.)
+
+[Illustration: Fig. 104.]
+
+Fig. 104 shows one form of coil. The battery wires are joined to the
+binding-posts at the left. The secondary coil ends in two rods, and the
+spark jumps from one to the other. The interrupter and a switch are
+shown at the left.
+
+Fig. 105 shows a small coil for medical purposes. A dry cell is placed
+under the coil and all is included in a neat box. The handles form the
+terminals of the secondary coil.
+
+=105. The Currents.= It should be noted that the current from the
+cell does not get into the secondary coil. The coils are thoroughly
+insulated from each other. The secondary current is an induced one,
+its voltage depending upon the relative number of turns of wire there
+are in the two coils. (See Transformers.) The secondary current is
+an alternating one; that is, it flows in one direction for an instant
+and then immediately reverses its direction. The rapidity of the
+alternations depends upon the speed of the interrupter. Coils are made
+that give a secondary current with an enormous voltage; so high, in
+fact, that the spark will pass many inches, and otherwise act like
+those produced by static electric machines.
+
+[Illustration: Fig. 105.]
+
+=106. Uses of Induction Coils.= Gas-jets can be lighted at a distance
+with the spark from a coil, by extending wires from the secondary
+coil to the jet. Powder can be fired at a distance, and other things
+performed, when a high voltage current is needed. Its use in medicine
+has been noted. It is largely used in telephone work. Of late, great
+use has been made of the secondary current in experiments with
+vacuum-tubes, X-ray work, etc.
+
+
+
+
+CHAPTER XIV.
+
+THE ELECTRIC TELEGRAPH, AND HOW IT SENDS MESSAGES.
+
+
+=107. The Complete Telegraph Line= consists of several instruments,
+switches, etc., etc., but its essential parts are: The _Line_, or wire,
+which connects the different stations; the _Transmitter_ or _Key_; the
+_Receiver_ or _Sounder_, and the _Battery_ or _Dynamo_.
+
+=108. The Line= is made of strong copper, iron, or soft steel wire. To
+keep the current in the line it is insulated, generally upon poles, by
+glass insulators. For very short lines two wires can be used, the line
+wire and the return; but for long lines the earth is used as a return,
+a wire from each end being joined to large metal plates sunk in the
+earth.
+
+[Illustration: Fig. 106.]
+
+=109. Telegraph Keys= are merely instruments by which the circuit
+can be conveniently and rapidly opened or closed at the will of the
+operator. An ordinary push-button may be used to turn the current off
+and on, but it is not so convenient as a key.
+
+Fig. 106 shows a side view of a simple key which can be put anywhere
+in the circuit, one end of the cut wire being attached to X and the
+other to Y. By moving the lever C up and down according to a previously
+arranged set of signals, a current will be allowed to pass to a
+distant station. As X and Y are insulated from each other, the current
+can pass only when C presses against Y.
+
+Fig. 107 shows a regular key, with switch, which is used to allow the
+current to pass through the instrument when receiving a message.
+
+[Illustration: Fig. 107.]
+
+=110. Telegraph Sounders= receive the current from some distant
+station, and with its electromagnet produce sounds that can be
+translated into messages.
+
+[Illustration: Fig. 108.]
+
+Fig. 108 shows simply an electromagnet H, the coil being connected in
+series with a key K and a cell D C. The key and D C are shown by a top
+view. The lever of K does not touch the other metal strap until it is
+pressed down. A little above the core of H is held a strip of iron, on
+armature I. As soon as the circuit is closed at K, the current rushes
+through the circuit, and the core attracts I making a distinct _click_.
+As soon as K is raised, I springs away from the core, if it has been
+properly held. In regular instruments a click is also made when the
+armature springs back again.
+
+The time between the two clicks can be short or long, to represent
+_dots_ or _dashes_, which, together with _spaces_, represent letters.
+(For Telegraph Alphabet and complete directions for home-made keys,
+sounders, etc., see "Apparatus Book," Chapter XIV.)
+
+[Illustration: Fig. 109.]
+
+[Illustration: Fig. 110.]
+
+Fig. 109 shows a form of home-made sounder. Fig. 110 shows one form of
+telegraph sounder. Over the poles of the horseshoe electromagnet is an
+armature fixed to a metal bar that can rock up and down. The instant
+the current passes through the coils the armature comes down until a
+stop-screw strikes firmly upon the metal frame, making the down click.
+As soon as the distant key is raised, the armature is firmly pulled
+back and another click is made. The two clicks differ in sound, and can
+be readily recognized by the operator.
+
+=111. Connections for Simple Line.= Fig. 111 shows complete connections
+for a home-made telegraph line. The capital letters are used for the
+right side, R, and small letters for the left side, L. Gravity cells,
+B and b, are used. The _sounders_, S and s, and the _keys_, K and k,
+are shown by a top view. The broad black lines of S and s represent the
+armatures which are directly over the electromagnets. The keys have
+switches, E and e.
+
+The two stations, R and L, may be in the same room, or in different
+houses. The _return wire_, R W, passes from the copper of b to the zinc
+of B. This is important, as the cells must help each other; that is,
+they are in series. The _line wire_, L W, passes from one station to
+the other, and the return may be through the wire, R W, or through the
+earth; but for short lines a wire is best.
+
+[Illustration: Fig. 111.]
+
+=112. Operation of Simple Line.= Suppose two boys, R (right) and L
+(left) have a line. Fig. 111 shows that R's switch, E, is open, while
+e is closed. The entire circuit, then, is broken at but one point. As
+soon as R presses his key, the circuit is closed, and the current from
+both cells rushes around from B, through K, S, L W, s, k, b, R W, and
+back to B. This makes the armatures of S and s come down with a click
+at the same time. As soon as the key is raised, the armatures lift and
+make the up-click. As soon as R has finished, he closes his switch E.
+As the armatures are then held down, L knows that R has finished, so
+he opens his switch e, and answers R. Both E and e are closed when the
+line is not in use, so that either can open his switch at any time and
+call up the other. Closed circuit cells must be used for such lines. On
+very large lines dynamos are used to furnish the current.
+
+=113. The Relay.= Owing to the large resistance of long telegraph
+lines, the current is weak when it reaches a distant station, and not
+strong enough to work an ordinary sounder. To get around this, relays
+are used; these are very delicate instruments that replace the sounder
+in the line wire circuit. Their coils are usually wound with many turns
+of fine wire, so that a feeble current will move its nicely adjusted
+armature. The relay armature merely acts as an automatic key to open
+and close a local circuit which includes a battery and sounder. The
+line current does not enter the sounder; it passes back from the relay
+to the sending station through the earth.
+
+[Illustration: Fig. 112.]
+
+Fig. 112 gives an idea of simple relay connections. The key K, and
+cell D C, represent a distant sending station. E is the electromagnet
+of the relay, and R A is its armature. L W and R W represent the line
+and return wires. R A will vibrate toward E every time K is pressed,
+and close the local circuit, which includes a local battery, L B, and
+a sounder. It is evident that as soon as K is pressed the sounder will
+work with a good strong click, as the local battery can be made as
+strong as desired.
+
+Fig. 113 shows a regular instrument which opens and closes the local
+circuit at the top of the armature.
+
+[Illustration: Fig. 113.]
+
+=114. Ink Writing Registers= are frequently used instead of sounders.
+Fig. 114 shows a writing register that starts itself promptly at the
+opening of the circuit, and stops automatically as soon as the circuit
+returns to its normal condition. A strip of narrow paper is slowly
+pulled from the reel by the machine, a mark being made upon it every
+time the armature of an inclosed electromagnet is attracted. When the
+circuit is simply closed for an instant, a short line, representing a
+_dot_, is made.
+
+Registers are built both single pen and double pen. In the latter case,
+as the record of one wire is made with a fine pen, and the other with
+a coarse pen, they can always be identified. The record being blocked
+out upon white tape in solid black color, in a series of clean-cut dots
+and dashes, it can be read at a glance, and as it is indelible, it may
+be read years afterward. Registers are made for local circuits, for
+use in connection with relays, or for direct use on main lines, as is
+usually desirable in fire-alarm circuits.
+
+[Illustration: Fig. 114.]
+
+
+
+
+CHAPTER XV.
+
+THE ELECTRIC BELL AND SOME OF ITS USES.
+
+
+[Illustration: Fig. 115.]
+
+[Illustration: Fig. 116.]
+
+=115. Automatic Current Interrupters= are used on most common bells,
+as well as on induction coils, etc. (See Sec. 104.) Fig. 115 shows a
+simple form of interrupter. The wire 1, from a cell D C, is joined to
+an iron strip I a short distance from its end. The other wire from D C
+passes to one end of the electromagnet coil H. The remaining end of H
+is placed in contact with I as shown, completing the circuit. As soon
+as the current passes, I is pulled down and away from the upper wire
+2, breaking the circuit. I, being held by its left-hand end firmly in
+the hand, immediately springs back to its former position, closing the
+circuit again. This action is repeated, the rapidity of the vibrations
+depending somewhat upon the position of the wires on I. In regular
+instruments a platinum point is used where the circuit is broken; this
+stands the sparking when the armature vibrates.
+
+=116. Electric Bells= may be illustrated by referring to Fig. 116,
+which shows a circuit similar to that described in Sec. 115, but which
+also contains a key K, in the circuit. This allows the circuit to
+be opened and closed at a distance from the vibrating armature. The
+circuit must not be broken at two places at the same time, so wires
+should touch at the end of I before pressing K. Upon pressing K the
+armature I will vibrate rapidly. By placing a small bell near the end
+of the vibrating armature, so that it will be struck by I at each
+vibration, we should have a simple electric bell. This form of electric
+bell is called a _trembling_ bell, on account of its vibrating armature.
+
+[Illustration: Fig. 117.]
+
+[Illustration: Fig. 118.]
+
+Fig. 117 shows a form of trembling bell with cover removed. Fig. 118
+shows a _single-stroke_ bell, used for fire-alarms and other signal
+work. In this the armature is attracted but once each time the current
+passes. As many taps of the bell can be given as desired by pressing
+the push-button. Fig. 119 shows a gong for railway crossings, signals,
+etc. Fig. 120 shows a circuit including cell, push-button, and bell,
+with extra wire for lengthening the line.
+
+[Illustration: Fig. 119.]
+
+_Electro-Mechanical Gongs_ are used to give loud signals for special
+purposes. The mechanical device is started by the electric current when
+the armature of the electromagnet is attracted. Springs, weights, etc.,
+are used as the power. Fig. 121 shows a small bell of this kind.
+
+[Illustration: Fig. 120.]
+
+=117. Magneto Testing Bells=, Fig. 122, are really small hand-power
+dynamos. The armature is made to revolve between the poles of strong
+permanent magnets, and it is so wound that it gives a current with a
+large E. M. F., so that it can ring through the large resistance of a
+long line to test it.
+
+_Magneto Signal Bells_, Fig. 123, are used as generator and bell in
+connection with telephones. The generator, used to ring a bell at a
+distant station, stands at the bottom of the box. The bell is fastened
+to the lid, and receives current from a distant bell.
+
+[Illustration: Fig. 121.]
+
+[Illustration: Fig. 122.]
+
+[Illustration: Fig. 123.]
+
+[Illustration: Fig. 124.]
+
+=118. Electric Buzzers= have the same general construction as electric
+bells; in fact, you will have a buzzer by removing the bell from an
+ordinary electric bell. Buzzers are used in places where the loud sound
+of a bell would be objectionable. Fig. 124 shows the usual form of
+buzzers, the cover being removed.
+
+
+
+
+CHAPTER XVI.
+
+THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.
+
+
+=119. The Telephone= is an instrument for reproducing sounds at a
+distance, and electricity is the agent by which this is generally
+accomplished. The part spoken to is called the _transmitter_, and
+the part which gives sound out again is called the _receiver_. Sound
+itself does not pass over the line. While the same apparatus can be
+used for both transmitter and receiver, they are generally different in
+construction to get the best results.
+
+[Illustration: Fig. 125.]
+
+[Illustration: Fig. 126.]
+
+[Illustration: Fig. 127.]
+
+=120. The Bell or Magneto-transmitter= generates its own current, and
+is, strictly speaking, a dynamo that is run by the voice. It depends
+upon induction for its action.
+
+[Illustration: Fig. 128.]
+
+Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the
+wires being connected to a delicate galvanoscope. If one pole of the
+magnet H M be suddenly moved up and down near the core, an alternating
+current will be generated in the coil, the circuit being completed
+through the galvanoscope. As H M approaches the core the current will
+flow in one direction, and as H M is withdrawn it will pass in the
+opposite direction. The combination makes a miniature alternating
+dynamo.
+
+[Illustration: Fig. 129.]
+
+If we imagine the soft iron core of H, Fig. 125, taken out, and one
+pole of H M, or preferably that of a bar magnet stuck through the coil,
+a feeble current will also be produced by moving the soft iron back and
+forth near the magnet's pole. This is really what is done in the Bell
+transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being
+made to vibrate by the voice immediately in front of a coil having
+a permanent magnet for a core. The disc, or _diaphragm_, as it is
+called, is fixed near, but it does not touch, the magnet. It is under
+a constant strain, being attracted by the magnet, so its slightest
+movement changes the strength of the magnetic field, causing more or
+less lines of force to shoot through the turns of the coil and induce a
+current. The coil consists of many turns of fine, insulated wire. The
+current generated is an alternating one, and although exceedingly small
+can force its way through a long length of wire.
+
+[Illustration: Fig. 130.]
+
+Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form
+of compound magnet frequently used in the transmitter. Fig. 129 shows a
+transmitter with cords which contain flexible wires.
+
+[Illustration: Fig. 131.]
+
+=121. The Receiver=, for short lines, may have the same construction as
+the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers,
+either being used as the transmitter and the other as the receiver.
+As the alternating current goes to the distant receiver, it flies
+through the coil first in one direction and then in the other. This
+alternately strengthens and weakens the magnetic field near the
+diaphragm, causing it to vibrate back and forth as the magnet pulls
+more or less. The receiver diaphragm repeats the vibrations in the
+transmitter. Nothing but the induced electric current passes over the
+wires.
+
+[Illustration: Fig. 132.]
+
+=122. The Microphone.= If a current of electricity be allowed to
+pass through a circuit like that shown in Fig. 131, which includes a
+battery, a Bell receiver, and a microphone, any slight sound near the
+microphone will be greatly magnified in the receiver. The microphone
+consists of pieces of carbon so fixed that they form loose contacts.
+Any slight movement of the carbon causes the resistance to the current
+to be greatly changed. The rapidly varying resistance allows more or
+less current to pass, the result being that this pulsating current
+causes the diaphragm to vibrate. The diaphragm has a constantly varying
+pull upon it when the carbons are in any way disturbed by the voice, or
+by the ticking of a watch, etc. This principle has been made use of in
+carbon transmitters, which are made in a large variety of forms.
+
+[Illustration: Fig. 133.]
+
+=123. The Carbon Transmitter= does not, in itself, generate a
+current like the magneto-transmitter; it merely produces changes in
+the strength of a current that flows through it and that comes from
+some outside source. In Fig. 132, X and Y are two carbon buttons, X
+being attached to the diaphragm D. Button Y presses gently against X,
+allowing a little current to pass through the circuit which includes
+a battery, D C, and a receiver, R. When D is caused to vibrate by the
+voice, X is made to press more or less against Y, and this allows more
+or less current to pass through the circuit. This direct undulating
+current changes the pull upon the diaphragm of R, causing it to vibrate
+and reproduce the original sounds spoken into the transmitter. In
+regular lines, of course, a receiver and transmitter are connected at
+each end, together with bells, etc., for signaling.
+
+[Illustration: Fig. 134.]
+
+=124. Induction Coils in Telephone Work.= As the resistance of long
+telephone lines is great, a high electrical pressure, or E.M.F. is
+desired. While the current from one or two cells is sufficient to work
+the transmitter properly, and cause undulating currents in the short
+line, it does not have power enough to force its way over a long line.
+
+To get around this difficulty, an induction coil, Fig. 133, is used
+to transform the battery current, that flows through the carbon
+transmitter and primary coil, into a current with a high E. M. F. The
+battery current in the primary coil is undulating, but always passes in
+the same direction, making the magnetic field around the core weaker
+and stronger. This causes an alternating current in the secondary coil
+and main line. In Fig. 133 P and S represent the primary and secondary
+coils. P is joined in series with a cell and carbon transmitter; S
+is joined to the distant receiver. One end of S can be grounded, the
+current completing the circuit through the earth and into the receiver
+through another wire entering the earth.
+
+[Illustration: Fig. 135.]
+
+=125. Various forms= of telephones are shown in Figs. 134, 135, 136.
+Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form
+of wall telephone; Fig. 136 shows head-telephones for switchboard
+operators.
+
+[Illustration: Fig. 136.]
+
+
+
+
+CHAPTER XVII.
+
+HOW ELECTRICITY IS GENERATED BY DYNAMOS.
+
+
+=126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a
+machine for converting mechanical energy into an electric current,
+through electromagnetic induction. The dynamo is a machine that will
+convert steam power, for example, into an electric current. Strictly
+speaking, a dynamo creates electrical pressure, or electromotive force,
+and not electricity, just as a force-pump creates water-pressure, and
+not water. They are generally run by steam or water power.
+
+[Illustration: Fig. 137.]
+
+=127. Induced Currents.= We have already spoken about currents being
+induced by moving a coil of wire in a magnetic field. We shall now
+see how this principle is used in the dynamo which is a generator of
+induced currents.
+
+[Illustration: Fig. 138.]
+
+Fig. 137 shows how a current can be generated by a bar magnet and
+a coil of wire. Fig. 138 shows how a current can be generated by a
+horseshoe magnet and a coil of wire having an iron core. The ends of
+the coil are to be connected to an astatic galvanoscope; this forms a
+closed circuit. The coil may be moved past the magnet, or the magnet
+past the coil.
+
+[Illustration: Fig. 139.]
+
+[Illustration: Fig. 140.]
+
+[Illustration: Fig. 141.]
+
+[Illustration: Fig. 142.]
+
+Fig. 139 shows how a current can be generated by two coils, H being
+connected to an astatic galvanoscope and E to a battery. By suddenly
+bringing E toward H or the core of E past that of H, a current is
+produced. We have in this arrangement the main features of a dynamo.
+We can reverse the operation, holding E in one position and moving H
+rapidly toward it. In this case H would represent the armature and E
+the field-magnet. When H is moved toward E, the induced current in H
+flows in one direction, and when H is suddenly withdrawn from E the
+current is reversed in H. (See "Study," Chapter XXV., for experiments.)
+
+[Illustration: Fig. 143.]
+
+=128. Induced Currents by Rotary Motion.= The motions of the coils in
+straight lines are not suitable for producing currents strong enough
+for commercial purposes. In order to generate currents of considerable
+strength and pressure, the coils of wire have to be pushed past
+magnets, or electromagnets, with great speed. In the dynamo the coils
+are so wound that they can be given a rapid rotary motion as they fly
+past strong electromagnets. In this way the coil can keep on passing
+the same magnets, in the same direction, as long as force is applied to
+the shaft that carries them.
+
+[Illustration: Fig. 144.]
+
+=129. Field-Magnets; Armature; Commutator.= What we need then, to
+produce an induced current by a rotary motion, is a strong magnetic
+field, a rotating coil of wire properly placed in the field, and some
+means of leading the current from the machine.
+
+[Illustration: Fig. 145.]
+
+[Illustration: Fig. 146.]
+
+If a loop of wire, Fig. 140, be so arranged on bearings at its ends
+that it can be made to revolve, a current will flow through it in
+one direction during one-half of the revolution, and in the opposite
+direction during the other half, it being insulated from all external
+conductors. This agrees with the experiments suggested in Sec. 127, when
+the current generated in a coil passed in one direction during its
+motion _toward_ the strongest part of the field, and in the opposite
+direction when the coil passed _out_ of it. A coil must be cut by
+lines of force to generate a current. A current inside of the machine,
+as in Fig. 140, would be of no value; it must be led out to external
+conductors where it can do work. Some sort of sliding contact is
+necessary to connect a revolving conductor with outside stationary
+ones. The magnet, called the _field-magnet_, is merely to furnish lines
+of magnetic force. The one turn of wire represents the simplest form of
+_armature_.
+
+Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated
+from each other, and rotating with the coil. The two stationary pieces
+of carbon, A, B, called _brushes_, press against the rings, and to
+these are joined wires, which complete the circuit, and which lead out
+where the current can do work. The arrows show the direction of the
+current during one-half of a revolution. The rings form a _collector_,
+and this arrangement gives an _alternating current_.
+
+[Illustration: Fig. 147.]
+
+In Fig. 142 the ends of the coil are joined to the two halves of a
+cylinder. These halves, X and Y, are insulated from each other, and
+from the axis. The current flows from X onto the brush A, through some
+external circuit, to do the work, and thence back through brush B onto
+Y. By the time that Y gets around to A, the direction of the current in
+the loop has reversed, so that it passes toward Y, but it still enters
+the outside circuit through A, because Y is then in contact with A.
+This device is called a _commutator_, and it allows a constant or
+_direct current_ to leave the machine.
+
+[Illustration: Fig. 148.]
+
+In regular machines, the field-magnets are electromagnets, the whole
+or a part of the current from the dynamo passing around them on its
+way out, to excite them and make a powerful field between the poles.
+To lessen the resistance to the lines of force on their way from the
+N to the S pole of the field-magnets, the armature coils are wound on
+an iron core; this greatly increases the strength of the field, as
+the lines of force have to jump across but two small air-gaps. There
+are many loops of wire on regular armatures, and many segments to the
+commutator, carefully insulated from each other, each getting its
+current from the coil attached to it.
+
+=130. Types of Dynamos.= While there is an almost endless number of
+different makes and shapes of dynamos, they may be divided into two
+great types; the _continuous_ or _direct current_, and the _alternating
+current_ dynamo. Direct current machines give out a current which
+constantly flows in one direction, and this is because a commutator is
+used. Alternating currents come from collectors or rings, as shown in
+Fig. 141; and as an alternating current cannot be used to excite the
+fields, an outside current from a small direct current machine must be
+used. These are called exciters.
+
+[Illustration: Fig. 149.]
+
+In direct current machines enough residual magnetism is left in the
+field to induce a slight current in the armature when the machine is
+started. This immediately adds strength to the field-magnets, which, in
+turn, induce a stronger current in the armature.
+
+=131. Winding of Dynamos.= There are several ways of winding dynamos,
+depending upon the special uses to be made of the current.
+
+The _series wound_ dynamo, Fig. 143, is so arranged that the entire
+current passes around the field-magnet cores on its way from the
+machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the
+current from the machine is carried around the field-magnet cores
+through many turns of fine wire. The _compound wound_ dynamo is really
+a combination of the two methods just given. In _separately-excited_
+dynamos, the current from a separate machine is used to excite the
+field-magnets.
+
+=132. Various Machines.= Fig. 145 shows a hand power dynamo
+which produces a current for experimental work. Fig. 146 shows a
+magneto-electrical generator which produces a current for medical use.
+Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps
+are connected in series to dynamos.
+
+[Illustration]
+
+
+
+
+CHAPTER XVIII.
+
+HOW THE ELECTRIC CURRENT IS TRANSFORMED.
+
+
+=133. Electric Current and Work.= The amount of work a current can do
+depends upon two factors; the strength (amperes), and the pressure,
+or E. M. F. (volts). A current of 10 amperes with a pressure of 1,000
+volts = 10 x 1,000 = 10,000 watts. This furnishes the same amount of
+energy as a current of 50 amperes at 200 volts; 50 x 200 = 10,000 watts.
+
+=134. Transmission of Currents.= It is often necessary to carry a
+current a long distance before it is used. A current of 50 amperes
+would need a copper conductor 25 times as large (sectional area) as one
+to carry the 10 ampere current mentioned in Sec. 133. As copper conductors
+are very expensive, electric light companies, etc., generally try to
+carry the current on as small a wire as possible. To do this, the
+voltage is kept high, and the amperage low. Thus, as seen in Sec. 133,
+the current of 1,000 volts and 10 amperes could be carried on a much
+smaller wire than the other current of equal energy. A current of
+1,000 volts, however, is not adapted for lights, etc., so it has to be
+changed to lower voltage by some form of transformer before it can be
+used.
+
+=135. Transformers=, like induction coils, are instruments for changing
+the E. M. F. and strength of currents. There is very little loss of
+energy in well-made transformers. They consist of two coils of wire on
+one core; in fact, an induction coil may be considered a transformer,
+but in this a direct current has to be interrupted. If the secondary
+coil has 100 times as many turns of wire as the primary, a current of
+100 volts can be taken from the secondary coil when the primary current
+is but 1 volt; but the _strength_ (amperes) of this new current will be
+but one-hundredth that of the primary current.
+
+By using the coil of fine wire as the primary, we can lower the voltage
+and increase the strength in the same proportion.
+
+[Illustration: Fig. 150.]
+
+[Illustration: Fig. 151.]
+
+Fig. 150 shows about the simplest form of transformer with a solid iron
+core, on which are wound two coils, the one, P, being the primary, and
+the other, S, the secondary. Fig. 151 shows the general appearance of
+one make of transformer. The operation of this apparatus, as already
+mentioned, is to reduce the high pressure alternating current sent out
+over the conductors from the dynamo, to a potential at which it can
+be employed with convenience and safety, for illumination and other
+purposes. They consist of two or more coils of wire most carefully
+insulated from one another. A core or magnetic circuit of soft iron,
+composed of very thin punchings, is then formed around these coils,
+the purpose of the iron core being to reduce the magnetic resistance
+and increase the inductive effect. One set of these coils is connected
+with the primary or high-pressure wires, while the other set, which are
+called the secondary coils, is connected to the house or low-pressure
+wires, or wherever the current is required for use. The rapidly
+alternating current impulses in the primary or high-pressure wires
+induce secondary currents similar in form but opposite in direction
+in the secondary coils. These current impulses are of a much lower
+pressure, depending upon the ratio of the number of turns of wire
+in the respective coils, it being customary to wind transformers in
+such a manner as to reduce from 1,000 or 2,000-volt primaries to 50
+or 100-volt secondaries, at which voltage the secondary current is
+perfectly harmless.
+
+[Illustration: Fig. 152.]
+
+=136. Motor-Dynamos.= Fig. 152. These consist essentially of two
+belt-type machines on a common base, direct coupled together, one
+machine acting as a motor to receive current at a certain voltage,
+and the other acting as a dynamo to give out the current usually
+at a different voltage. As they transform current from one voltage
+to another, motor-dynamos are sometimes called Double Field Direct
+Current Transformers. The larger sizes have three bearings, one bearing
+being between the two machines, while the smaller sizes have but two
+bearings, the two armatures being fastened to a common spider.
+
+[Illustration: Fig. 153.]
+
+_Applications._ The uses to which motor-dynamos are put are very
+various. They are extensively used in the larger sizes as "Boosters,"
+for giving the necessary extra force on long electric supply circuits
+to carry the current to the end with the same pressure as that which
+reaches the ends of the shorter circuits from the station.
+
+Motor-dynamos have the advantage over dynamotors, described later, of
+having the secondary voltage easily and economically varied over wide
+ranges by means of a regulator in the dynamo field.
+
+=137. Dynamotors.= Fig. 153. In Dynamotors the motor and dynamo
+armatures are combined in one, thus requiring a single field only.
+The primary armature winding, which operates as a motor to drive the
+machine, and the secondary or dynamo winding, which operates as a
+generator to produce a new current, are upon the same armature core,
+so that the armature reaction of one winding neutralizes that of the
+other. They therefore have no tendency to spark, and do not require
+shifting of the brushes with varying load. Having but one field and two
+bearings, they are also more efficient than motor-dynamos.
+
+_Applications._ They have largely displaced batteries for telegraph
+work. The size shown, occupying a space of about 8-inch cube, and
+having an output of 40 watts, will displace about 800 gravity cells,
+occupying a space of about 10 feet cube. The cost of maintenance of
+such a battery per year, exclusive of rent, is about $800, whereas the
+1-6 dynamotor can be operated at an annual expense of $150.
+
+Dynamotors are largely used by telephone companies for charging storage
+batteries, and for transforming from direct to alternating current, for
+ringing telephone bells. Electro-cautery, electroplating, and electric
+heating also give use to dynamotors.
+
+
+
+
+CHAPTER XIX.
+
+HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.
+
+
+[Illustration: Fig. 154.]
+
+[Illustration: Fig. 155.]
+
+[Illustration: Fig. 156.]
+
+=138. Conductors and Insulators.= To carry the powerful current from
+the generating station to distant places where it is to give heat,
+power, or light, or even to carry the small current of a single cell
+from one room to another, _conductors_ must be used. To keep the
+current from passing into the earth before it reaches its destination
+_insulators_ must be used. The form of conductors and insulators used
+will depend upon the current and many other conditions. It should be
+remembered that the current has to be carried to the lamp or motor,
+through which it passes, and then back again to the dynamo, to form a
+complete circuit. A break anywhere in the circuit stops the current.
+Insulators are as important as conductors.
+
+[Illustration: Fig. 157.]
+
+[Illustration: Fig. 158.]
+
+=139. Mains, Service Wires, etc.= From the switchboard the current
+flows out through the streets in large conductors, or _mains_, the
+supply being kept up by the dynamos, just as water-pressure is kept up
+by the constant working of pumps. Branches, called _service wires_, are
+led off from the mains to supply houses or factories, one wire leading
+the current into the house from one main, and a similar one leading it
+out of the house again to the other main.
+
+[Illustration: Fig. 159.]
+
+[Illustration: Fig. 160.]
+
+In large buildings, pairs of wires, called _risers_, branch out from
+the service wires and carry the current up through the building. These
+have still other branches--_floor mains_, _etc._, that pass through
+halls, etc., smaller branches finally reaching the lamps. The sizes of
+all of these wires depend upon how much current has to pass through
+them. The mains in large cities are usually placed underground. In some
+places they are carried on poles.
+
+[Illustration: Fig. 161.]
+
+=140. Electric Conduits= are underground passages for electric wires,
+cables, etc. There are several ways of insulating the conductors.
+Sometimes they are placed in earthenware or iron tubes, or in wood that
+has been treated to make it water-proof. At short distances are placed
+man-holes, where the different lengths are joined, and where branches
+are attached.
+
+[Illustration: Fig. 162.]
+
+Fig. 154 shows creosoted wooden pipes; Fig. 155 shows another form of
+wooden pipe. Fig. 156 shows a coupling-box used to join Edison tubes.
+The three wires, used in the three-wire system, are insulated from each
+other, the whole being surrounded by an iron pipe of convenient length
+for handling. Fig. 157 shows sections of man-holes and various devices
+used in conduit work.
+
+[Illustration: Fig. 163.]
+
+=141. Miscellaneous Appliances.= When the current enters a house for
+incandescent lighting purposes, for example, quite a number of things
+are necessary. To measure the current a meter is usually placed in the
+cellar. In new houses the insulated conductors are usually run through
+some sort of tube which acts as a double protection, all being hidden
+from view. Fig. 158 shows a short length of iron tube with a lining of
+insulating material. Wires are often run through tubes made of rubber
+and various other insulating materials.
+
+Where the current is to be put into houses after the plastering has
+been done, the wires are usually run through _mouldings_ or supported
+by _cleats_. Fig. 159 shows a cross-section of moulding. The insulated
+wires are placed in the slots, which are then covered.
+
+[Illustration: Fig. 164.]
+
+[Illustration: Fig. 165.]
+
+[Illustration: Fig. 166.]
+
+[Illustration: Fig. 167.]
+
+Fig. 160 shows a form of porcelain cleat. These are fastened to
+ceilings or walls, and firmly hold the insulated wires in place. Fig.
+161 shows a wood cleat. Fig. 162 shows small porcelain _insulators_.
+These may be screwed to walls, etc., the wire being then fastened to
+them. Fig. 163 shows how telegraph wires are supported and insulated.
+Fig. 164 shows how wires may be carried by tree and insulated from them.
+
+[Illustration: Fig. 168.]
+
+[Illustration: Fig. 169.]
+
+[Illustration: Fig. 170.]
+
+=142. Safety Devices.= We have seen that when too large a current
+passes through a wire, the wire becomes heated and may even be melted.
+Buildings are wired to use certain currents, and if from any cause much
+more current than the regular amount should suddenly pass through the
+service wires into the house, the various smaller wires would become
+overheated, and perhaps melt or start a fire. An accidental short
+circuit, for example, would so reduce resistance that too much current
+would suddenly rush through the wires. There are several devices by
+which the over-heating of wires is obviated.
+
+[Illustration: Figs. 171 to 175.]
+
+Fig. 165 shows a _safety fuse_, or _safety cut-out_, which consists of
+a short length of easily fusible wire, called _fuse wire_, placed in
+the circuit and supported by a porcelain block. These wires are tested,
+different sizes being used for different currents. As soon as there
+is any tendency toward over-heating, the fuse _blows_; that is, it
+promptly melts and opens the circuit before any damage can be done to
+the regular conductors. Fig. 166 shows a cross-section of a _fuse plug_
+that can be screwed into an ordinary socket. The fuse wire is shown
+black.
+
+Fig. 167 shows a _fuse link_. These are also of fusible material, and
+so made that they can be firmly held under screw-heads. For heavy
+currents _fuse ribbons_ are used, or several wires or links may be
+used side by side. Fig. 168 shows a _fusible rosette_. Fig. 169 shows
+two fuse wires fixed between screw-heads, the current passing through
+them in opposite directions, both sides of the circuit being included.
+Fig. 170 shows various forms of cut-outs.
+
+[Illustration: Fig. 176.]
+
+=143. Wires and Cables= are made in many sizes. Figs. 171 to 175 show
+various ways of making small conductors. They are made very flexible,
+for some purposes, by twisting many small copper wires together, the
+whole being then covered with insulating material.
+
+[Illustration: Fig. 177.]
+
+Figs. 176, 177, show sections of submarine cables. Such cables consist
+of copper conductors insulated with pure gutta-percha. These are then
+surrounded by hempen yarn or other elastic material, and around the
+whole are placed galvanized iron armor wires for protection. Each core,
+or conductor, contains a conductor consisting of a single copper wire
+or a strand of three or more twisted copper wires.
+
+=144. Lamp Circuits.= As has been noted before, in order to have the
+electric current do its work, we must have a complete circuit. The
+current must be brought back to the dynamo, much of it, of course,
+having been used to produce light, heat, power, etc. For lighting
+purposes this is accomplished in two principal ways.
+
+[Illustration: Fig. 178.]
+
+Fig. 178 shows a number of lamps so arranged, "in series," that the
+same current passes through them all, one after the other. The total
+resistance of the circuit is large, as all of the lamp resistances are
+added together.
+
+[Illustration: Fig. 179.]
+
+Fig. 179 shows lamps arranged side by side, or "in parallel," between
+the two main wires. The current divides, a part going through each lamp
+that operates. The total resistance of the circuit is not as large
+as in the series arrangement, as the current has many small paths in
+going from one main wire to the other. Fig. 179 also shows the ordinary
+_two-wire system_ for incandescent lighting, the two main wires having
+usually a difference of potential equal to 50 or 110 volts. These
+comparatively small pressures require fairly large conductors.
+
+_The Three-Wire System_, Fig. 180, uses the current from two dynamos,
+arranged with three main wires. While the total voltage is 220, one of
+the wires being neutral, 110 volts can be had for ordinary lamps. This
+voltage saves in the cost of conductors.
+
+[Illustration: Fig. 180.]
+
+[Illustration: Fig. 181.]
+
+_The Alternating System_, Fig. 181, uses transformers. The high
+potential of the current allows small main wires, from which branches
+can be run to the primary coil of the transformer. The secondary coil
+sends out an induced current of 50 or 110 volts, while that in the
+primary may be 1,000 to 10,000 volts.
+
+
+
+
+CHAPTER XX.
+
+HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.
+
+
+=145. Resistance and Heat.= We have seen that all wires and conductors
+offer resistance to the electric current. The smaller the wire the
+greater its resistance. Whenever resistance is offered to the current,
+heat is produced. By proper appliances, the heat of resistance can be
+used to advantage for many commercial enterprises. Dynamos are used to
+generate the current for heating and lighting purposes.
+
+[Illustration: Fig. 182.]
+
+Fig. 182 shows how the current from two strong cells can be used to
+heat a short length of very fine platinum or German-silver wire.
+The copper conductors attached to the cells do not offer very much
+resistance.
+
+It will be seen from the above that in all electrical work the sizes
+of the wires used have to be such that they do not overheat. The coils
+of dynamos, motors, transformers, ampere-meters, etc., etc., become
+somewhat heated by the currents passing through them, great care being
+taken that they are properly designed and ventilated so that they will
+not burn out.
+
+[Illustration: Fig. 183.]
+
+[Illustration: Fig. 184.]
+
+=146. Electric Welding.= Fig. 183 shows one form of electric welding
+machine. The principle involved in the art of electric welding is
+that of causing currents of electricity to pass through the abutting
+ends of the pieces of metal which are to be welded, thereby generating
+heat at the point of contact, which also becomes the point of greatest
+resistance, while at the same time mechanical pressure is applied
+to force the parts together. As the current heats the metal at the
+junction to the welding temperature, the pressure follows up the
+softening surface until a complete union or weld is effected; and, as
+the heat is first developed in the interior of the parts to be welded,
+the interior of the joint is as efficiently united as the visible
+exterior. With such a method and apparatus, it is found possible to
+accomplish not only the common kinds of welding of iron and steel, but
+also of metals which have heretofore resisted attempts at welding, and
+have had to be brazed or soldered.
+
+[Illustration: Figs. 185 to 189.]
+
+The introduction of the electric transformer enables enormous currents
+to be so applied to the weld as to spend their energy just at the point
+where heating is required. They need, therefore, only to be applied
+for a few seconds, and the operation is completed before the heat
+generated at the weld has had time to escape by conduction to any other
+part.
+
+Although the quantity of the current so employed in the pieces to be
+welded is enormous, the potential at which it is applied is extremely
+low, not much exceeding that of the batteries of cells used for ringing
+electric bells in houses.
+
+[Illustration: Fig. 190.]
+
+=147. Miscellaneous Applications.= Magneto Blasting Machines are now
+in very common use for blasting rocks, etc. Fig. 184 shows one, it
+being really a small hand dynamo, occupying less than one-half a cubic
+foot of space. The armature is made to revolve rapidly between the
+poles of the field-magnet by means of a handle that works up and down.
+The current is carried by wires from the binding-posts to fuses. The
+heat generated by resistance in the fuse ignites the powder or other
+explosive.
+
+_Electric soldering irons_, _flat-irons_, _teakettles_, _griddles_,
+_broilers_, _glue pots_, _chafing-dishes_, _stoves_, etc., etc., are
+now made. Figs. 185 to 189 show some of these applications. The coils
+for producing the resistance are inclosed in the apparatus.
+
+[Illustration: Fig. 191.]
+
+Fig. 190 shows a complete electric kitchen. Any kettle or part of the
+outfit can be made hot by simply turning a switch. Fig. 191 shows an
+electric heater placed under a car seat. Many large industries that
+make use of the heating effects of the current are now being carried
+on.
+
+
+
+
+CHAPTER XXI.
+
+HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.
+
+
+[Illustration: Fig. 192.]
+
+[Illustration: Fig. 193.]
+
+=148. Incandescence.= We have just seen that the electric current
+produces heat when it flows through a conductor that offers
+considerable resistance to it. As soon as this was discovered men
+began to experiment to find whether a practical light could also be
+produced. It was found that a wire could be kept hot by constantly
+passing a current through it, and that the light given out from it
+became whiter and whiter as the wire became hotter. The wire was said
+to be _incandescent_, or glowing with heat. As metal wires are good
+conductors of electricity, they had to be made extremely fine to offer
+enough resistance; too fine, in fact, to be properly handled.
+
+=149. The Incandescent Lamp.= Many substances were experimented upon
+to find a proper material out of which could be made a _filament_
+that would give the proper resistance and at the same time be strong
+and lasting. It was found that hair-like pieces of carbon offered the
+proper resistance to the current. When heated in the air, however,
+carbon burns; so it became necessary to place the carbon filaments in a
+globe from which all the air had been pumped before passing the current
+through them. This proved to be a success.
+
+[Illustration: Fig. 194.]
+
+[Illustration: Fig. 195.]
+
+[Illustration: Fig. 196.]
+
+Fig. 192 shows the ordinary form of lamp. The _carbon filament_ is
+attached, by carbon paste, to short platinum wires that are sealed in
+the glass, their lower ends being connected to short copper wires that
+are joined to the terminals of the lamp. When the lamp is screwed
+into its socket, the current can pass up one side of the filament
+and down the other. The filaments used have been made of every form
+of carbonized vegetable matter. Bamboo has been largely used, fine
+strips being cut by dies and then heated in air-tight boxes containing
+fine carbon until they were thoroughly carbonized. This baking of the
+bamboo produces a tough fiber of carbon. Various forms of thread have
+been carbonized and used. Filaments are now made by pressing finely
+pulverized carbon, with a binding material, through small dies. The
+filaments are made of such sizes and lengths that will adapt them to
+the particular current with which they are to be used. The longer the
+filament, the greater its resistance, and the greater the voltage
+necessary to push the current through it.
+
+[Illustration: Fig. 197.]
+
+[Illustration: Fig. 198.]
+
+After the filaments are properly attached, the air is pumped from the
+bulb or globe. This is done with some form of mercury pump, and the air
+is so thoroughly removed from the bulb that about one-millionth only of
+the original air remains. Before sealing off the lamp, a current is
+passed through the filament to drive out absorbed air and gases, and
+these are carried away by the pump. By proper treatment the filaments
+have a uniform resistance throughout, and glow uniformly when the
+current passes.
+
+[Illustration: Fig. 199.]
+
+[Illustration: Fig. 200.]
+
+=150. Candle-Power.= A lamp is said to have 4, 8, 16 or more
+candle-power. A 16-candle-power lamp, for example, means one that will
+give as much light as sixteen standard candles. A standard sperm candle
+burns two grains a minute. The candle-power of a lamp can be increased
+by forcing a strong current through it, but this shortens its life.
+
+_The Current_ used for incandescent lamps has to be strong enough to
+force its way through the filament and produce a heat sufficient to
+give a good light. The usual current has 50 or 110 volts, although
+small lamps are made that can be run by two or three cells. If the
+voltage of the current is less than that for which the lamp was made,
+the light will be dim. The filament can be instantly burned out by
+passing a current of too high pressure through it.
+
+Even with the proper current, lamps soon begin to deteriorate, as small
+particles of carbon leave the filament and cling to the glass. This is
+due to the evaporation, and it makes the filament smaller, and a higher
+pressure is then needed to force the current through the increased
+resistance; besides this, the darkened bulb does not properly let the
+light out. The current may be direct or alternating.
+
+[Illustration: Fig. 201.]
+
+[Illustration: Fig. 202.]
+
+=151. The Uses= to which incandescent lamps are put are almost
+numberless. Fig. 193 shows a decorative lamp. Fancy lamps are made in
+all colors. Fig. 194 shows a conic candle lamp, to imitate a candle.
+What corresponds to the body of the candle (see figure B to C) is a
+delicately tinted opal glass tube surmounted (see figure A to B) by a
+finely proportioned conic lamp with frosted globe. C to D in the figure
+represents the regular base, and thus the relative proportions of the
+parts are shown. Fig. 195 shows another form of candelabra lamp. Fig.
+196 shows small dental lamps. Fig. 197 shows a small lamp with mirror
+for use in the throat. Fig. 198 shows lamp with half shade attached,
+used for library tables. Fig. 199 shows an electric pendant for several
+lamps, with shade. Fig. 200 shows a lamp guard. Fig. 201 shows a lamp
+socket, into which the lamp is screwed. Fig. 202 shows incandescent
+bulbs joined in parallel to the + and - mains. Fig. 203 shows how the
+lamp cord can be adjusted to desired length. Fig. 204 shows a lamp
+with reflector placed on a desk. Fig. 205 shows a form of shade and
+reflector.
+
+[Illustration: Fig. 203.]
+
+[Illustration: Fig. 204.]
+
+[Illustration: Fig. 205.]
+
+
+
+
+CHAPTER XXII.
+
+HOW LIGHT IS PRODUCED BY THE ARC LAMP.
+
+
+=152. The Electric Arc.= When a strong current passes from one carbon
+rod to another across an air-space, an _electric arc_ is produced.
+When the ends of two carbon rods touch, a current can pass from one to
+the other, but the imperfect contact causes resistance enough to heat
+the ends red-hot. If the rods be separated slightly, the current will
+continue to flow, as the intensely heated air and flying particles of
+carbon reduce the resistance of the air-space.
+
+Fig. 206 shows two carbon rods which are joined to the two terminals
+of a dynamo. The upper, or positive, carbon gradually wears away and
+becomes slightly hollow. The heated _crater_, as it is called, is the
+hottest part. The negative carbon becomes pointed. The arc will pass in
+a vacuum, and even under water.
+
+[Illustration: Fig. 206.]
+
+As the electric arc is extremely hot, metals are easily vaporized in
+it; in fact, even the carbon rods themselves slowly melt and vaporize.
+This extreme heat is used for many industrial purposes.
+
+[Illustration: Fig. 207.]
+
+[Illustration: Fig. 208.]
+
+"The phenomenon of the electric arc was first noticed by Humphrey
+Davy in 1800, and its explanation appears to be the following: Before
+contact the difference of potential between the points is insufficient
+to permit a spark to leap across even 1/10000 of an inch of air-space,
+but when the carbons are made to touch, a current is established.
+On separating the carbons, the momentary extra current due to
+self-induction of the circuit, which possesses a high electromotive
+force, can leap the short distance, and in doing so volatilizes a small
+quantity of carbon between the points. Carbon vapor, being a partial
+conductor, allows the current to continue to flow across the gap,
+provided it be not too wide; but as the carbon vapor has a very high
+resistance it becomes intensely heated by the passage of the current,
+and the carbon points also grow hot. Since, however, solid matter is a
+better radiator than gaseous matter, the carbon points emit far more
+light than the arc itself, though they are not so hot. It is observed,
+also, that particles of carbon are torn away from the + electrode,
+which becomes hollowed out to a cup-shape, and some of these are
+deposited on the - electrode."
+
+[Illustration: Fig. 209.]
+
+=153. Arc Lamps.= As the carbons gradually wear away, some device is
+necessary to keep their ends the right distance apart. If they are too
+near, the arc is very small; and if too far apart, the current can not
+pass and the light goes out. The positive carbon gives the more intense
+light and wears away about twice as fast as the - carbon, so it is
+placed above the - carbon, to throw the light downwards.
+
+[Illustration: Fig. 210.]
+
+[Illustration: Fig. 211.]
+
+Arc lamps contain some device by which the proper distance between
+the carbons can be kept. Most of them grip the upper carbon and pull
+it far enough above the lower one to establish the arc. As soon as
+the distance between them gets too great again, the grip on the upper
+carbon is loosened, allowing the carbon to drop until it comes in
+contact with the lower one, thus starting the current again. These
+motions are accomplished by electromagnets. Fig. 207 shows a form of
+arc lamp with _single carbons_ that will burn from 7 to 9 hours.
+
+[Illustration: Fig. 212.]
+
+[Illustration: Fig. 213.]
+
+[Illustration: Fig. 214.]
+
+Fig. 208 shows the mechanism by which the carbons are regulated. Fig.
+209 shows a form of _double carbon_, or _all-night_ lamp, one set of
+carbons being first used, the other set being automatically switched in
+at the proper time.
+
+[Illustration: Fig. 215.]
+
+Figs. 210, 211 show forms of _short arc lamps_, for use under low
+ceilings, so common in basements, etc.
+
+Fig. 212 shows a _hand-feed focussing_ type of _arc lamp_. In regular
+street lamps, the upper carbon only is fed by mechanism, as it burns
+away about twice as fast as the lower one, thus bringing the arc lower
+and lower. When it is desired to keep the arc at the focus of a
+reflector, both carbons must be fed.
+
+Fig. 213 shows a _theatre arc lamp_, used to throw a strong beam of
+light from the balcony to the stage.
+
+Fig. 214 shows the arc lamp used as a search-light. The reflector
+throws a powerful beam of light that can be seen for miles; in
+fact, the light is used for signalling at night. Fig. 215 shows how
+search-lights are used at night on war-vessels.
+
+
+
+
+CHAPTER XXIII.
+
+X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.
+
+
+[Illustration: Fig. 216.]
+
+[Illustration: Fig. 217.]
+
+=154. Disruptive Discharges.= We have seen, in the study of induction
+coils, that a spark can jump several inches between the terminals
+of the secondary coil. The attraction between the two oppositely
+charged terminals gets so great that it overcomes the resistance of
+the air-space between them, a brilliant spark passes, and they are
+discharged. This sudden discharge is said to be _disruptive_, and it
+is accompanied by a flash of light and a loud report. The _path_ of
+the discharge may be nearly straight, or crooked, depending upon the
+nature of the material in the gap between the terminals.
+
+[Illustration: Fig. 218.]
+
+[Illustration: Fig. 219.]
+
+=155. Effect of Air Pressure on Spark.= The disruptive spark takes
+place in air at ordinary pressures. The nature of the spark is greatly
+changed when the pressure of the air decreases. Fig. 216 shows an
+air-tight glass tube so arranged that the air can be slowly removed
+with an air-pump. The upper rod shown can be raised or lowered to
+increase the distance between it and the lower rod, these acting as the
+terminals of an induction coil. Before exhausting any air, the spark
+will jump a small distance between the rods and act as in open air. As
+soon as a small amount of air is removed, a change takes place. The
+spark is not so intense and has no definite path, there being a general
+glow throughout the tube. As the air pressure becomes still less, the
+glow becomes brighter, until the entire tube is full of purple light
+that is able to pass the entire length of it; that is, the discharge
+takes place better in rarefied air than it does in ordinary air.
+
+=156. Vacuum-Tubes.= As electricity passes through rarefied gases much
+easier than through ordinary air, regular tubes, called _vacuum-tubes_,
+are made for such study. Fig. 217 shows a plain tube of this kind,
+platinum terminals being fused in the glass for connections. These
+tubes are often made in complicated forms, Fig. 218, with colored
+glass, and are called _Geissler tubes_. They are often made in such a
+way that the electrodes are in the shape of discs, etc., and are called
+_Crookes tubes_, Fig. 219. A slight amount of gas is left in the tubes.
+
+[Illustration: Fig. 220.]
+
+[Illustration: Fig. 220-A.]
+
+=157. Cathode Rays.= The _cathode_ is the electrode of a vacuum-tube
+by which the current leaves the tube, and it has been known for some
+time that some kind of influence passes in straight lines from this
+point. Shadows, Fig. 219, are cast by such rays, a screen being placed
+in their path.
+
+=158. X-Rays.= Professor Roentgen of Wuerzburg discovered that when the
+cathode rays are allowed to fall upon a solid body, the solid body
+gives out still other rays which differ somewhat from the original
+cathode rays. They can penetrate, more or less, through many bodies
+that are usually considered opaque. The hand, for example, may be used
+as a negative for producing a photograph of the bones, as the rays do
+not pass equally well through flesh and bone.
+
+[Illustration: Fig. 221.]
+
+Fig. 220 shows a Crookes tube fitted with a metal plate, so that
+the cathode rays coming from C will strike it. The X-rays are given
+out from P. These rays are invisible and are even given out where
+the cathode rays strike the glass. Some chemical compounds are made
+luminous by these rays; so screens are made and coated with them in
+order that the shadows produced by the X-rays can be seen by the
+eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a
+_fluoroscope_ that contains a screen covered with proper chemicals.
+
+[Illustration: Fig. 222.]
+
+[Illustration: Fig. 223.]
+
+=159. X-Ray Photographs.= Bone does not allow the X-rays to pass
+through it as readily as flesh, so if the hand be placed over a
+sensitized photographic plate, Fig. 221, and proper connections be
+made with the induction coil, etc., the hand acts as a photographic
+negative. Upon developing the plate, as in ordinary photography,
+a picture or shadow of the bones will be seen. Fig. 222 shows the
+arrangement of battery, induction coil, focus tube, etc., for examining
+the bones of the human body.
+
+Fig. 223 shows the bones of a fish. Such photographs have been very
+valuable in discovering the location of bullets, needles, etc., that
+have become imbedded in the flesh, as well as in locating breaks in the
+bones.
+
+
+
+
+CHAPTER XXIV.
+
+THE ELECTRIC MOTOR, AND HOW IT DOES WORK.
+
+
+=160. Currents and Motion.= We have seen, Chapter XII., that when coils
+of wire are rapidly moved across a strong magnetic field, a current
+of electricity is generated. We have now to deal with the opposite of
+this; that is, we are to study how _motion_ can be produced by allowing
+a current of electricity to pass through the armature of a machine.
+
+[Illustration: Fig. 224.]
+
+[Illustration: Fig. 225.]
+
+Fig. 224 shows, by diagram, a coil H, suspended so that it can move
+easily, its ends being joined to a current reverser, and this, in turn,
+to a dry cell D C. A magnet, H M, will attract the core of H when
+no current passes. When the current is allowed to pass first in one
+direction and then in the opposite direction, by using the reverser,
+the core of H will jump back and forth from one pole of H M to the
+other. There are many ways by which motion can be produced by the
+current, but to have it practical, the motion must be a rotary one.
+(See "Study," Chapter XXVI., for numerous experiments.)
+
+[Illustration: Fig. 226.]
+
+=161. The Electric Motor= is a machine for transforming electric
+energy into mechanical power. The construction of motors is very
+similar to that of dynamos. They have field-magnets, armature coils,
+commutator, etc.; in fact, the armature of an ordinary direct current
+dynamo will revolve if a current be passed through it, entering by one
+brush and leaving by the other. There are many little differences of
+construction, for mechanical and electrical reasons, but we may say
+that the general construction of dynamos and motors is the same.
+
+Fig. 225 shows a coil of wire, the ends of which are connected to
+copper and zinc plates. These plates are floated in dilute sulphuric
+acid, and form a simple cell which sends a current through the wire, as
+shown by the arrows.
+
+[Illustration: Fig. 227.]
+
+We have seen that a current-carrying wire has a magnetic field and
+acts like a magnet; so it will be easily seen that if a magnet be held
+near the wire it will be either attracted or repelled, the motion
+depending upon the poles that come near each other. As shown in the
+figure, the N pole of the magnet repels the field of the wire, causing
+it to revolve. We see that this action is just the reverse to that in
+galvanometers, where the coil is fixed, and the magnet, or magnetic
+needle, is allowed to move. As soon as the part of the wire, marked A
+in Fig. 225, gets a little distance from the pole, the opposite side
+of the wire, B, begins to be attracted by it, the attraction getting
+stronger and stronger, until it gets opposite the N pole. If the N pole
+were still held in place, B would vibrate back and forth a few times,
+and finally come to rest near the pole. If, however, as soon as B gets
+opposite N the S pole of the magnet be quickly turned toward B, the
+coil will be repelled and the rotary motion will continue.
+
+[Illustration: Fig. 228.]
+
+[Illustration: Figs. 229 to 231.]
+
+[Illustration: Fig. 232.]
+
+[Illustration: Fig. 233.]
+
+Let us now see how this helps to explain electric motors. We may
+consider the wire of Fig. 225 as one coil of an armature, and the
+plates, C and Z, as the halves of a commutator. In this arrangement, it
+must be noted, the current always flows through the armature coil in
+the same direction, the rotation being kept up by reversing the poles
+of the field-magnet. In ordinary simple motors the current is reversed
+in the armature coils, the field-magnets remaining in one position
+without changing the poles. This produces the same effect as the above.
+The current is reversed automatically as the brushes allow the current
+to enter first one commutator bar and then the opposite one as the
+armature revolves. The regular armatures have many coils and many
+commutator bars, as will be seen by examining the illustrations shown.
+
+The ordinary galvanometer may be considered a form of motor. By
+properly opening and closing the circuit, the rotary motion of the
+needle can be kept up as long as current is supplied. Even an electric
+bell or telegraph sounder may be considered a motor, giving motion
+straight forward and back.
+
+=162. The Uses of Motors= are many. It would be impossible to mention
+all the things that are done with the power from motors. A few
+illustrations will give an idea of the way motors are attached to
+machines.
+
+Fig. 226 shows one form of motor, the parts being shown in Fig. 227.
+
+[Illustration: Fig. 234.]
+
+Fig. 228 shows a fan motor run by a battery. They are generally run
+by the current from the street. Figs. 229-231 show other forms of fan
+motors. Fig. 232 shows an electric hat polisher. A church organ bellows
+is shown in Fig. 233, so arranged that it can be pumped by an electric
+motor. Fig. 234 shows a motor direct connected to a drill press.
+
+=163. Starting Boxes.= If too much current were suddenly allowed to
+pass into the armature of a motor, the coils would be over-heated,
+and perhaps destroyed, before it attained its full speed. A rapidly
+revolving armature will take more current, without being overheated,
+than one not in motion. A motor at full speed acts like a dynamo, and
+generates a current which tends to flow from the machine in a direction
+opposite to that which produces the motion. It is evident, then, that
+when the armature is at rest, all the current turned on passes through
+it without meeting with this opposing current.
+
+[Illustration: Fig. 235.]
+
+[Illustration: Fig. 236.]
+
+Fig. 235 shows a starting, stopping, and regulating box, inside of
+which are a number of German-silver resistance coils properly connected
+to contact-points at the top. By turning the knob, the field of the
+motor is immediately charged first through resistance, then direct, and
+then the current is put on the armature gradually through a series of
+coils, the amount of current depending upon the distance the switch is
+turned. Fig. 236 shows a cross section of the same.
+
+
+
+
+CHAPTER XXV.
+
+ELECTRIC CARS, BOATS, AND AUTOMOBILES.
+
+
+=164. Electric Cars=, as well as boats, automobiles, etc., etc., are
+moved by the power that comes from electric motors, these receiving
+current from the dynamos placed at some "central station." We have
+already seen how the motor can do many kinds of work. By properly
+gearing it to the car wheels, motion can be given to them which will
+move the car.
+
+[Illustration: Fig. 237.]
+
+Fig. 237 shows two dynamos which will be supposed to be at a power
+house and which send out a current to propel cars. From the figure
+it will be seen that the wires over the cars, called trolley-wires,
+are connected to the positive (+) terminals of the dynamos, and that
+the negative (-) terminals are connected to the tracks. In case a
+wire were allowed to join the trolley-wire and track, we should have
+a short circuit, and current would not only rush back to the dynamo
+without doing useful work, but it would probably injure the machines.
+When some of the current is allowed to pass through a car, motion is
+produced in the motors, as has been explained. As the number of cars
+increases, more current passes back to the dynamos, which must do more
+work to furnish such current.
+
+_Trolley-poles_, fastened to the top of the cars and which end in
+grooved wheels, called _trolley-wheels_, are pressed by springs against
+the trolley-wires. The current passes down these through switches to
+_controllers_ at each end of the car, one set being used at a time.
+
+[Illustration: Fig. 238.]
+
+[Illustration: Fig. 239.]
+
+=165. The Controllers=, as the name suggests, control the speed of the
+car by allowing more or less current to pass through the motors. The
+motors, resistance coils and controllers are so connected with each
+other that the amount of current used can be regulated.
+
+[Illustration: Fig. 240.]
+
+[Illustration: Fig. 241.]
+
+When the motorman turns the handle of the controller to the first
+notch, the current passes through all of the resistance wires placed
+under the car, then through one motor after the other. The motors being
+joined in series by the proper connections at the controller, the
+greatest resistance is offered to the current and the car runs at the
+slowest speed at this first notch. As more resistance is cut out by
+turning the handle to other notches, the car increases its speed; but
+as the resistance wires become heated and the heat passes into the air,
+there is a loss of energy. It is not economical to run a car at such a
+speed that energy is wasted as heat. As soon as the resistance is all
+cut out, the current simply passes through the motors joined in series.
+This gives a fairly slow speed and one that is economical because all
+the current tends to produce motion.
+
+By allowing the current to pass through the motors joined in parallel,
+that is, by allowing each to take a part of the current, the resistance
+is greatly reduced, and a higher speed attained. This is not instantly
+done, however, as too much strain would be put upon the motors. As soon
+as the next notch is reached, the motors are joined in parallel and
+the resistance also thrown in again. By turning the handle still more,
+resistance is gradually cut out, and the highest speed produced when
+the current passes only through the motors in parallel.
+
+[Illustration: Fig. 242.]
+
+[Illustration: Fig. 243.]
+
+Fig. 238 represents a controller, by diagram, showing the relative
+positions of the controller cylinder, reversing and cut-out cylinders,
+arrangements for blowing out the short electric arcs formed, etc. A
+ratchet and pawl is provided, which indicates positively the running
+notches, at the same time permitting the cylinder to move with ease.
+Fig. 239 shows a top view of the controller.
+
+[Illustration: Fig. 244.]
+
+=166. Overhead and Underground Systems.= When wires for furnishing
+current are placed over the tracks, as in Fig. 237, we have the
+overhead system. In cities the underground system is largely used.
+The location of the conducting wires beneath the surface of the
+street removes all danger to the public, and protects them from all
+interference, leaving the street free from poles and wires.
+
+Fig. 240 shows a cross-section of an underground conduit. The rails,
+R R, are supported by cast-iron yokes, A, placed five feet apart, and
+thoroughly imbedded in concrete. The conduit has sewer connections
+every 100 feet. Conducting bars, C C, are placed on each side of
+the conduit, and these are divided into sections of about 500 feet.
+Insulators, D D, are placed every 15 feet. They are attached to, and
+directly under, the slot-rails, the stem passing through the conductor
+bar.
+
+[Illustration: Fig. 245.]
+
+Figs. 240 and 241 show the plow E. The contact plates are carried on
+coiled springs to allow a free motion. Two guide-wheels, F F, are
+attached to the leg of the plow. The conducting wires are carried up
+through the leg of the plow.
+
+=167. Appliances.= A large number of articles are needed in the
+construction of electric railroads. A few, only, can be shown that are
+used for the overhead system. Fig. 242 shows a pole insulator. Fig. 243
+shows a feeder-wire insulator. Fig. 244 shows a line suspension. Fig.
+245 shows a form of right-angle cross which allows the trolley-wheels
+of crossing lines to pass. Fig. 246 shows a switch. In winter a part of
+the current is allowed to pass through electric heaters placed under
+the seats of electric cars.
+
+[Illustration: Fig. 246.]
+
+=168. Electric Boats= are run by the current from storage batteries
+which are usually placed under the seats. An electric motor large
+enough to run a small boat takes up very little room and is generally
+placed under the floor. This leaves the entire boat for the use of
+passengers. The motor is connected to the shaft that turns the screw.
+Fig. 247 shows one design.
+
+=169. Electric Automobiles= represent the highest type of electrical
+and mechanical construction. The _running-gear_ is usually made of the
+best cold-drawn seamless steel tubing, to get the greatest strength
+from a given weight of material. The wheels are made in a variety of
+styles, but nearly all have ball bearings and pneumatic tires. In the
+lightest styles the wheels have wire spokes.
+
+The _electric motors_, supported by the running-gear, are geared to
+the rear wheels. The motors are made as nearly dust-proof as possible.
+
+_Storage batteries_ are put in a convenient place, depending upon the
+design of the carriage, and from these the motors receive the current.
+These can be charged from the ordinary 110-volt lighting circuits or
+from private dynamos. The proper plugs and attachments are usually
+furnished by the various makers for connecting the batteries with the
+street current, which is shut off when the batteries are full by an
+automatic switch.
+
+[Illustration: Fig. 247.]
+
+_Controllers_ are used, as on electric cars, the lever for starting,
+stopping, etc., being usually placed on the left-hand side of the seat.
+The _steering_ is done by a lever that moves the front wheels. Strong
+brakes, and the ability to quickly reverse the motors, allow electric
+carriages to be stopped suddenly in case of accidents.
+
+Electric automobiles are largely used in cities, or where the current
+can be easily had. The batteries must be re-charged after they have
+run the motors for a certain time which depends upon the speed and
+road, as well as upon the construction. Where carriages are to be run
+almost constantly, as is the case with those used for general passenger
+service in cities, duplicate batteries are necessary, so that one or
+two sets can be charged while another is in use. Fig. 248 shows one
+form of electric vehicle, the storage batteries being placed under and
+back of the seat.
+
+[Illustration: Fig. 248.]
+
+
+
+
+CHAPTER XXVI.
+
+A WORD ABOUT CENTRAL STATIONS.
+
+
+=170. Central Stations=, as the word implies, are places where, for
+example, electricity is generated for the incandescent or arc lights
+used in a certain neighborhood; where telephone or telegraph messages
+are sent to be resent to some other station; where operators are kept
+to switch different lines together, so that those on one line can
+talk to those on another, etc., etc. There are many kinds of central
+stations, each requiring a large amount of special apparatus to carry
+on the work. Fig. 249 gives a hint in regard to the way car lines
+get their power from a central power station. As a large part of the
+apparatus required in ordinary central stations has already been
+described, it is not necessary to go into the details of such stations.
+
+[Illustration: Fig. 249.]
+
+In lighting stations, for example, we have three principal kinds of
+apparatus. Boilers produce the steam that runs the steam engines, and
+these run the dynamos that give the current. Besides these there are
+many other things needed. The electrical energy that goes over the
+wires to furnish light, heat, and power, really comes indirectly from
+the coal that is used to boil water and convert it into steam. The
+various parts of the central station merely aid in this transformation
+of energy.
+
+[Illustration: Fig. 250.]
+
+[Illustration: Fig. 251.]
+
+The dynamos are connected to the engines by belts, or they are direct
+connected. Figs. 250, 251, show dynamos connected to engines without
+belts.
+
+The current from the dynamos is led to large switchboards which contain
+switches, voltmeters, ammeters, lightning arresters, and various other
+apparatus for the proper control and measurement of the current. From
+the switchboard it is allowed to pass through the various street mains,
+from which it is finally led to lamps, motors, etc.
+
+Water-power is frequently used to drive the dynamos instead of steam
+engines. The water turns some form of water-wheel which is connected
+to the dynamos. At Niagara Falls, for example, immense quantities of
+current are generated for light, heat, power, and industrial purposes.
+
+[Illustration]
+
+
+
+
+CHAPTER XXVII.
+
+MISCELLANEOUS USES OF ELECTRICITY.
+
+
+=171. The Many Uses= to which the electric current is put are almost
+numberless. New uses are being found for it every day. Some of the
+common applications are given below.
+
+=172. Automatic Electric Program Clocks=, Fig. 252, are largely used
+in all sorts of establishments, schools, etc., for ringing bells at
+certain stated periods. The lower dial shown has many contact-points
+that can be inserted to correspond to given times. As this revolves,
+the circuits are closed, one after the other, and it may be so set that
+bells will be rung in different parts of the house every five minutes,
+if desired.
+
+[Illustration: Fig. 252.]
+
+[Illustration: Fig. 253.]
+
+=173. Call Boxes= are used to send in calls of various kinds to
+central stations. Fig. 253 shows one form. The number of different
+calls provided includes messenger, carrier, coupe, express wagon,
+doctor, laborer, police, fire, together with three more, which may be
+made special to suit the convenience of the individual customer. The
+instruments are provided with apparatus for receiving a return signal,
+the object of which is to notify the subscriber that his call has been
+received and is having attention.
+
+[Illustration: Fig. 254.]
+
+[Illustration: Fig. 255.]
+
+Fig. 254 shows another form of call box, the handle being moved around
+to the call desired. As it springs back to the original position, an
+interrupted current passes through the box to the central station,
+causing a bell to tap a certain number of times, giving the call and
+location of the box.
+
+=174. Electric Gas-Lighters.= Fig. 255 shows a _ratchet burner_. The
+first pull of the chain turns on the gas through a four-way gas-cock,
+governed by a ratchet-wheel and pawl. The issuing gas is lighted by a
+wipe-spark at the tip of the burner. Alternate pulls shut off the gas.
+As the lever brings the attached wire A, in contact with the wire B,
+a bright spark passes, which ignites the gas, the burner being joined
+with a battery and induction or spark coil.
+
+_Automatic burners_ are used when it is desired to light gas at
+a distance from the push-button. Fig. 256 shows one form. Two
+electromagnets are shown, one being generally joined to a white
+push-button for turning on the gas and lighting it, the other being
+joined to a black button which turns off the gas when it is pressed.
+The armatures of the magnets work the gas-valve. Sparks ignite the gas,
+as explained above.
+
+[Illustration: Fig. 256.]
+
+[Illustration: Fig. 257.]
+
+=175. Door Openers.= Fig. 257 shows one form. They contain
+electromagnets so arranged that when the armature is attracted by the
+pushing of a button anywhere in the building, the door can be pushed
+open.
+
+=176. Dental Outfits.= Fig. 258 shows a motor arranged to run dental
+apparatus. The motor can be connected to an ordinary incandescent light
+socket. In case the current gives out, the drills, etc., can be run by
+foot power.
+
+[Illustration: Fig. 258.]
+
+=177. Annunciators= of various kinds are used in hotels, factories,
+etc., to indicate a certain room when a bell rings at the office.
+The bell indicates that some one has called, and the annunciator
+shows the location of the call by displaying the number of the room
+or its location. Fig. 259 shows a small annunciator. They contain
+electromagnets which are connected to push-buttons located in the
+building, and which bring the numbers into place as soon as the current
+passes through them.
+
+[Illustration: Fig. 259.]
+
+
+
+
+INDEX.
+
+
+Numbers refer to paragraphs. See Table of Contents for the titles of
+the various chapters.
+
+    Action of magnets upon each other, 32.
+
+    Adjuster, for lamp cords, 151.
+
+    Air pressure, effect of spark upon, 155.
+
+    Aluminum-leaf, for electroscopes, 5.
+
+    Alternating current, 129, 130;
+      system of wiring for, 144.
+
+    Amalgamation of zincs, 47.
+
+    Amber, electrification upon, 3.
+
+    Ammeter, the, 74;
+      how placed in circuit, 77.
+
+    Ampere, the, 72.
+
+    Annunciators, 177.
+
+    Anode, 79, 82.
+
+    Apparatus for electrical measurements, Chap. VI.
+
+    Appliances, for distribution of currents, 141;
+      for electric railways, 167;
+      for heating by electricity, 147.
+
+    Arc, the electric, 152.
+
+    Arc lamp, the, 153;
+      how light is produced by, Chap. XXII.;
+      double carbon, 153;
+      hand-feed focussing, 153;
+      for search-lights, 153;
+      short, for basements, 153;
+      single carbon, 153;
+      for theater use, 153.
+
+    Armature, of dynamo, 127, 129;
+      of electromagnets, 98;
+      of horseshoe magnet, 26;
+      of motors, 161;
+      uses of, 39.
+
+    Artificial magnets, 25.
+
+    Astatic, detectors, 94;
+      galvanometer, 73;
+      needles, 94.
+
+    Aurora borealis, 23.
+
+    Automatic, current interrupters, 104, 115;
+      gas lighters, 174;
+      program clocks, 172.
+
+    Automobiles, 169;
+      controllers for, 169;
+      motors for, 169;
+      steering of, 169;
+      storage batteries for, 169.
+
+
+    Bamboo filaments, 149.
+
+    Bar magnets, 27;
+      magnetic figures of, 38.
+
+    Batteries, large plunge, 54;
+      plunge, 53;
+      secondary, 86;
+      storage, and how they work, Chap. IX.
+
+    Bell, the electric, and some of its uses, Chap. XV.;
+      electric, 116;
+      magneto testing, 117;
+      trembling, etc., 116.
+
+    Bell transmitter, 120.
+
+    Belts, electricity generated by friction upon, 1.
+
+    Benjamin Franklin, 18.
+
+    Bichromate of potash cells, 51, etc.
+
+    Binding-posts, Chap. V.;
+      common forms of, 63.
+
+    Blasting, by electricity, 147;
+      electric machines for, 147.
+
+    Bluestone cell, 56.
+
+    Boats, electric, 168.
+
+    Boilers, use of in central stations, 170.
+
+    Bones, photographed by x-rays, Chap. XXIII.
+
+    Boosters, 136.
+
+    Brushes, 129.
+
+    Bunsen cells, 56_a_.
+
+    Burner, automatic, 174;
+      for gas-lights, 174;
+      ratchet, 174.
+
+    Buzzers, electric, 118.
+
+
+    Cables and wires, 143.
+
+    Call boxes, electric, 173.
+
+    Carbon, in arc lamps, 152, 153;
+      filament, 149;
+      transmitter, 123.
+
+    Carpet, electricity generated upon, 1.
+
+    Cars, electric, 164;
+      controllers for, 165;
+      heating by electricity, 167;
+      overhead system for, 166;
+      underground system for, 166.
+
+    Cat, electricity generated upon, 1.
+
+    Cathode, definition of, 79;
+      rays, 157.
+
+    Cells, Bunsen, 56_a_;
+      bichromate of potash, 51;
+      closed circuit, 50;
+      dry, 58;
+      Edison-Lelande, 59;
+      electricity generated by, Chap. III.;
+      Fuller, 55;
+      Gonda, 57;
+      gravity, 56;
+      Grenet, 52;
+      Leclanche, 57;
+      open circuit, 50;
+      plates and poles of, 45_a_;
+      polarization of, 48;
+      simple, 45, 49;
+      single-fluid, 49;
+      two-fluid, 49;
+      various voltaic, Chap. IV.
+
+    Central stations, 170;
+      a word about, Chap. XXVI.
+
+    Chain lightning, 19.
+
+    Chafing-dishes, electrical, 147.
+
+    Charging condensers, 15.
+
+    Chemical action, and electricity, 81.
+
+    Chemical effects of electric current, Chap. VII.
+
+    Chemical meters, 78.
+
+    Church organs, pumped by motors, 162.
+
+    Circuits, electric, 50;
+      for lamps, 144.
+
+    Cleats, porcelain, 141;
+      wooden, 141.
+
+    Clocks, automatic electric, 172.
+
+    Closed circuit cells, 50.
+
+    Coils, induction, and how they work, Chap. XIII.;
+      induction, construction of, 104;
+      method of joining, 98;
+      primary and secondary, 103;
+      resistance, 69;
+      rotation of, 95;
+      of transformers, 135.
+
+    Collectors on dynamos, 129.
+
+    Commutators, 129.
+
+    Compasses, magnetic, 31.
+
+    Compound, magnets, 28;
+      wound dynamo, 131.
+
+    Condensation of static electricity, 15.
+
+    Condensers, 15;
+      for induction coils, 104.
+
+    Conductors, and insulators, 4, 138.
+
+    Conduits, electric, 140.
+
+    Connections, electrical, 60;
+      for telegraph lines, 111.
+
+    Controllers, for automobiles, 169;
+      for electric cars, 165.
+
+    Copper sulphate, effects of current on, 82;
+      formula of, 79.
+
+    Copper voltameters, 75.
+
+    Cords, adjustable for lamps, 151.
+
+    Coulomb, the, 76.
+
+    Crater of hot carbons, 152.
+
+    Crookes tubes, 156, 158.
+
+    Current, detectors, 93;
+      direction of in cell, 46;
+      from magnet and coil, 100;
+      from two coils, 102;
+      induced, 127;
+      of induction coils, 105;
+      interrupters, automatic, 104, 115;
+      local, 47;
+      primary and secondary, 102;
+      transformation of, Chap. XVIII.;
+      transmission of, 134.
+
+    Currents, and motion, 160;
+      how distributed for use, Chap. XIX.
+
+    Current strength, 71;
+      measurement of, 73;
+      unit of, 72.
+
+    Cylinder electric machines, 9.
+
+
+    Daniell cell, 56.
+
+    D'Arsonval galvanometer, 73.
+
+    Declination, 41.
+
+    Decorative incandescent lamps, 151.
+
+    Dental, lamps, 151;
+      outfits, 176.
+
+    Detectors, astatic, 94;
+      current, 93.
+
+    Diamagnetic bodies, 29.
+
+    Diaphragm for telephones, 120.
+
+    Dip, of magnetic needle, 42.
+
+    Direct current, 129, 130.
+
+    Direction of current in cell, 46.
+
+    Discharging condensers, 15.
+
+    Disruptive discharges, 154.
+
+    Distribution of currents for use, Chap. XIX.
+
+    Door opener, electric, 175.
+
+    Dots and dashes, 110.
+
+    Drill press, run by motor, 162.
+
+    Dry cells, 58.
+
+    Dynamo, the, 126;
+      alternating current, 130;
+      commutator of, 129;
+      compound wound, 131;
+      direct current, 130;
+      lamps connected to, 132;
+      series wound, 131;
+      shunt wound, 131;
+      used as motor, 161;
+      use of in central stations, 170;
+      used with water power, 170.
+
+    Dynamos, electricity generated by, Chap. XVII.;
+      types of, 130;
+      various machines, 132;
+      winding of, 131.
+
+    Dynamotors, 137.
+
+
+    Earth, inductive influence of, 43;
+      lines of force about, 40, 42.
+
+    Ebonite, electricity by friction upon, 3, 4.
+
+    Edison-Lelande cells, 59.
+
+    Electric, automobiles, 169;
+      bell, and some of its uses, Chap. XV.;
+      boats, 168;
+      buzzers, 118;
+      cars, 164;
+      conduits, 140;
+      fans, 162;
+      flat-irons, 146;
+      gas lighters, 174;
+      griddles, 147;
+      kitchen, 147;
+      lights, arc, Chap. XXII.;
+      lights, incandescent, Chap. XXI.;
+      machines, static, 7 to 13;
+      machines, uses of, 14;
+      motor, the, 161;
+      motor, and how it does work, Chap. XXIV.;
+      soldering irons, 146;
+      telegraph, and how it sends messages, Chap. XIV.;
+      telephone, and how it transmits speech, Chap. XVI.;
+      welding, 146.
+
+    Electric current, and work, 133;
+      and chemical action, 81;
+      chemical effects of, Chap. VII.;
+      how distributed for use, Chap. XIX.;
+      magnetic effects of, Chap. XI.;
+      how transformed, Chap. XVIII.
+
+    Electrical, connections, 60;
+      horse-power, 77;
+      measurements, Chap. VI.;
+      resistance, 68;
+      resistance, unit of, 69;
+      units, Chap. VI.
+
+    Electricity, about frictional, Chap. I.;
+      and chemical action, 81;
+      atmospheric, 18;
+      heat produced by, Chap. XX.;
+      history of, 3;
+      how generated upon cat, 1;
+      how generated by dynamos, Chap. XVII.;
+      how generated by heat, Chap. X.;
+      how generated by induction, Chap. XII.;
+      how generated by voltaic cell, Chap. III.;
+      origin of name, 2.
+
+    Electrification, kinds of, 6;
+      laws of, 7.
+
+    Electrolysis, 79.
+
+    Electrolyte, 79.
+
+    Electromagnetic induction, 99.
+
+    Electromagnetism, 91.
+
+    Electromagnets, 96;
+      forms of, 97.
+
+    Electro-mechanical gong, 116.
+
+    Electromotive force, defined, 65, 71;
+      measurement of, 67;
+      of polarization, 85;
+      of static electricity, 17;
+      unit of, 66.
+
+    Electrophorus, the, 8.
+
+    Electroplating, 82.
+
+    Electroscopes, 5.
+
+    Electrotyping, 83.
+
+    Experiments, early, with currents, 44;
+      some simple, 1.
+
+    External resistance, 68.
+
+
+    Fan motors, 162.
+
+    Field, magnetic, 37.
+
+    Field-magnets, 129.
+
+    Figures, magnetic, 38.
+
+    Filaments, carbon, 149;
+      bamboo, etc., 149.
+
+    Fire, St. Elmo's, 22.
+
+    Flat-irons, electric, 147.
+
+    Floor mains, 139.
+
+    Fluoroscope, 158.
+
+    Force, and induced currents, 101;
+      lines of magnetic, 38;
+      lines of about a wire, 92, 96;
+      lines of about a magnet, 37, 38.
+
+    Frictional electricity, about, Chap, I.;
+      location of charge of, 4;
+      sparks from, 4.
+
+    Fuller cell, the, 55.
+
+    Fuse, link, 142;
+      plug, 142;
+      ribbons, 142;
+      wire, 142.
+
+    Fusible rosettes, 142.
+
+
+    Galvani, early experiments of, 44.
+
+    Galvanometers, 73;
+      astatic, 73;
+      considered as motor, 161;
+      D'Arsonval, 73;
+      tangent, 73.
+
+    Galvanoscope, 73;
+      astatic, 94.
+
+    Gas lighters, electric, 174.
+
+    Geissler tubes, 156.
+
+    Generators, electric, 126.
+
+    Glass, electricity generated upon, 4.
+
+    Glue pots, electric, 147.
+
+    Gold-leaf, for electroscopes, 5.
+
+    Gold plating, 82.
+
+    Gonda cell, 57.
+
+    Gong, electro-mechanical, 116.
+
+    Gravity cell, the, 56;
+      replaced by dynamotors, 137.
+
+    Grenet cell, 52.
+
+    Griddles, electric, 147.
+
+    Guard, for lamps, 151.
+
+
+    Heat, how generated by electricity, Chap. X.;
+      and magnetism, 35;
+      and resistance, 145.
+
+    Heat lightning, 19.
+
+    Heaters, for cars, 167.
+
+    History of electricity, 3.
+
+    Horse-power, electrical, 77.
+
+    Horseshoe, permanent magnets, 26;
+      electromagnets, 97, 98.
+
+    Human body, bones of, photographed by x-rays, Chap. XXIII.
+
+    Hydrogen, action of in cell, 48;
+      attraction of for oxygen, 85.
+
+    Incandescence, 148.
+
+    Incandescent lamp, 149;
+      candle-power of, 150;
+      current for, 150;
+      light produced by, Chap. XXI.;
+      construction of, 149;
+      uses of, 151.
+
+    Inclination of magnetic needle, 42.
+
+    Indicating push-button, 61.
+
+    Induced currents, 127;
+      and lines of force, 101;
+      by rotary motion, 128;
+      of induction coils, 105;
+      of transformers, 135.
+
+    Induced magnetism, 36.
+
+    Induction, electricity generated by, Chap. XII.;
+       electromagnetic, 99.
+
+    Induction coils, condensers for, 104;
+      construction of, 104;
+      currents of, 105;
+      how they work, Chap. XIII.;
+      in telephone work, 124;
+      uses of, 106.
+
+    Inductive influence of earth, 43.
+
+    Influence machines for medical purposes, 13.
+
+    Ink writing registers, 114.
+
+    Insulating tubing, 141.
+
+    Insulators, 141;
+      and conductors, 4, 138;
+      feeder-wire, 167;
+      for poles, 167;
+      porcelain, 141.
+
+    Internal resistance, 68.
+
+    Interrupters, automatic current, 104, 115.
+
+    Ions, 80.
+
+    Iron, electricity upon, by friction, 4.
+
+
+    Jar, Leyden, 15.
+
+    Jarring magnets, effects of, 33.
+
+
+    Keeper of magnets, 26.
+
+    Keys, telegraph, 109.
+
+    Kinds of electrification, 6.
+
+    Kitchen, electric, 147.
+
+    Knife switch, 62.
+
+
+    Lamp, incandescent, candle-power of, 150;
+      cord, adjustable, 151;
+      current for, 150;
+      dental, 151;
+      for desks, 151;
+      for throat, 151;
+      guard for, 151;
+      incandescent, 149;
+      socket, 151;
+      with half shade, 151.
+
+    Lamp, the arc, 153;
+      how light is produced by, Chap. XXII.;
+      double carbon, 153;
+      hand-feed focussing, 153;
+      for search-lights, 153;
+      single carbon, 153;
+      short, for basements, 153;
+      for theater use, 153.
+
+    Lamp circuits, alternating system, 144.
+
+    Lamps, in parallel, 144;
+      lamps in series, 144;
+      three-wire system, 144;
+      two-wire system, 144.
+
+    Laws, of electrification, 7;
+      of magnetic attraction, 32;
+      of resistance, 70.
+
+    Leaf electroscopes, 5.
+
+    Leclanche cell, 57.
+
+    Leyden, battery, 16;
+      jar, 15.
+
+    Light, how produced by arc lamp, Chap. XXII.;
+      how produced by incandescent lamp, Chap. XXI.
+
+    Lightning, 19;
+      rods, 21.
+
+    Line, telegraph, Chap. XIV.;
+      connections for, 111;
+      operation of, 112.
+
+    Line suspension, for trolley-wires, 167.
+
+    Line wire, 111.
+
+    Lines of force, conductors of, 39, 96;
+      about the earth, 40, 42;
+      and induced currents, 101;
+      about a magnet, 38;
+      about a wire, 92.
+
+    Local currents, 47.
+
+
+    Magnetic, bodies, 29;
+      declination, 41;
+      effects of electric current, Chap. XI.;
+      field, 37;
+      figure of one bar magnet, 38;
+      figure of two bar magnets, 38;
+      figure of horseshoe magnet, 38;
+      needle, dip of, 42;
+      needles and compasses, 31.
+
+    Magnetism, and heat, 35;
+      induced, 36;
+      laws of, 32;
+      residual, 34;
+      retentivity, 34;
+      temporary, 36;
+      terrestrial, 40;
+      theory of, 33.
+
+    Magneto, signal bells, 117;
+      testing bells, 117;
+      transmitter, 120.
+
+    Magnets, action upon each other, 32;
+      artificial, 25;
+      bar, 27;
+      compound, 28;
+      effects of jarring, 33;
+      electro, 96;
+      electro, forms of, 97;
+      horseshoe, 26;
+      and magnetism, about, Chap. II.;
+      making of, 30;
+      natural, 24.
+
+    Mains, electric, 139.
+
+    Man-holes, in conduits, 140.
+
+    Measurements, electric, Chap. VI.;
+      of current strength, 73;
+      of E.M.F., 67.
+
+    Meters, chemical, 78;
+      permanent record, 77.
+
+    Microphone, the, 122.
+
+    Motion and currents, 160.
+
+    Motor, acting like dynamo, 163;
+      armature of, 161;
+      controlling speed of, 165;
+      electric, 161;
+      electric, and how it does work, Chap. XXIV.;
+      fans, 162;
+      for automobiles, 169;
+      for boats, 168;
+      for pumping bellows, 162;
+      for running drill press, 162;
+      parts of, 162;
+      starting boxes for, 163;
+      uses of, 162.
+
+    Motor-dynamos, 136.
+
+    Mouldings, for wires, 141.
+
+
+    Name, electricity, origin of, 2.
+
+    Natural magnets, 24.
+
+    Needles, astatic, 94;
+      dipping, 42;
+      magnetic, 31.
+
+    Negative electrification, 5.
+
+    Non-conductors, 4.
+
+    North pole, magnetic of earth, 40;
+      of magnets, 26.
+
+    Northern lights, 23.
+
+
+    Ohm, the, 69.
+
+    Open circuit cells, 50.
+
+    Openers, for doors, 175.
+
+    Outfits, dental, 175.
+
+    Overhead trolley system, 166.
+
+    Oxygen, attraction for hydrogen, 85.
+
+
+    Parallel arrangement of lamps, 144.
+
+    Peltier effect, 89.
+
+    Pendant, electric, 151.
+
+    Pith-ball electroscope, 5.
+
+    Plate electrical machine, 10.
+
+    Plates of cells, 45_a_.
+
+    Plunge batteries, 53;
+      large, 54.
+
+    Polarity of coils, 95.
+
+    Polarization, 84;
+      electromotive force of, 85;
+      of cells, 48.
+
+    Pole-changing switch, 62.
+
+    Poles, of cells, 45_a_;
+      of horseshoe magnet, 26.
+
+    Positive electrification, 6.
+
+    Potential, defined, 65.
+
+    Push-buttons, Chap. V.;
+      indicating, 61;
+      modifications of, 61;
+      table clamp, 61.
+
+
+    Quantity of electricity, 76;
+      unit of, 76.
+
+    Rays, cathode, 157;
+      x-rays, 158.
+
+    Receiver, telephone, 121.
+
+    Reflectors, for lamps, 151.
+
+    Registers, ink writing, 114.
+
+    Relay, the, 113.
+
+    Residual magnetism, 34.
+
+    Resistance, coils and boxes, 69;
+      electrical, 68;
+      external, 68;
+      and heat, 145;
+      internal, 68;
+      laws of, 70;
+      unit of, 69.
+
+    Retentivity, 34.
+
+    Risers, in buildings, 139.
+
+    Rods, lightning, 21.
+
+    Roentgen, Prof., 158.
+
+    Rosette, fusible, 142.
+
+    Running-gear, of automobiles, 169.
+
+
+    Safety, devices, 142;
+      fuse, 142;
+      fuse link, 142;
+      fuse plug, 142;
+      fuse ribbon, 142;
+      fuse wire, 142.
+
+    Search-lights, 153;
+      signals sent by, 153.
+
+    Secondary batteries, 86;
+      uses of, 87.
+
+    Series arrangement of lamps, 144.
+
+    Series wound dynamo, 131.
+
+    Service wires, 139.
+
+    Shunt-wound dynamo, 131.
+
+    Signal bells, magneto, 117.
+
+    Simple cell, the, 45, 49.
+
+    Single-fluid cells, 49.
+
+    Single-point switch, 62.
+
+    Single-stroke bell, 116.
+
+    Socket, for incandescent lamps, 151.
+
+    Soldering irons, electric, 147.
+
+    Sounders, telegraph, 110;
+      home-made, 110.
+
+    Spark, effect of air pressure on, 155.
+
+    Sparks, from cells, 17;
+      from frictional electricity, 4.
+
+    St. Elmo's fire, 22.
+
+    Starting boxes, for motors, 163.
+
+    Static electric machines, 8.
+
+    Static electricity, condensation of, 15;
+      electromotive force of, 17;
+      to test presence of, 5;
+      uses of, 14.
+
+    Steam engines, in central stations, 170.
+
+    Steel, inductive influence of earth upon, 43;
+      retentivity of, 26.
+
+    Storage batteries, the, and how they work, Chap. IX.;
+      for automobiles, 169;
+      for boats, 168;
+      for natural sources of power, 87.
+
+    Stoves, electric, 147.
+
+    Strength of current, 71;
+      measurement of, 73;
+      unit of, 72.
+
+    Switchboards, 62.
+
+    Switches, Chap. V.;
+      knife, 62;
+      pole-changing, 62;
+      single point, 62;
+      for trolley lines, 167.
+
+    Table clamp-push, 61.
+
+    Tangent galvanometer, 73.
+
+    Teakettles, electric, 147.
+
+    Telegraph, electric, and how it sends messages, Chap. XIV.;
+      ink writing registers, 114;
+      keys, 109;
+      relay, 113;
+      sounders, 110.
+
+    Telegraph line, 107, 108;
+      operation of, 112;
+      simple connections of, 111.
+
+    Telephone, the, and how it transmits speech, Chap. XVI.;
+      receiver, 121;
+      transmitter, 120;
+      use of induction coil with, 124;
+      various forms of, 125.
+
+    Temporary magnetism, 36.
+
+    Terrestrial magnetism, 40.
+
+    Theory of magnetism, 33.
+
+    Thermoelectricity, 88.
+
+    Thermopiles, 90.
+
+    Three-wire system, 144.
+
+    Throat, lamp for, 151.
+
+    Thunder, 20.
+
+    Toepler-Holtz machines, 11.
+
+    Transformers, 135.
+
+    Transforming electric current, Chap. XVIII.;
+      for electric welding, 146.
+
+    Transmission of currents, 134.
+
+    Transmitter, Bell, 120;
+      carbon, 123.
+
+    Trembling bell, 116.
+
+    Trolley-wires, 164;
+      -poles, 164;
+      -wheels, 164.
+
+    Tubes, Crookes, 156, 158;
+      Geissler, 156;
+      vacuum, 156.
+
+    Two-fluid cells, 49.
+
+    Two-wire system, 144.
+
+
+    Underground trolley system 166;
+      conduits for, 166.
+
+    Unit, of current strength, 72;
+      of electromotive force, 66;
+      of quantity, 76;
+      of resistance, 69.
+
+    Units, electrical, Chap. VI.
+
+    Uses, of armatures, 39;
+      of electricity, miscellaneous, Chap. XXVII.;
+      of induction coils, 106;
+      of motors, 162;
+      of storage batteries, 87.
+
+
+    Vacuum-tubes, 156.
+
+    Variation, angle of, 41.
+
+    Volt, the, 66.
+
+    Volta, 66;
+      early experiments of, 44.
+
+    Voltaic cell, electricity generated by, Chap. III.
+
+    Voltaic pile, 44.
+
+    Voltameters, 75;
+      copper, 75;
+      water, 75.
+
+    Voltmeters, 67, 77.
+
+
+    Water, decomposition of, 79;
+      power, source of energy, 170;
+      voltameters, 73.
+
+    Watt, the, 77.
+
+    Wattmeters, 77.
+
+    Welding, electric, 146.
+
+    Wimshurst electric machine, 12.
+
+    Wires and cables, 143.
+
+    Wiring, for alternating system, 144;
+      three-wire system, 144;
+      two-wire system, 144.
+
+    Work, and electric current, 133.
+
+
+    X-ray photographs, 159.
+
+    X-rays, 156;
+      and how the bones of the human body are photographed, Chap. XXIII.
+
+
+    Yokes, 97, 98.
+
+
+    Zincs, amalgamation of, 47.
+
+
+
+
+THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.
+
+
+    By THOMAS M. ST. JOHN, Met. E.
+
+
+    The book contains 180 pages, and 260 illustrations; it measures
+    5 x 7-1/2 in., and is bound in cloth.
+
+    PRICE, POST-PAID, $1.00.
+
+    =CONTENTS:= _Chapter_ I. About Frictional Electricity.--II.
+    About Magnets and Magnetism.--III. How Electricity
+    is Generated by the Voltaic Cell.--IV. Various
+    Voltaic Cells.--V. About Push-Buttons, Switches and
+    Binding-Posts.--VI. Units and Apparatus for Electrical
+    Measurements.--VII. Chemical Effects of the Electric
+    Current.--VIII. How Electroplating and Electrotyping are
+    Done.--IX. The Storage Battery and How it Works.--X. How
+    Electricity is Generated by Heat.--XI. Magnetic Effects of
+    the Electric Current.--XII. How Electricity is Generated
+    by Induction.--XIII. How the Induction Coil Works.--XIV.
+    The Electric Telegraph, and How it Sends Messages.--XV. The
+    Electric Bell and Some of its Uses.--XVI. The Telephone,
+    and How it Transmits Speech.--XVII. How Electricity
+    is Generated by Dynamos.--XVIII. How the Electric
+    Current is Transformed.--XIX. How Electric Currents are
+    Distributed for Use.--XX. How Heat is Produced by the
+    Electric Current.--XXI. How Light is Produced by the
+    Incandescent Lamp.--XXII. How Light is Produced by the Arc
+    Lamp.--XXIII. X-Rays, and How the Bones of the Human Body
+    are Photographed.--XXIV. The Electric Motor and How it Does
+    Work.--XXV. Electric Cars, Boats and Automobiles.--XXVI. A
+    Word About Central Stations.--XXVII. Miscellaneous Uses of
+    Electricity.
+
+This book explains, in simple, straightforward language, many things
+about electricity; things in which the American boy is intensely
+interested; things he wants to know; things he should know.
+
+It is free from technical language and rhetorical frills, but it tells
+how things work, and why they work.
+
+It is brimful of illustrations--the best that can be had--illustrations
+that are taken directly from apparatus and machinery, and that show
+what they are intended to show.
+
+This book does not contain experiments, or tell how to make apparatus;
+our other books do that. After explaining the simple principles of
+electricity, it shows how these principles are used and combined to
+make electricity do every-day work.
+
+    _Everyone Should Know About Electricity._
+
+    A VERY APPROPRIATE PRESENT
+
+
+
+
+THIRD EDITION
+
+How Two Boys Made Their Own Electrical Apparatus.
+
+
+    Containing complete directions for making all kinds of
+    simple electrical apparatus for the study of elementary
+    electricity. By PROFESSOR THOMAS M. ST. JOHN, New York City.
+
+    The book measures 5 x 7-1/2 in., and is beautifully bound in
+    cloth. It contains 141 pages and 125 illustrations. Complete
+    directions are given for making 152 different pieces of
+    Apparatus for the practical use of students, teachers, and
+    others who wish to experiment.
+
+    PRICE, POST-PAID, $1.00.
+
+The shocking coils, telegraph instruments, batteries, electromagnets,
+motors, etc., etc., are so simple in construction that any boy of
+average ability can make them; in fact, the illustrations have been
+made directly from apparatus constructed by young boys.
+
+The author has been working along this line for several years, and he
+has been able, _with the help of boys_, to devise a complete line of
+simple electrical apparatus.
+
+
+  =_THE APPARATUS IS SIMPLE because the designs and methods
+      of construction have been worked out practically in the
+      school-room, absolutely no machine-work being required._=
+
+  =_THE APPARATUS IS PRACTICAL because it has been designed
+      for real use in the experimental study of elementary
+      electricity._=
+
+  =_THE APPARATUS IS CHEAP because most of the parts can be
+      made of old tin cans and cracker boxes, bolts, screws, wires
+      and wood._=
+
+
+    =Address, THOMAS M. ST. JOHN,=
+                =407 West 51st Street,=
+                                  =New York.=
+
+
+
+
+How Two Boys Made Their Own Electrical Apparatus.
+
+
+=CONTENTS:= _Chapter_ I. Cells and Batteries.--II. Battery Fluids
+and Solutions.--III. Miscellaneous Apparatus and Methods of
+Construction.--IV. Switches and Cut-Outs.--V. Binding-Posts and
+Connectors.--VI. Permanent Magnets,--VII. Magnetic Needles and
+Compasses.--VIII. Yokes and Armatures.--IX. Electro-Magnets.--X.
+Wire-Winding Apparatus.--XI. Induction Coils and Their
+Attachments.--XII. Contact Breakers and Current Interrupters.--XIII.
+Current Detectors and Galvanometers.--XIV. Telegraph Keys and
+Sounders.--XV. Electric Bells and Buzzers.--XVI. Commutators and
+Current Reversers.--XVII. Resistance Coils.--XVIII. Apparatus for
+Static Electricity.--XIX. Electric Motors.--XX. Odds and Ends.--XXI.
+Tools and Materials.
+
+"The author of this book is a teacher and wirier of great ingenuity,
+and we imagine that the effect of such a book as this falling into
+juvenile hands must be highly stimulating and beneficial. It is
+full of explicit details and instructions in regard to a great
+variety of apparatus, and the materials required are all within the
+compass of very modest pocket-money. Moreover, it is systematic and
+entirely without rhetorical frills, so that the student can go right
+along without being diverted from good helpful work that will lead
+him to build useful apparatus and make him understand what he is
+about. The drawings are plain and excellent. We heartily commend the
+book."--_Electrical Engineer._
+
+
+"Those who visited the electrical exhibition last May cannot have
+failed to notice on the south gallery a very interesting exhibit,
+consisting, as it did, of electrical apparatus made by boys. The
+various devices there shown, comprising electro-magnets, telegraph keys
+and sounders, resistance coils, etc., were turned out by boys following
+the instructions given in the book with the above title, which is
+unquestionably one of the most practical little works yet written that
+treat of similar subjects, for with but a limited amount of mechanical
+knowledge, and by closely following the instructions given, almost any
+electrical device may be made at very small expense. That such a book
+fills a long-felt want may be inferred from the number of inquiries
+we are constantly receiving from persons desiring to make their own
+induction coils and other apparatus."--_Electricity._
+
+
+"At the electrical show in New York last May one of the most
+interesting exhibits was that of simple electrical apparatus made by
+the boys in one of the private schools in the city. This apparatus,
+made by boys of thirteen to fifteen years of age, was from designs
+by the author of this clever little book, and it was remarkable to
+see what an ingenious use had been made of old tin tomato-cans,
+cracker-boxes, bolts, screws, wire, and wood. With these simple
+materials telegraph instruments, coils, buzzers, current detectors,
+motors, switches, armatures, and an almost endless variety of apparatus
+were made, In this book Mr. St. John has given directions in simple
+language for making and using these devices, and has illustrated
+these directions with admirable diagrams and cuts. The little volume
+is unique, and will prove exceedingly helpful to those of our young
+readers who are fortunate enough to possess themselves of a copy. For
+schools where a course of elementary science is taught, no better
+text-book in the first-steps in electricity is obtainable."--_The Great
+Round World._
+
+
+
+
+Exhibit of Experimental Electrical Apparatus
+
+AT THE ELECTRICAL SHOW, MADISON SQUARE GARDEN, NEW YORK.
+
+
+While only 40 pieces of simple apparatus were shown in this exhibit, it
+gave visitors something of an idea of what young boys can do if given
+proper designs.
+
+[Illustration: "HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS"
+
+Gives Proper Designs--Designs for over 150 Things.]
+
+
+
+
+Fun With Photography
+
+BOOK AND COMPLETE OUTFIT.
+
+
+[Illustration]
+
+=PHOTOGRAPHY= is now an educational amusement, and to many it is the
+most fascinating of all amusements. The magic of sunshine, the wonders
+of nature, and the beauties of art are tools in the hand of the amateur
+photographer.
+
+A great many things can be done with this outfit, and it will give an
+insight into this most popular pastime.
+
+
+    =THE OUTFIT= contains everything necessary for making
+    ordinary prints--together with other articles to be used
+    in various ways. The following things are included:
+    One Illustrated Book of Instructions, called "Fun With
+    Photography;" 1 Package of Sensitized Paper; 1 Printing
+    Frame, including Glass, Back, and Spring; 1 Set of Masks for
+    Printing Frame; 1 Set of Patterns for Fancy Shapes; 1 Book
+    of Negatives (Patent Pending) Ready for Use; 6 Sheets of
+    Blank Negative Paper; 1 Alphabet Sheet; 1 Package of Card
+    Mounts; 1 Package of Folding Mounts; 1 Package of "Fixo."
+
+    =CONTENTS OF BOOK:=--=Chapter I.
+    Introduction.=--Photography.--Magic Sunshine.--The
+    Outfit.--=II. General Instructions.=--The
+    Sensitized Paper.--How the Effects are
+    Produced.--Negatives.--Prints.--Printing Frames.--Our
+    Printing Frame.--Putting Negatives in Printing
+    Frame.--Printing.--Developing.--Fixing.--Drying.--Trimming.--Fancy
+    Shapes.--Mounting.--=III. Negatives and How to Make
+    Them.=--The Paper.--Making Transparent Paper.--Making
+    the Negatives.--Printed Negatives.--Perforated
+    Negatives.--Negatives Made from Magazine Pictures.--Ground
+    Glass Negatives.--=IV. Nature Photography.=--Aids
+    to Nature Study.--Ferns and Leaves.--Photographing
+    Leaves.--Perforating Leaves.--Drying Leaves, Ferns,
+    etc., for Negatives.--Flowers.--=V. Miscellaneous
+    Photographs.=--Magnetic Photographs.--Combination
+    Pictures.--Initial Pictures.--Name Plates.--Christmas,
+    Easter and Birthday Cards.
+
+    _The Book and Complete Outfit will be sent, by mail or
+    express, Charges Prepaid, upon receipt of 65 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+Fun With Magnetism.
+
+BOOK AND COMPLETE OUTFIT FOR SIXTY-ONE EXPERIMENTS IN MAGNETISM...
+
+
+[Illustration]
+
+Children like to do experiments; and in this way, better than in any
+other, _a practical knowledge of the elements of magnetism_ may be
+obtained.
+
+These experiments, although arranged to _amuse_ boys and girls, have
+been found to be very _useful in the class-room_ to supplement the
+ordinary exercises given in text-books of science.
+
+To secure the _best possible quality of apparatus_, the horseshoe
+magnets were made at Sheffield, England, especially for these sets.
+They are new and strong. Other parts of the apparatus have also been
+selected and made with great care, to adapt them particularly to these
+experiments.--_From the author's preface._
+
+
+    =CONTENTS.=--Experiments With Horseshoe Magnet.--Experiments
+    With Magnetized Needles.--Experiments With Needles,
+    Corks, Wires, Nails, etc.--Experiments With Bar
+    Magnets.--Experiments With Floating Magnets.--Miscellaneous
+    Experiments.--Miscellaneous Illustrations showing what very
+    small children can do with the Apparatus.--Diagrams showing
+    how Magnetized Needles may be used by little children to
+    make hundreds of pretty designs upon paper.
+
+
+    =AMUSING EXPERIMENTS.=--Something for Nervous People to
+    Try.--The Jersey Mosquito.--The Stampede.--The Runaway.--The
+    Dog-fight.--The Whirligig.--The Naval Battle.--A
+    String of Fish.--A Magnetic Gun.--A Top Upsidedown.--A
+    Magnetic Windmill.--A Compass Upsidedown.--The Magnetic
+    Acrobat.--The Busy Ant-hill.--The Magnetic Bridge.--The
+    Merry-go-Round.--The Tight-rope Walker.--A Magnetic Motor
+    Using Attractions and Repulsions.
+
+    _The Book and Complete Outfit will be sent, Post-paid,
+    upon receipt of 35 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+FUN WITH SHADOWS
+
+BOOK AND COMPLETE OUTFIT FOR SHADOW PICTURES, PANTOMIMES,
+ENTERTAINMENTS, Etc., Etc.
+
+
+[Illustration]
+
+=Shadow Making= has been a very popular amusement for several
+centuries. There is a great deal of _fun_ and instruction in it, and
+its long life is due to the fact that it has always been a source of
+keen delight to grown people as well as to children.
+
+In getting material together for this little book, the author has been
+greatly aided by English, French and American authors, some of whom are
+professional shadowists. It has been the author's special effort to get
+the subject and apparatus into a practical, cheap form for boys and
+girls.
+
+
+    =THE OUTFIT= contains everything necessary for all ordinary
+    shadow pictures, shadow entertainments, shadow plays, etc.
+    The following articles are included:
+
+    One book of Instructions called "Fun with Shadows"; 1 Shadow
+    Screen; 2 Sheets of Tracing Paper; 1 Coil of Wire for
+    Movable Figures; 1 Cardboard Frame for Circular Screen; 1
+    Cardboard House for Stage Scenery; 1 Jointed Wire Fish-pole
+    and Line; 2 Bent Wire Scenery Holders; 4 Clamps for Screen;
+    1 Wire Figure Support; 1 Wire for Oar; 2 Spring Wire Table
+    Clamps; 1 Wire Candlestick Holder; 5 Cardboard Plates
+    containing the following printed figures that should be cut
+    out with shears: 12 Character Hats; 1 Boat; 1 Oar-blade; 1
+    Fish; 1 Candlestick; 1 Cardboard Plate containing printed
+    parts for making movable figures.
+
+    =CONTENTS OF BOOK:= One Hundred Illustrations and Diagrams,
+    including Ten Full-page Book Plates, together with Six
+    Full-page Plates on Cardboard.
+
+    _Chapter_ I. Introduction.--II. General Instructions.--III.
+    Hand Shadows of Animals.--IV. Hand Shadows of Heads,
+    Character Faces, etc.--V. Moving Shadow Figures and How
+    to Make Them.--VI. Shadow Pantomimes.--VII. Miscellaneous
+    Shadows.
+
+    _The Book and Complete Outfit will be sent, =POST-PAID=,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+Fun With Electricity.
+
+BOOK AND COMPLETE OUTFIT FOR SIXTY EXPERIMENTS IN ELECTRICITY....
+
+
+[Illustration]
+
+Enough of the principles of electricity are brought out to make the
+book instructive as well as amusing. The experiments are systematically
+arranged, and make a fascinating science course. No chemicals, no
+danger.
+
+The book is conversational and not at all "schooly," Harry and Ned
+being two boys who perform the experiments and talk over the results as
+they go along.
+
+"The book reads like a story."--"An appropriate present for a
+boy or girl."--"Intelligent parents will appreciate 'Fun With
+Electricity.'"--"Very complete, because it contains both book and
+apparatus."--"There is no end to the fun which a boy or girl can have
+with this fascinating amusement."
+
+
+    =THERE IS FUN IN THESE EXPERIMENTS.=--Chain Lightning.--An
+    Electric Whirligig.--The Baby Thunderstorm.--A Race
+    with Electricity.--An Electric Frog Pond.--An Electric
+    Ding-Dong.--The Magic Finger.--Daddy Long-Legs.--Jumping
+    Sally.--An Electric Kite.--Very Shocking.--Condensed
+    Lightning.--An Electric Fly-Trap.--The Merry Pendulum.--An
+    Electric Ferry-Boat.--A Funny Piece of Paper.--A Joke on the
+    Family Cat.--Electricity Plays Leap-Frog.--Lightning Goes
+    Over a Bridge.--Electricity Carries a Lantern.--And _=40
+    Others=_.
+
+    The =_OUTFIT_= contains 20 different articles. The =_BOOK
+    OF INSTRUCTION=_ measures 5 x 7-1/2 inches, and has 38
+    illustrations, 55 pages, good paper and clear type.
+
+    _The Book, and Complete Outfit will be sent, by mail or
+    express, Charges Prepaid, upon receipt of 65 Cents, by_
+
+    =THOMAS M. ST. JOHN, 407 W. 51st St., New York.=
+
+
+
+
+Fun With Puzzles.
+
+BOOK, KEY, AND COMPLETE OUTFIT FOR FOUR HUNDRED PUZZLES...
+
+
+The BOOK measures 5 x 7-1/2 inches. It is well printed, nicely bound,
+and contains 15 chapters, 80 pages, and 128 illustrations. The KEY is
+illustrated. It is bound with the book, and contains the solution of
+every puzzle. The COMPLETE OUTFIT is placed in a neat box with the
+book. It consists of numbers, counters, figures, pictures, etc., for
+doing the puzzles.
+
+    =CONTENTS:= _Chapter_ (1) Secret Writing. (2) Magic
+    Triangles, Squares, Rectangles, Hexagons, Crosses, Circles,
+    etc. (3) Dropped Letter and Dropped Word Puzzles. (4) Mixed
+    Proverbs, Prose and Rhyme. (5) Word Diamonds, Squares,
+    Triangles, and Rhomboids. (6) Numerical Enigmas. (7)
+    Jumbled Writing and Magic Proverbs. (8) Dissected Puzzles.
+    (9) Hidden and Concealed Words. (10) Divided Cakes, Pies,
+    Gardens, Farms, etc. (11) Bicycle and Boat Puzzles. (12)
+    Various Word and Letter Puzzles. (13) Puzzles with Counters.
+    (14) Combination Puzzles. (15) Mazes and Labyrinths.
+
+"Fun With Puzzles" is a book that every boy and girl should have. It
+is amusing, instructive,--educational. It is just the thing to wake up
+boys and girls and make them think. They like it, because it is real
+fun. This sort of educational play should be given in every school-room
+and in every home.
+
+"Fun With Puzzles" will puzzle your friends, as well as yourself; it
+contains some real brain-splitters. Over 300 new and original puzzles
+are given, besides many that are hundreds of years old.
+
+=Secret Writing.= Among the many things that "F. W. P." contains, is
+the key to _secret writing_. It shows you a very simple way to write
+letters to your friends, and it is simply impossible for others to read
+what you have written, unless they know the secret. This, alone is a
+valuable thing for any boy or girl who wants to have some fun.
+
+    _The Book, Key, and Complete Outfit will be sent, postpaid,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+Fun With Soap-Bubbles.
+
+BOOK AND COMPLETE OUTFIT FOR FANCY BUBBLES AND FILMS....
+
+
+[Illustration]
+
+=THE OUTFIT= contains everything necessary for thousands of beautiful
+bubbles and films. All highly colored articles have been carefully
+avoided, as cheap paints and dyes are positively dangerous in
+children's mouths. The outfit contains the following articles:
+
+One Book of Instructions, called "Fun With Soap-Bubbles," 1 Metal Base
+for Bubble Stand, 1 Wooden Rod for Bubble Stand, 3 Large Wire Rings for
+Bubble Stand, 1 Small Wire Ring, 3 Straws, 1 Package of Prepared Soap,
+1 Bubble Pipe, 1 Water-proof Bubble Horn. The complete outfit is placed
+in a neat box with the book. (Extra Horns, Soap, etc., furnished at
+slight cost.)
+
+    =CONTENTS OF BOOK.=--Twenty-one
+    Illustrations.--Introduction.--The Colors of
+    Soap-bubbles.--The Outfit.--Soap Mixture.--Useful
+    Hints.--Bubbles Blown With Pipes.--Bubbles Blown
+    With Straws.--Bubbles Blown With the Horn.--Floating
+    Bubbles.--Baby Bubbles.--Smoke Bubbles.--Bombshell
+    Bubbles.--Dancing Bubbles.--Bubble Games.--Supported
+    Bubbles.--Bubble Cluster.--Suspended Bubbles.--Bubble
+    Lamp Chimney.--Bubble Lenses.--Bubble Basket.--Bubble
+    Bellows.--To Draw a Bubble Through a Ring.--Bubble
+    Acorn.--Bubble Bottle.--A Bubble Within a Bubble.--Another
+    Way.--Bubble Shade.--Bubble Hammock.--Wrestling
+    Bubbles.--A Smoking Bubble.--Soap Films.--The Tennis
+    Racket Film.--Fish-net Film.--Pan-shaped Film.--Bow and
+    Arrow Film.--Bubble Dome.--Double Bubble Dome.--Pyramid
+    Bubbles.--Turtle-back Bubbles.--Soap-bubbles and Frictional
+    Electricity.
+
+
+"There is nothing more beautiful than the airy-fairy soap-bubble with
+its everchanging colors."
+
+    _THE BEST POSSIBLE AMUSEMENT FOR OLD
+    AND YOUNG._
+
+
+    _The Book and Complete Outfit will be sent, =POST-PAID=,
+    upon receipt of 35 cents, by_
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City.=
+
+
+
+
+The Study of Elementary Electricity and
+
+Magnetism by Experiment.
+
+
+    By THOMAS M. ST. JOHN, Met. E.
+
+    The book contains 220 pages and 168 illustrations;
+    it measures 5 x 7-1/2 in. and is bound in green cloth.
+
+    PRICE, POST-PAID, $1.25.
+
+This book is designed as a text-book for amateurs, students, and others
+who wish to take up a systematic course of elementary electrical
+experiments at home or in school. Full directions are given for.......
+
+    _Two Hundred Simple Experiments._
+
+The experiments are discussed by the author, after the student has been
+led to form his own opinion about the results obtained and the points
+learned.
+
+In selecting the apparatus for the experiments in this book, the author
+has kept constantly in mind the fact that the average student will not
+buy the expensive pieces usually described in text-books.
+
+    The two hundred experiments given can be performed with
+    simple apparatus; in fact, the student should make at least
+    a part of his own apparatus, and for the benefit of those
+    who wish to do this, the author has given, throughout the
+    work, explanations that will aid in the construction of
+    certain pieces especially adapted to these experiments. For
+    those who have the author's "How Two Boys Made Their Own
+    Electrical Apparatus," constant references have been made to
+    it as the "Apparatus Book," as this contains full details
+    for making almost all kinds of simple apparatus needed
+    in "The Study of Elementary Electricity and Magnetism by
+    Experiment."
+
+_If you wish to take up a systematic course of experiments--experiments
+that may be performed with simple, inexpensive apparatus,--this book
+will serve as a valuable guide._
+
+
+
+
+Condensed List of Apparatus
+
+FOR
+
+"The Study of Elementary Electricity and Magnetism by Experiment."
+
+
+_Number_ 1. Steel Needles; package of twenty-five.--2. Flat Cork.--3.
+Candle.--4-15. Annealed Iron Wires; assorted lengths.--16. Horseshoe
+Magnet; best quality; English.--17. Iron Filings.--18. Parts for
+Compass.--19, 20. Wire Nails; soft steel.--21, 22. Spring Steel; for
+bar magnets.--23. Iron Ring.--24. Sifter; for iron filings.--25.
+Spring Steel; for flexible magnet.--26, 27. Ebonite Sheets; with
+special surface.--28. Ebonite Rod.--29. Ebonite Rod; short.--30.
+Flannel Cloth.--31. Tissue Paper.--32. Cotton Thread.--33. Silk
+Thread.--34. Support Base.--35. Support Rod.--36. Support Wire.--37.
+Wire Swing.--38. Sheet of Glass.--39. Hairpin.--40. Circular
+Conductor.--41. Circular Conductor.--42. Electrophorus Cover.--43.
+Insulating Table.--44. Insulated Copper Wire.--45. Rubber Band.--46.
+Bent Wire Clamps.--47. Cylindrical Conductor.--48. Discharger; for
+condenser.--49. Aluminum-Leaf.--50. Wires.
+
+51. Dry Cell.--52. Mercury.--53. Insulated Copper Wire; for
+connections.--54. Spring Connectors; two dozen.--55. Parts
+for Key.--56. Metal Connecting Plates.--57. Parts for Current
+Reverser.--58. Parts for Galvanoscope.--59. Parts for Astatic
+Galvanoscope.--60-63. Zinc Strips.--64. Carbon Rod.--65, 66. Glass
+Tumblers.--67, 68. Copper Strips.--69. Galvanized Iron Nail.--70,
+71. Wooden Cross-Pieces.--72. Brass Screws; one dozen.--73. Porous
+Cup.--74. Zinc Rod.--75. Copper Plate.--76. Iron Strip.--77, 78. Lead
+Strips.--79. Parts for Resistance Coil.--80. Parts for Wheatstone's
+Bridge.--81. German-Silver Wire; Size No. 30.--82. German-Silver Wire;
+No. 28.--83--85. Plate Binding-Posts.--86. Copper Sulphate.--87. Copper
+Burs; one dozen.--88. Combination Rule.--89. Coil of Wire; on spool
+for electromagnet.--90. Coil of Wire; on spool for electromagnet.--91.
+Carbon Rod.--92, 93. Soft Iron Cores with Screws.--94. Combined
+Base and Yoke.--95. Combination Connecting Plates.--96. Long Iron
+Core.--97. Round Bar Magnet, 5 x 3/8 in.--98. Thin Electromagnet.--99.
+Degree-Card; for galvanoscope.--100. Scale for Bridge.--101, 102. Soft
+Iron Cores with Heads.--103, 104. Flat Bar Magnets; these are 6 x 1/2
+x 1/4 in.; highly polished steel; poles marked.--105. Compass.
+
+    =_Illustrated Price Catalogue upon Application._=
+
+
+
+
+Electrical Apparatus For Sale
+
+A COMPLETE ELECTRIC AND MAGNETIC CABINET FOR STUDENTS, SCHOOLS AND
+AMATEURS. SIX EXTRAORDINARY OFFERS
+
+
+=This Cabinet of Electrical Experiments= contains three main parts:
+(_A_) Apparatus; (_B_) Text-Book; (_C_) Apparatus List.
+
+(_A_) =The Apparatus= furnished consists of one hundred and five
+pieces. Over three hundred separate articles are used in making up this
+set. Most of it is ready for use when received. Seven pieces, however,
+are not assembled; but the parts can be readily finished and put
+together. (Sold, also, _all_ pieces assembled.)
+
+(_B_) =The Text-Book=--called "The Study of Elementary Electricity
+and Magnetism by Experiment"--gives full directions for two hundred
+experiments. (See table of contents, etc.) Price, post-paid, $1.25.
+
+(_C_) =The Apparatus List= is an illustrated book devoted entirely to
+this special set of apparatus. Not given with first offer.
+
+  _THE APPARATUS IS SIMPLE because the designs and methods of
+     construction have been worked out with great care._
+
+  _THE APPARATUS IS PRACTICAL because it has been designed
+     for real use in "The Study of Elementary Electricity and
+     Magnetism by Experiment."_
+
+  _THE APPARATUS IS CHEAP because the various parts are
+     so designed that they can be turned out in quantity by
+     machinery._
+
+    =1st Offer:= Pieces 1 to 50                     $1.00
+    =2d Offer:= Pieces 51 to 105, with part (_C_)      3.50
+    =3d Offer:= Pieces 1 to 105, with part (_C_)      4.00
+    =4th Offer:= Complete Cabinet, parts (_A_), (_B_), (_C_)      5.00
+    =5th Offer:= Apparatus only, all pieces assembled      4.60
+    =6th Offer:= Complete Cabinet, all pieces assembled      5.60
+
+    =_Express charges must be paid by you. Estimates given._=
+
+A "Special Catalogue," pertaining to the above, with complete
+price-list, will be mailed upon application.
+
+    =THOMAS M. ST. JOHN, 407 West 51st St., New York City=
+
+
+
+
+Fun With Telegraphy
+
+BOOK AND COMPLETE OUTFIT.
+
+
+[Illustration]
+
+=TELEGRAPHY= is of the greatest importance to all civilized nations,
+and upon it depend some of the world's most important enterprises.
+Every boy and girl can make practical use of telegraphy in one way or
+another, and the time it takes to learn it will be well spent.
+
+
+=THE OUTFIT.=--Mr. St. John has worked for a number of years to produce
+a telegraph outfit that would be simple, cheap, and practical for those
+who wish to make a study of telegraphy. After making and experimenting
+with nearly one hundred models, many of which were good, he has at last
+perfected an instrument so simple, original, and effective that it is
+now being made in large quantities.
+
+The sounders are so designed that they will work properly with any dry
+cell of ordinary strength, and this is a great advantage for practice
+lines. Dry batteries are cheap and clean, and there are no dangers from
+acids.
+
+The outfit consists of the following articles, placed in a neat box:
+One Book of Instruction, called "Fun With Telegraphy"; one Telegraph
+"Key"; one Telegraph "Sounder"; Insulated Copper Wires for connections.
+The "key" and "sounder" are mounted, with proper "binding-posts," upon
+a base of peculiar construction, which aids in giving a large volume of
+sound.
+
+
+=CONTENTS OF BOOK.=--Telegraphy.--The Outfit.--A Complete Telegraph
+Line.--Connections.--The Telegraph Key.--The Sounder.--The Battery.--A
+Practice Line.--A Two-instrument Line.--Operation of Line.--The Morse
+Telegraph Alphabet.--Aids to Learning Alphabet.--Cautions.--Office
+Calls.--Receiving Messages.--Remember.--Extra Parts.
+
+
+=ABOUT BATTERIES.=--For those who cannot easily secure batteries, we
+will furnish small dry cells, post-paid, at 15 cents each, in order to
+deliver the outfits complete to our customers. This price barely covers
+the total cost to us, postage alone being 6 cents.
+
+    _=FUN WITH TELEGRAPHY, including Book, Key, Sounder,
+    and Wire (no battery), post-paid, 50 cents, by=_
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+Tool Sets for Students
+
+
+The following tool sets have been arranged especially for those who
+wish to make use of the designs contained in "How Two Boys Made Their
+Own Electrical Apparatus," "Real Electric Toy-Making for Boys,"
+"Electric Instrument-Making," etc. It is very poor economy to waste
+valuable time and energy in order to save the cost of a few extra tools.
+
+=NOTE.=--Save money by buying your tools in sets. We do not pay express
+or freight charges at the special prices below.
+
+=FOR $1.00.=--One _Steel Punch_; round, knurled head.--One light
+_Hammer_; polished, nickel-plated, varnished handle.--One _Iron Clamp_;
+japanned, 2-1/4 in.--One _Screw-Driver_; tempered and polished blade,
+cherry stained hardwood handle, nickel ferrule.--One _Wrench_; retinned
+skeleton frame, gilt adjusting wheel.--One _Awl_; tempered steel
+point, turned and stained wood handle, with ferrule.--One _Vise_; full
+malleable, nicely retinned, 1-3/8 in. jaws, full malleable screw with
+spring.--One pair _Steel Pliers_; 4 in. long, polished tool steel,
+unbreakable, best grooved jaw.--One pair of _Shears_; carbonized steel
+blades, hardened edge, nickel-plated, heavy brass nut and bolt.--One
+_File_; triangular, good steel.--One _File Handle_; good wood, brass
+ferrule.--One _Foot Rule_; varnished wood, has English and metric
+system.--One _Soldering Set_; contains soldering iron, solder, resin,
+sal ammoniac, and directions. One _Center-Punch_; finely tempered steel.
+
+=FOR $2.00.=--All that is contained in the $1.00 set of tools, together
+with the following: One pair of _Tinner's Shears_; cut, 2-3/4 in., cast
+iron, hardened, suitable for cutting thin metal.--One _Hollow Handle
+Tool Set_; very useful; polished handle holds 10 tools, gimlet,
+brad-awls, chisel, etc.--One _Try Square_; 6-in. blue steel blade,
+marked in 1/8s, strongly riveted.--One 1-lb. _Hammer_; full size,
+polished head, wedged varnished hardwood handle.--One _Hack Saw_; steel
+frame, 9-1/2-in. polished steel blade, black enamel handle; very useful.
+
+=FOR $3.50.=--Two _Steel Punches_; different sizes, one solid round,
+knurled head, polished; the other, point and head brightly polished,
+full nickel, center part knurled.--One _Light Hammer_; polished and
+nickel plated, varnished handle.--One regular _Machinist's Hammer_;
+ball peen, solid cast steel, with varnished hardwood handle; a
+superior article.--Two _Iron Clamps_; one opens 2-1/4 in., the other
+3 in., japanned.--One _Screw-Driver_; tempered and polished blade,
+firmly set in cherry stained hardwood handle with nickel ferrule.--One
+_Wrench_; retinned, skeleton frame, gilt adjusting wheel.--One _Awl_;
+tempered steel blade, ground to point, firmly set in turned and stained
+handle with ferrule.--One _Steel Vise_; 2-1/4-in., jaws, steel screw,
+bright polished jaws and handle; a good strong vise.--One pair of
+_Steel Pliers_; 6 in. long, bright steel, flat nose, 2 wire-cutters,
+practically unbreakable.--One pair of _Shears_; carbonized steel
+blades, hardened edges, nickel plated, heavy brass nut and bolt.--One
+_File_; triangular and of good steel.--One _File Handle_; good wood,
+with brass ferrule.--One _Foot Rule_; varnished wood, has both the
+English and metric systems.--One _Soldering Set_; contains soldering
+iron, solder, resin, sal ammoniac, and directions; a very handy
+article.--One _Center-Punch_; finely tempered steel.--One pair of
+_Tinner's Shears_; these are best grade, inlaid steel cutting edges,
+polished and tempered, japanned handles; thoroughly reliable.--One
+_Hollow Handle Tool Set_; very useful; the polished handle holds 10
+tools, gimlet, chisel, brad-awl, etc.--One _Try Square_; 6-in. blue
+steel blade, marked both sides in 1/8s, strongly riveted with brass
+rivets.--One _Hack Saw_; steel frame, 9-1/2-in. polished steel blade,
+black enamel handle; very useful for sawing small pieces of wood.
+
+=FOR $5.00= will be included everything in the $3.50 offer, and the
+following: One _Glue-Pot_; medium size, with brush and best wood
+glue; inside pot has hinge cover.--One _Ratchet Screw-Driver_; great
+improvement over ordinary screw-drivers; well made and useful.--One
+_Hand Drill_; frame malleable iron; hollow screw top holding 6 drills;
+bores from 1-16 to 3-16-in. holes; solid gear teeth; 3-jawed nickel
+plated chuck; a superior tool, and almost a necessity.
+
+    =GIVE THE BOY A SET OF TOOLS=
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+REAL ELECTRIC TOY-MAKING FOR BOYS
+
+    _By_ THOMAS M. ST. JOHN, Met. E.
+
+
+    This book contains 140 pages and over one hundred
+    original drawings, diagrams, and full-page plates.
+    It measures 5 x 7-1/2 in., and is bound in cloth.
+
+    Price, post-paid, $1.00
+
+
+=CONTENTS:= _Chapter_ I. Toys Operated by Permanent Magnets.--II.
+Toys Operated by Static Electricity.--III. Making Electromagnets for
+Toys.--IV. Electric Batteries.--V. Circuits and Connections.--VI. Toys
+Operated by Electromagnets. VII. Making Solenoids for Toys.--VIII.
+Toys Operated by Solenoids.--IX. Electric Motors.--X. Power,
+Speed, and Gearing.--XI. Shafting and Bearings.--XII. Pulleys and
+Winding-Drums.--XIII. Belts and Cables.--XIV. Toys Operated by
+Electric Motors.--XV. Miscellaneous Electric Toys.--XVI. Tools.--XVII.
+Materials.--XVIII. Various Aids to Construction.
+
+While planning this book, Mr. St. John definitely decided that he would
+not fill it with descriptions of complicated, machine-made instruments
+and apparatus, under the name of "Toy-Making," for it is just as
+impossible for most boys to get the parts for such things as it is
+for them to do the required machine work even after they have the raw
+materials.
+
+Great care has been taken in designing the toys which are described
+in this book, in order to make them so simple that any boy of average
+ability can construct them out of ordinary materials. The author can
+personally guarantee the designs, for there is no guesswork about
+them. Every toy was made, changed, and experimented with until it was
+as simple as possible; the drawings were then made from the perfected
+models.
+
+As the result of the enormous amount of work and experimenting which
+were required to originate and perfect so many new models, the author
+feels that this book may be truly called "Real Electric Toy-Making for
+Boys."
+
+    =Every Boy Should Make Electrical Toys.=
+
+
+
+
+The Electric Shooting Game>
+
+A MOST ORIGINAL AND FASCINATING GAME PATENT APPLIED FOR AND COPYRIGHTED
+
+
+[Illustration]
+
+_=SHOOTING BY ELECTRICITY=_
+
+=The Electric Shooting Game= is an entirely new idea, and one that
+brings into use that most mysterious something--_electricity_. The
+game is so simple that small children can play it, and as there are
+no batteries, acids, or liquids of any kind, there is absolutely no
+danger. The electricity is of such a nature that it is perfectly
+harmless--but very active.
+
+The "_game-preserve_" is neat and attractive, being printed in colors,
+and the birds and animals are well worth hunting. Each has a fixed
+value--and some of them must not be shot at all--so there is ample
+opportunity for a display of skill in bringing down those which count
+most.
+
+"_Electric bullets_" are actually shot from the "_electric gun_" by
+electricity. This instructive game will furnish a vast amount of
+amusement to all.
+
+ _=The "Game-Preserve,"--the "Electric Gun,"--the
+    "Shooting-Box,"--the "Electric Bullets,"--in fact, the
+    entire electrical outfit, together with complete illustrated
+    directions, will be sent in a neat box, Post-Paid, upon
+    receipt of 50 cents, by=_
+
+    =THOMAS M. ST. JOHN, 848 Ninth Ave., New York=
+
+
+
+
+       *       *       *       *       *
+
+
+
+
+Transcriber's note:
+
+Obvious punctuation errors were corrected.
+
+Page 46, "turnnd" changed to "turned" (be turned to 1)
+
+Page 66, word "a" added to text (in a glass jar)
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK THINGS A BOY SHOULD KNOW ABOUT
+ELECTRICITY***
+
+
+******* This file should be named 44665.txt or 44665.zip *******
+
+
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