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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:
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+      or
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+
+
+      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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