initial commit of ebook 38526HEADmain

1 files changed, 5413 insertions, 0 deletions

diff --git a/38526.txt b/38526.txt
new file mode 100644
index 0000000..ac891ed
--- /dev/null
+++ b/38526.txt
@@ -0,0 +1,5413 @@
+The Project Gutenberg EBook of Hertzian Wave Wireless Telegraphy, by
+John Ambrose Fleming
+
+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: Hertzian Wave Wireless Telegraphy
+
+Author: John Ambrose Fleming
+
+Release Date: January 8, 2012 [EBook #38526]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HERTZIAN WAVE WIRELESS TELEGRAPHY ***
+
+
+
+
+Produced by Robert Cicconetti, Tim Madden and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/American Libraries.)
+
+
+
+
+
+
+ [Transcriber's Notes
+
+ All apparent printer's errors and variations in spelling have been
+ retained, there are also some inconsistencies in the hyphenation of
+ words. All these have been detailed at the end of the text.
+
+ There are a number of mathematical equations in the text, these have
+ been rendered into a text representation that attempts to make them
+ as clear as possible while matching the text as closely as possible.
+ Multi-line fractions have been rendered on one line with the addition
+ of extra brackets if required to ensure clarity. Superscripts are
+ denoted with ^ followed by the superscripted term in {}. Subscripts
+ are denoted by _ with the subscripted term in {}. Square roots are
+ denoted by [\sq] followed by the rooted term in {}. Some equations
+ have following punctuation, both commas or full stops, in the original
+ text as though the equation is part of the sentence. The middle dot
+ has been used in the original text, both as a multiplication symbol
+ and as a decimal point. These have been kept but the middle dot as a
+ multiplication symbol in formulae is surrounded by single spaces.
+
+ Italicised text in the original text is represented by _text_, bold
+ text is represented by =bold=.]
+
+                  *       *       *       *       *
+
+
+
+
+                 HERTZIAN WAVE WIRELESS TELEGRAPHY.
+
+                    BY DR. J. A. FLEMING, F.R.S.
+
+      [From the POPULAR SCIENCE MONTHLY, June-December, 1903.]
+
+                  *       *       *       *       *
+
+         [From the "Popular Science Monthly," June, 1903.]
+
+
+
+
+               HERTZIAN WAVE WIRELESS TELEGRAPHY.[1]
+
+                   BY DR. J. A. FLEMING, F.R.S.,
+
+  PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON.
+
+
+The immense public interest which has been aroused of late years in
+the subject of telegraphy without connecting wires has undoubtedly
+been stimulated by the achievements of Mr. Marconi in effecting
+communication over great distances by means of Hertzian waves. The
+periodicals and daily journals, which are the chief avenues through
+which information reaches the public, whilst eager to describe in a
+sensational manner these wonderful applications of electrical
+principles, have done little to convey an intelligible explanation of
+them. Hence it appeared probable that a service would be rendered by
+an endeavour to present an account of the present condition of
+electric wave telegraphy in a manner acceptable to those unversed in
+the advanced technicalities of the subject, but acquainted at least
+with the elements of electrical science. It is the purpose of these
+articles to attempt this task. We shall, however, limit the discussion
+to an account of the scientific principles underlying the operation of
+this particular form of wireless telegraphy, omitting, as far as
+possible, references to mere questions of priority and development.
+
+The practical problem of electric wave wireless telegraphy, which has
+been variously called Hertzian wave telegraphy, Marconi telegraphy, or
+spark telegraphy (_Funkentelegraphie_), is that of the production of
+an effect called an electric wave or train of electric waves, which
+can be sent out from one place, controlled, detected at another place,
+and interpreted into an alphabetic code. Up to the present time, the
+chief part of that intercommunication has been effected by means of
+the Morse code, in which a group of long and short signs form the
+letter or symbol. Some attempts have been made with more or less
+success to work printing telegraphs and even writing or drawing
+telegraphs by Hertzian waves, but have not passed beyond the
+experimental stage, whilst wireless telephony by this means is still a
+dream of the future.
+
+We shall, in the first place, consider the transmitting arrangements
+and, incidentally, the nature of the effect or wave transmitted; in
+the second place, the receiving appliances; and, finally, discuss the
+problem of the isolation or secrecy of the intelligence conveyed
+between any two places.
+
+The transmitter used in Hertzian wave telegraphy consists essentially
+of a device for producing electric waves of a type which will travel
+over the surface of the land or sea without speedy dissipation, and
+the important element in this arrangement is the _radiator_, by which
+these waves are sent out. It will be an advantage to begin by
+explaining the electrical action of the radiator, and then proceed to
+discuss the details of the transmitting appliances.
+
+It will probably assist the reader to arrive most easily at a general
+idea of the functions of the various portions of the transmitting
+arrangements, and in particular of the radiator, if we take as our
+starting point an analogy which exists between electric wave
+generation for telegraphic purposes and air wave generation for sound
+signal purposes. Most persons have visited some of the large
+lighthouses which exist around our coasts and have there seen a steam
+or air _siren_, as used for the production of sound signals during
+fogs. If they have examined this appliance, they will know that it
+consists, in the first place, of a long metal tube, generally with a
+trumpet-shaped mouthpiece. At the bottom of this tube there is a fixed
+plate with holes in it, against which revolves another similarly
+perforated plate. These two plates separate a back chamber or wind
+chest from the tube, and the wind chest communicates with a reservoir
+of compressed air or a high-pressure steam boiler. In the
+communication pipe there is a valve which can be suddenly opened for a
+longer or shorter time. When the movable plate revolves, the
+coincidence or non-coincidence of the holes in the two plates opens or
+shuts the air passage way very rapidly. Hence when the blast of air or
+steam is turned on, the flow is cut up by the revolving plates into a
+series of puffs which inflict blows upon the stationary air in the
+siren tube. If these blows come at the rate, say, of a hundred a
+second, they give rise to aerial oscillations in the tube, which
+impress the ear as a deep, musical note or roar; and this continuous
+sound can be cut up by closing the valve intermittently into long and
+short periods, and so caused to signal a letter according to the Morse
+code, denoting the name of the lighthouse. In this case the object is
+to produce: first, aerial vibrations in the tube, giving rise to a
+train of powerful air waves; secondly, to intermit this wave-train so
+as to produce an intelligible signal; and thirdly, to transmit this
+wave as far as possible through space.
+
+The production of a sound or air wave can only be achieved by
+administering a very sudden blow to the general mass of the air in the
+tube. This impulse must be sufficient to call into operation the
+inertia and elastic qualities of the air. It is found, moreover, that
+the amplitude of the resulting wave, or the loudness of the sound, is
+increased by suitably proportioning the length of the siren pipe and
+the frequency of the air puffs; whilst the distance at which it is
+heard depends also in some degree upon the form of the mouthpiece.
+
+Inside the siren tube, when it is in operation, the air molecules are
+in rapid vibratory motion in the direction of the length of the tube.
+If we could at any one instant examine the distribution and changes of
+air pressure in the tube, we should find that at some places there are
+large, and at others small, variations in air pressure. These latter
+places are called the _nodes_ of pressure. At the pressure nodes,
+however, we should find large variations in the velocity of the air
+particles, and these points are called the _antinodes_ of velocity. In
+those places at which the pressure variation is greatest, the velocity
+changes are least, and _vice versa_. Outside the tube, as a result of
+these air motions in it, we have a hemispherical air wave produced,
+which travels out from the mouthpiece as a centre; and if we could
+examine the distribution of air pressure and velocity through all
+external space, we should find a distribution which is periodic in
+space as well as time, constituting the familiar phenomenon of an air
+wave.
+
+Turning then to consider the production of an electric, instead of an
+air wave, we notice in the first place that the medium with which we
+are concerned is the _ether_ filling all space. This ether permits the
+production of physical changes in it which are analogous to, but not
+identical in nature with, the pressures and movements which constitute
+a sound wave. The Hertzian radiator is an appliance for acting on the
+ether as the siren acts on the air. It produces a wave in it, and it
+can be shown that all the parts of the above described siren apparatus
+have their electrical equivalents in the transmitter employed in
+Hertzian wave wireless telegraphy.
+
+To understand the nature of an electric wave we must consider, in the
+first place, some properties of the ether. In this medium we can at
+any place produce a state called _electric displacement_ or _ether
+strain_ as we can produce compression or rarefaction in air; and, just
+as the latter changes are said to be created by mechanical force, so
+the former is said to be due to _electric force_. We can not define
+more clearly the nature of this ether strain or displacement until we
+know much more about the structure of the ether than we do at present.
+We can picture to ourselves the operation of compressing air as an
+approximation of the air molecules, but the difficulty of
+comprehending the nature of an electric wave arises from the fact that
+we cannot yet definitely resolve the notion of electric strain into
+any simpler or more familiar ideas.
+
+We have to be content, therefore, to disguise our present ignorance by
+the use of some descriptive term, such as _electric strain_,
+_electrostatic strain_ or _ether strain_, to describe the directed
+condition of the space around a body in a state of electrification
+which is produced by electric force. This electric strain is certainly
+not of the nature of a compression in the ether, but much more akin to
+a twist or rotational strain in a solid body.
+
+For our present purpose it is not so necessary to postulate any
+particular theory of the ether as it is to possess some consistent
+hypothesis, in terms of which we can describe the phenomena which
+will concern us. These effects are, as we shall see, partly states of
+electrification on the surface or distributions of electric current in
+wires or rods, and partly conditions in the space outside them, which
+we are led to recognise as distributions of electric strain and of an
+associated effect called _magnetic flux_.
+
+We find such a theory at hand at the present time in the electronic
+theory of electricity, which has now been sufficiently developed and
+popularised to make it useful as a descriptive hypothesis.[2] This
+theory has the great recommendation that it offers a means of
+abolishing the perplexing dualism of ether and ponderable matter, and
+gives a definite and, in a sense, objective meaning to the word
+electricity. In this physical speculation, the chief subject of
+contemplation is the electron, or ultimate particle of negative
+electricity, which, when associated in greater or less number with a
+matrix of some description, forms the atom of ponderable matter. To
+avoid further hypothesis, this matrix may be called the _co-electron_;
+and we shall adopt the view that a single chemical atom is a union of
+a _co-electron_ with a surrounding envelope or group of electrons, one
+or more of the latter being detachable. We need not stop to speculate
+on the structure of the atomic core or co-electron, whether it is
+composed of positive and negative electrons or of something entirely
+different. The single electron is the indivisible unit or atomic
+element of so-called negative electricity, and the neutral chemical
+atom deprived of one electron is the unit of positive electricity. On
+this hypothesis, the chemical atom is to be regarded as a microcosm, a
+sort of a solar system in miniature, the component electrons being
+capable of vibration relatively to the atomic centre of mass.
+Furthermore, from this point of view it is the electron which is the
+effective cause of radiation. It alone has a grip on the ether whereby
+it is able to establish wave motion in the latter.
+
+Dr. Larmor has developed in considerable detail an hypothesis of the
+nature of an electron which makes it the centre or convergence-point
+of lines of a self-locked ether strain of a torsional type. The notion
+of an atom merely as a "centre of force" was one familiar to Faraday
+and much supported by Boscovich and others. The fatal objection to the
+validity of this notion as originally stated was that it offers no
+possibility of explaining the inertia of matter. On the electronic
+hypothesis, the source of all inertia is the inertia of the ether, and
+until we are able to dissect this last quality into anything simpler
+than the time-element involved in the production of an ether strain or
+displacement, we must accept it as an ultimate fact, not more
+elucidated because we speak of it as the inductance of the electron.
+
+We postulate, therefore, the following ideas: We have to think of the
+ether as a homogeneous medium in which a strain of some kind, most
+probably of a rotational type, is possible. This strain appears only
+under the influence of an appropriate stress called the electric
+force, and disappears when the force is removed. Hence to create this
+strain necessitates the expenditure of energy. An electron is a
+centre or convergence-point of lines of permanent ether strain of such
+nature that it cannot release itself. To obtain some idea of the
+nature of such a structure, let us imagine a flat steel band formed
+into a ring by welding the ends together. There is then no torsional
+strain. If, however, we suppose the band cut in one place, one end
+then given half a turn and the cut ends again welded, we shall have on
+the band a self-locked twist, which can be displaced on the band, but
+which can not release itself or be released except by cutting the
+ring. Hence we see that to make an electron in an ether possessing
+torsional elasticity would require creative energy, and when made, the
+electron cannot destroy itself except by occupying simultaneously the
+same place as an electron of opposite type. Every electron extends,
+therefore, as Faraday said of the atom, throughout the universe, and
+the properties that we find in the electron are only there because
+they are first in the universal medium, the ether. Every line of ether
+or electric strain must, therefore, be a self-closed line, or else it
+must terminate on an electron and a co-electron.
+
+So far we have only considered the electron at rest. If, however, it
+moves, it can be mathematically demonstrated that it must give rise to
+a second form of ether strain which is related to the electric strain
+as a twist is related to a thrust or a vortex ring to a squirt in
+liquid or a rotation to a linear progression. The ether strain which
+results from the lateral movement of lines of electric strain is
+called the _magnetic flux_, and it can be mathematically shown that
+the movement of an electron, consisting when a rest of a radial
+convergence of lines of electric strain, must be accompanied by the
+production of self-closed lines of magnetic flux, distributed in
+concentric circles or rings round it, the planes of these circles
+being perpendicular to the direction of motion of the electron.
+
+This electronic hypothesis, therefore, affords a basis on which we can
+build up a theory affording an explanation of the nature of the
+intimate connection known to exist between ether, matter and
+electricity. The electron is the connecting link between them all, for
+it is in itself a centre of convergent ether strain; isolated, it
+presents itself as electricity of the negative or resinous kind; and,
+in combination with co-electrons and other electrons, it forms the
+atoms of ponderable matter. At rest the electron or the co-electron
+constitutes an electric charge, and when in motion it is an electric
+current. A steady flux or drift of electrons in one direction and
+co-electrons in the opposite direction is a continuous electric
+current, whilst their mere oscillation about a mean position is an
+alternating current. Furthermore, the vibration of an electron, if
+sufficiently rapid, enables it to establish what are called electric
+waves in the ether, but which are really detached and self-closed
+lines of ether strain distributed in a periodic manner through space.
+
+We have, therefore, to start with, three conceptions concerning the
+electron, viz.: Its condition when at rest; its state when in uniform
+motion; and its operations when in vibration or rapid oscillation. In
+the first case, by our fundamental supposition, it consists of lines
+of ether strain of a type called the electric strain, radiating
+uniformly in all directions. When in uniform motion, it can be shown
+that these lines of electric strain tend to group themselves in a
+plane perpendicular to the line of motion drawn through the electron,
+and their lateral motion generates another class of strain called the
+magnetic strain, disposed in concentric circles described round the
+electron and lying in this equatorial plane.
+
+The proof of the above propositions cannot be given verbally, but
+requires the aid of mathematical analysis of an advanced kind. The
+reader must be referred for the complete demonstration to the writings
+of Professor J. J. Thomson[3] and Mr. Oliver Heaviside.[4]
+
+In the third case, when the electron vibrates, we have a state in
+which self-closed lines of electric strain and magnetic flux are
+thrown off and move away through the ether constituting electric
+radiation, The manner in which this happens was first described by
+Hertz in a Paper on "Electric Oscillations treated according to the
+Method of Maxwell."[5] As this phenomenon lies at the very root of
+Hertzian wave wireless telegraphy, we must spend a moment or two in
+its careful examination.
+
+Let us imagine two metal rods placed in line and constituting what is
+called a linear oscillator. Let these rods have adjacent ends
+separated by a very small air space, and let one rod be charged with
+positive and the other with negative electricity. On the electronic
+theory this is explained by stating that there is an accumulation of
+electrons in one and of co-electrons in the other. These charges
+create a distribution of electric strain throughout their
+neighbourhood, which follows approximately the same law of
+distribution as the lines of magnetic force of a bar magnet, and may
+be roughly represented as in Fig. 1. Suppose then that the air gap is
+destroyed, these charges move towards each other and disappear by
+uniting, the lines of electric strain then collapse, and as they
+shrink in give rise to circular lines of magnetic flux embracing the
+rods. This external distribution of magnetism constitutes an electric
+current in the rods produced by the movement of the two opposite
+electric charges. At this stage it may be explained that the electrons
+or atoms of electricity can in some cases make their way freely
+between the atoms of ponderable matter. The former are incomparably
+smaller than the latter, and in those cases in which this electronic
+movement can take place easily, we call the material a good conductor.
+
+[Illustration: FIG. 1.--LINES OF ELECTRIC STRAIN BETWEEN A POSITIVE
+AND NEGATIVE ELECTRON AT REST.]
+
+Suppose then the electric charges reappear in reversed positions and
+go through an oscillatory motion. The result in the external space
+would be the alternate production of lines of electric strain and
+magnetic flux, the direction of these lines being reversed each half
+cycle. Inside the rods we have a movement of electrons and
+co-electrons to and fro, electric charges at the ends of the rods
+alternating with electric currents in the rods, the charges being at a
+maximum when the current is zero, and the current at a maximum when
+the charges have for the moment disappeared. Outside the rods we have
+a corresponding set of charges, lines of electric strain stretching
+from end to end of the rod, alternating with rings of magnetic flux
+embracing the rod. So far we have supposed the oscillation to be
+relatively a slow one.
+
+[Illustration: FIG. 2.--SUCCESSIVE STAGES IN THE DEFORMATION OF A LINE
+OF STRAIN BETWEEN POSITIVE AND NEGATIVE ELECTRONS IN RAPID
+OSCILLATION, SHOWING CLOSED LOOP OF ELECTRIC STRAIN THROWN OFF.]
+
+Imagine next that the to and fro movement of the electrons or charges
+is sufficiently rapid to bring into play the inertia quality of the
+medium. We then have a different state of affairs. The lines of strain
+in the external medium cannot contract or collapse quickly enough to
+keep up with the course of events, or movements of the electrons in
+the rods, and hence their regular contraction and absorption is
+changed into a process of a different kind. As the electrons and
+co-electrons, _i.e._, the electric charges, vibrate to and fro, the
+lines of electric strain connecting them are nipped in and thrown off
+as completely independent and closed lines of electric strain, and at
+each successive alternation, groups or batches of these loops of
+strain are detached from the rod, and, so to speak, take on an
+independent existence. The whole process of the formation of these
+self-closed lines of electric strain is best understood by examining a
+series of diagrams which roughly represent the various stages of the
+process. In Fig. 2 we have a diagram (_a_) the curved line in which
+delineates approximately the form of one line of electric strain round
+a linear oscillator, with spark gap in the centre, one half being
+charged positively and the other negatively. Let us then suppose that
+the insulation of the spark gap is destroyed, so that the opposite
+electric charges rush together and oscillate to and fro. The strain
+lines at each oscillation are then crossed or decussate, and the
+result, as shown in Fig. 2, _d_, is that a portion of the energy of
+the field is thrown off in the form of self-closed lines of strain
+(see Fig. 2, _e_). At each oscillation of the charges the direction of
+the lines of strain springing from end to end of the radiator is
+reversed. It is a general property of lines of strain whether electric
+or magnetic, that there is a tension along the line and a pressure at
+right angles. In other words, these lines of electric strain are like
+elastic threads, they tend to contract in the direction of their
+length and press sideways on each other when in the same direction.
+Hence it is not difficult to see that as each batch of self-closed
+lines of strain is thrown off, the direction of the strain round each
+loop is alternately in one direction and in the other. Hence these
+loops of electric strain press each other out, and each one that is
+formed squeezes the already formed loops further and further from the
+radiator. The loops, therefore, march away into space (see Fig. 2,
+_f_). If we imagine ourselves standing at a little distance at a point
+on the equatorial line and able to see these loops of strain as they
+pass, we should recognise a procession of loops, consisting of
+alternately directed strain lines marching past. This movement through
+the ether of self-closed lines of electric strain constitutes what is
+called electric radiation.
+
+Hence along a line drawn perpendicular to the radiator through its
+centre, there is a distribution of electric strain normal to that
+line, which is periodic in space and in time. Moreover, in addition to
+these lines of electric strain, there are at right angles to them
+another set of self-closed lines of magnetic flux. Alternated between
+the instants when the electric charges at the ends of the radiator are
+at their maximum, we have instants when the radiator rod is the seat
+of an electric current, and hence the field round it is filled with
+circular lines of magnetic flux coaxial with the radiator. As the
+current alternates in direction each half period, these rings of
+magnetic flux alternate in direction as regards the flux, and hence we
+must complete our mental picture of the space round the radiator rods
+when the charges are oscillating by supposing it filled with
+concentric rings of magnetic flux which are periodically reversed in
+direction, and have their maximum values at those instants and places
+where the lines of electric strain have their zero values.
+Accordingly, along the equatorial line we have two sets of strains in
+the ether, distributed periodically in space and in time. First, the
+lines of electric strain in the plane of the radiator, and, secondly,
+the lines of magnetic flux at right angles to these. At any one point
+in space these two changes, the strain and the flux, succeed each
+other periodically, being, however, at right angles in direction. At
+any one moment these two effects are distributed periodically or
+cyclically through space, and these changes in time and space
+constitute an _electric wave_ or electromagnetic wave.
+
+We may then summarise the above statements by saying that the most
+recent hypothesis as to the nature of electrical action and of
+electricity itself is briefly comprised in the following statements:
+The universally diffused medium called the ether has had created in
+it certain centres of strain or radiating points from which proceed
+lines of strain, and these centres of force are called electrons.
+Electrons must, therefore, be of two kinds, positive and negative,
+according to the direction of the strain radiating from the centre.
+These electrons in their free condition constitute what we call
+electricity, and the electrons themselves are the atoms of electricity
+which, in one sense, is, therefore, as much material as that which we
+call ordinary gross or ponderable matter.
+
+Collocations of these electrons constitute the atoms of gross matter,
+and we must consider that the individuality of any atom is not
+determined merely by the identity of the electrons composing it, but
+by the permanence of their arrangement or form. In any mass of
+material substance there is probably a continual exchange of electrons
+from one atom to another, and hence at any one given moment, whilst a
+number of the electrons are an association forming material atoms,
+there will be a further number of isolated but intermingled electrons,
+which are called the free electrons. In substances which we call good
+conductors, we must imagine that the free electrons have the power of
+moving freely through or between the material atoms, and this movement
+of the electrons constitutes a current of electricity; whilst a
+superfluity of electrons of either type in any one mass of matter
+constitutes what we call a charge of electricity. Hence an electrical
+oscillation, which is merely a very rapid alternating current taking
+place in a conductor, is on this hypothesis assumed to consist in a
+rapid movement to and fro of the free electrons. We may picture to
+ourselves, therefore, a rod of metal in which electrical oscillations
+are taking place, as similar to an organ-pipe or siren tube in which
+movements of the air particles are taking place to and fro, the free
+electrons corresponding with the air particles.
+
+Owing to the nature of the structure of an electron, it follows,
+however, that every movement of an electron is accompanied by changes
+in the distribution of the electric strain or ether strain taking
+place throughout all surrounding space, and, as already explained,
+certain very rapid movements of the electrons have the effect of
+detaching closed lines of strain in the ether which move off through
+space, forming, when cyclically distributed, an electric wave.
+
+[Illustration: FIG. 3.--SIMPLE MARCONI RADIATOR. B, battery; I,
+induction coil; K, signalling key; S, spark gap; A, aerial wire; E,
+earth plate.]
+
+We may next proceed to apply these principles to the explanation of
+the action of the simplest form of Hertzian wave telegraphic radiator,
+viz., the Marconi aerial wire. In its original form this consists of a
+long vertical insulated wire, A, the lower end of which is attached to
+one of the spark balls S of an induction coil, I, the other spark ball
+being connected to earth E, and the two spark balls being placed a few
+millimetres apart (see Fig. 3). When the coil is set in action
+oscillatory or Hertzian sparks pass between the balls, electric
+oscillations are set up in the wire and electric waves are radiated
+from it. Deferring for the moment a more detailed examination of the
+operations of the coil and at the spark gap, we may here say that the
+action which takes place in the aerial wire is as follows: The wire is
+first charged to a high potential, let us suppose, with negative
+electricity. We may imagine this process to consist in forcing
+additional electrons into it, the induction coil acting as an electron
+pump. Up to a certain pressure the spark gap is a perfect insulator,
+but at a critical pressure, which for spark gap lengths of four or
+five millimetres and balls about one centimetre in diameter
+approximates to three thousand volts per millmetre, the insulation of
+the air gives way, and the charge in the wire rushes into the earth.
+In consequence, however, of the inertia of the medium or of the
+electrons, the charge, so to speak, overshoots the mark, and the wire
+is then left with a charge of opposite sign. This again in turn
+rebounds, and so the wire is discharged by a series of electrical
+oscillations, consisting of alternations of static charge and electric
+discharge. We may fasten our attention either on the events taking
+place in the vertical wire or in the medium outside, but the two sets
+of phenomena are inseparably connected and go on together. When the
+aerial wire is statically charged, we may describe it by saying that
+there is an accumulation of electrons or co-electrons in it. Outside
+the wire there is, however, a distribution of electric strain the
+strain lines proceeding from the wire to the earth (see Fig. 4).
+
+[Illustration: FIG. 4.--LINES OF ELECTRIC STRAIN (DOTTED LINES)
+EXTENDING BETWEEN A MARCONI AERIAL, A, AND THE EARTH _ee_ BEFORE
+DISCHARGE.]
+
+The wire has _capacity_ with respect to the earth, and it acts like
+the inner coating of a Leyden jar, of which the dielectric is the air
+and ether around it, and the outer coating is the earth's surface.
+When the discharge takes place, we may consider that electrons rush
+out of the wire and then rush back again into it. At the moment when
+the electrons rush out of or into the aerial wire, we say there is an
+electric current flowing into or out of the wire, and this electron
+movement, therefore, creates the magnetic flux which is distributed in
+concentric circles round the wire. This current, and, therefore,
+motion of electrons, can be proved to exist by its heating effect upon
+a fine wire inserted in series with the aerial, and in the case of
+large aerials it may have a mean value of many amperes and a maximum
+value of hundreds of amperes. Inside the aerial wire we have,
+therefore, alternations of electric potential or charge and electric
+current, or we may call it electron-pressure and electron-movement.
+
+There is, therefore, an oscillation of electrons in the aerial wire,
+just as in the case of an organ-pipe there is an oscillation of air
+molecules in the pipe. Outside the aerial we have variations and
+distributions of electric strain and magnetic flux. The resemblance
+between the closed organ-pipe and the simple Marconi aerial is, in
+fact, very complete. In the case of the closed organ-pipe, we have a
+longitudinal oscillation of air molecules in the pipe. At the open end
+or mouthpiece, where we have air moving in and out, the air movement
+is alternating and considerable, but there is little or no variation
+of air pressure. At the upper or closed end of the pipe we have great
+variation of air pressure, but little or no air movement (see Fig. 5).
+
+Compare this now with the electrical phenomena of the aerial. At the
+spark ball or lower end we have little or no variation of potential or
+electron pressure, but we have electrons rushing into and out of the
+aerial at each half oscillation, forming the electric discharge or
+current. At the upper or insulated end we have little or no current,
+but great variations of potential or electron pressure. Supposing we
+could examine the wire inch by inch, all the way up from the spark
+balls at the bottom to the top, we should find at each stage of our
+journey that the range of variation and maximum value of the current
+in the wire became less and those of the potential became greater. At
+the bottom we have nearly zero potential or no electric pressure, but
+large current, and at the top end, no current, but great variation of
+potential.
+
+[Illustration: FIG. 5.--AMPLITUDE OF PRESSURE VARIATION IN A CLOSED
+ORGAN PIPE, INDICATED BY THE ORDINATES OF THE DOTTED LINE _xy_.]
+
+We can represent the amplitude of the current and potential values
+along the aerial by the ordinates of a dotted line so drawn that its
+distance from the aerial represents the potential oscillation or
+current oscillation at that point (see Fig. 6).
+
+This distribution of potential and current along the wire does not
+necessarily imply that any one electron moves far from its normal
+position. The actual movement of any particular air molecule in the
+case of a sound wave is probably very small, and reckoned in
+millionths of an inch. So also we must suppose that any one electron
+may have a small individual amplitude of movement, but the
+displacement is transferred from one to another. Conduction in a solid
+may be effected by the movement of free electrons intermingled with
+the chemical atoms, but any one electron may be continually passing
+from a condition of freedom to one of combination.
+
+[Illustration: FIG. 6.--(_a_) DISTRIBUTION OF ELECTRIC PRESSURE IN A
+MARCONI AERIAL, A, (_b_) DISTRIBUTION OF ELECTRIC CURRENT IN A MARCONI
+AERIAL, AS SHOWN BY THE ORDINATES OF THE DOTTED LINE _xy_.]
+
+So much for the events inside the wire, but now outside the wire its
+electric charge is represented by lines of electric strain springing
+from the aerial to the earth. It must be remembered that every line of
+strain terminates on an electron or a co-electron. Hence, when the
+discharge or spark takes place between the spark balls, the rapid
+movement of the electrons in the wire is accompanied by a
+redistribution and movement of the lines of strain outside. As the
+negative charge flows out of the aerial the ends of the strain lines
+abutting on to it run down the wire and are transferred to the earth,
+and at the next instant this semi-loop of electric or ether strain,
+with its ends on the earth, is pushed out sideways from the wire by
+the growth of a new set of lines of ether strain in an opposite
+direction. The process is best understood by consulting a series of
+diagrams which represent the distribution and approximate form of a
+few of the strain lines at successive instants (see Fig. 7). In
+between the lines of formation of the successive strain lines between
+the aerial and the earth, corresponding to the successive alternate
+electric charges of the aerial with opposite sign, there are a set of
+concentric rings of magnetic flux formed round it which are
+alternately in opposite directions, and these expand out, keeping step
+with the progress of the detached strain loops and having their planes
+at right angles to the latter. As the semi-loops of electric strain
+march outwards with their feet on the ground, these strain lines must
+always be supposed to terminate on electrons, but not continually on
+the same electrons. Since the earth is a conductor, we must suppose
+that there is a continual migration of the electrons forming the atoms
+of the earth, and that when one electron enters an atom, another
+leaves it. Hence, corresponding to the electric wave in the space
+above, there are electrical changes in the ground beneath. This view
+is confirmed by the well-known fact that the achievement of Hertzian
+wave telegraphy is much dependent on the nature of the surface over
+which it is conducted, and can be carried on more easily over good
+conducting material, like sea water, than over badly conducting dry
+land.
+
+[Illustration: FIG. 7.--SUCCESSIVE STAGES IN THE PRODUCTION OF A
+SEMI-LOOP OF ELECTRIC STRAIN BY A MARCONI AERIAL RADIATOR.]
+
+The matter may be viewed, however, from another standpoint. Good
+conductors are opaque to Hertzian waves; in other words, are
+non-absorptive. The energy of the electric wave is not so rapidly
+absorbed when it glides over a sea surface as when it is passing over
+a surface which is an indifferent conductor, like dry land. In fact,
+it is possible by the improvement of the signals to detect a heavy
+fall of rain in the space between two stations separated only by dry
+land. It is, however, clear that on the electronic theory the
+progression of the lines of electric strain can only take place if the
+surface over which they move is a fairly good conductor, unless these
+lines of strain form completely closed loops. Hence we may sum up by
+saying that there are three set of phenomena to which we must pay
+attention in formulating any complete theory of the aerial. The first
+is the operation taking place in the vertical wire, which is described
+by saying that electrical oscillations or vibratory movements of
+electrons are taking place in it, and, on our adopted theory, it may
+be said to consist in a longitudinal vibration of electrons of such a
+nature that we may appropriately call the aerial an ether organ-pipe.
+Then in the next place, we have the distribution and movement of the
+lines of electric strain and magnetic flux in the space outside the
+wire, constituting the electric wave; and lastly, there are the
+electrical changes in the conductor over which the wave travels, which
+is the earth or water surrounding the aerial. In subsequently dealing
+with the details of transmitting arrangements, attention will be
+directed to the necessity for what telegraphists call a "good earth"
+in connection with Hertzian wave telegraphy. This only means that
+there must be a perfectly free egress and ingress for the electrons
+leaving or entering the aerial, so that nothing hinders their access
+to the conducting surface over which the wave travels. There must be
+nothing to stop or throttle the rush of electrons into or out of the
+aerial wire, or else the lines of strain cannot be detached and and
+travel away.
+
+We may next consider more particularly the energy which is available
+for radiation and which is radiated. In the original form of simple
+Marconi aerial, the aerial itself when insulated forms one coating or
+surface of a condenser, the dielectric being the air and ether around
+it, and the other conductor being the earth. The electric energy
+stored up in it just before discharge takes place is numerically equal
+to the product of the capacity of the aerial and half the square of
+the potential to which it is charged.
+
+If we call C the capacity of the aerial in microfarads, and V the
+potential in volts to which it is raised before discharge, then the
+energy storage in joules E is given by the equation,
+
+    E = (CV^{2}) / (2 . 10^{6}).
+
+Since one joule is nearly equal to three-quarters of a foot-pound, the
+energy storage in foot-pounds F is roughly given by the rule F =
+(3/8)CV^{2}/10^{6}. For spark lengths of the order of five to fifteen
+millimetres, the disruptive voltage in air of ordinary pressure is at
+the rate of 3,000 volts per millimetre. Hence, if S stands for the
+spark length in millimetres, and C for the aerial capacity in
+microfarads, it is easy to see that the energy storage in foot-pound
+is
+
+    F = (27CS^{2}) / 8.
+
+If the aerial consists of a stranded wire formed of 7/22 and has a
+length of 150 feet, and is insulated and held vertically with its
+lower end near the earth, it would have a capacity of about one three
+ten-thousandths of a microfarad or 0.0003 mfd.[6] Hence, if it is used
+as a Marconi aerial and operated with a spark gap of one centimetre in
+length, the energy stored up in the wire before each discharge would
+be only one-tenth (0.1) of a foot-pound.
+
+By no means can all of this energy be radiated as Hertzian waves; part
+of it is dissipated as heat and light in the spark, and yet such an
+aerial can, with a sensitive receiver such as that devised by Mr.
+Marconi, make itself felt for a hundred miles over sea in every
+direction. This fact gives us an idea of the extremely small energy
+which, when properly imparted to the ether, can effect wireless
+telegraphy over immense distances. Of course, the minimum telegraphic
+signal, say the Morse dot, may involve a good many, perhaps
+half-a-dozen, discharges of the wire, but even then the amount of
+energy concerned in affecting the receiver at the distant place is
+exceedingly small.
+
+The problem, therefore, of long-distance telegraphy by Hertzian waves
+is largely, though not entirely, a matter of associating sufficient
+energy with the aerial wire or radiator. There are obviously two
+things which may be done; first, we may increase the capacity of the
+aerial, and secondly, we may increase the charging voltage or, in
+other words, lengthen the spark gap. There is, however, a well-defined
+limit to this last achievement. If we lengthen the spark gap too much,
+its resistance becomes too great and the spark ceases to be
+oscillatory. We can make a discharge, but we obtain no radiation. When
+using an induction coil, about a centimetre, or at most a centimetre
+and a half, is the limiting length of oscillatory sparks; in other
+words, our available potential difference is restricted to 30,000 or
+40,000 volts. By other appliances we can, however, obtain oscillatory
+sparks having a voltage of 100,000 or 200,000 volts, and so obtain
+what Hertz called "active sparks" five or six centimetres in length.
+
+Turning then to the question of capacity, we may enquire in the next
+place how the capacity of an aerial wire can be increased. This has
+generally been done by putting up two or more aerial wires in
+contiguity and joining them together, and so making arrangements
+called in the admitted slang of the subject "multiple aerials." The
+measurement of the capacity of insulated wires can be easily carried
+out by means of an appliance devised by the author and Mr. W. C.
+Clinton, consisting of a rotating commutator which alternately charges
+the insulated wire at a source of known electromotive force and then
+discharges it through a galvanometer. If this galvanometer is
+subsequently standardised, so that the ampere value of its deflection
+is known, we can determine easily the capacity C of the aerial or
+insulated conductor, reckoned in microfarads, when it is charged to a
+potential of V volts, and discharged _n_ times a second through a
+galvanometer. The series of discharges are equivalent to a current, of
+which the value in amperes A is given by the equation
+
+    A = (nVC) / (10^{6}),
+
+and hence, if the value of the current resulting is known, we have the
+capacity of the aerial or conductor expressed in microfarads, given by
+the formula
+
+    C = (A10^{6}) / (nV).
+
+A series of experiments made on this plan have revealed the fact that
+if a number of vertical insulated wires are hung up in the air and
+rather near together, the electrical capacity of the whole of the
+wires in parallel is not nearly equal to the sum of their individual
+capacities. If a number of parallel insulated wires are separated by a
+distance equal to about 3 per cent. of their length, the capacity of
+the whole lot together varies roughly as the square root of their
+number. Thus, if we call the capacity of one vertical wire in free
+space unity, then the capacity of four wires placed rather near
+together will only be about twice that of one wire, and that of
+twenty-five wires will only be about five times one wire.
+
+This approximate rule has been confirmed by experiments made with long
+wires one hundred or two hundred feet in length in the open air. Hence
+it points to the fact that the ordinary plan of endeavouring to obtain
+a large capacity by putting several wires in parallel and not very far
+apart is very uneconomical in material. The diagrams in Fig. 8 show
+the various methods which have been employed by Mr. Marconi and others
+in the construction of such multiple wire aerials. If, for instance,
+we put four insulated stranded 7/22 wires each 100 feet long, about
+six feet apart, all being held in a vertical position, the capacity of
+the four together is not much more than twice that of a single wire.
+In the same manner, if we arrange 150 similar wires, each 100 feet
+long, in the form of a conical aerial, the wires being distributed at
+the top round a circle 100 feet in diameter, the whole group will not
+have much more than twelve times the capacity of one single wire,
+although it weighs 150 times as much.
+
+[Illustration: FIG. 8.--VARIOUS FORMS OF AERIAL RADIATOR. _a_, single
+wire; _b_, multiple wire; _c_, fan shape; _d_, cylindrical; _g_,
+Conical.]
+
+The author has designed an aerial in which the wires, all of equal
+length, are arranged sufficiently far apart not to reduce each other's
+capacity.
+
+As a rough guide in practice, it may be borne in mind that a wire
+about one tenth of an inch in diameter and one hundred feet long, held
+vertical and insulated, with its bottom end about six feet from the
+ground, has a capacity of 0.0002 of a microfarad, if no other earthed
+vertical conductors are very near it. The moral of all this is that
+the amount of electric energy which can be stored up in a simple
+Marconi aerial is very limited, and is not much more than one-tenth of
+a joule or one-fourteenth of a foot-pound, per hundred feet of 7/22
+wire. The astonishing thing is that with so little storage of energy
+it should be possible to transmit intelligence to a distance of a
+hundred miles without connecting wires.
+
+One consequence, however, of the small amount of energy which can be
+accumulated in a simple Marconi aerial is that this energy is almost
+entirely radiated in one oscillation or wave. Hence, strictly
+speaking, a simple aerial of this type does not create a train of
+waves in the ether, but probably at most a single impulse or two.
+
+[Illustration: FIG. 9.--MARCONI-BRAUN SYSTEM OF INDUCING ELECTROMOTIVE
+FORCE IN AN AERIAL, A. B, battery; K, key; I, induction coil; S, spark
+gap; C, Leyden jar; E, earth plate; _ps_, oscillation transformer.]
+
+We shall later on consider some consequences which follow from this
+fact. Meanwhile, it may be explained that there are methods by which
+not only a much larger amount of energy can be accumulated in
+connection with an aerial, but more sustained oscillations created
+than by the original Marconi method. One of these methods originated
+with Professor Braun, of Strasburg, and a modification was first
+described by Mr. Marconi in a lecture before the Society of Arts of
+London.[7] In this method the charge in the aerial is not created by
+the direct application to it of the secondary electromotive force of
+an induction coil, but by means of an induced electromotive force
+created in the aerial by an oscillation transformer. The method due to
+Professor Braun is as follows: A condenser or Leyden jar has one
+terminal, say, its inside, connected to one spark ball of an induction
+coil. The other spark ball is connected to the outside of the Leyden
+jar or condenser through the primary coil of a transformer of a
+particular kind, called an oscillation transformer (see Fig. 9). The
+spark balls are brought within a few millimetres of each other. When
+the coil is set in operation, the jar is charged and discharged
+through the spark gap, and electrical oscillations are set up in the
+circuit consisting of the dielectric of the jar, the primary coil of
+the oscillation transformer and the spark gap. The secondary circuit
+of this oscillation transformer is connected in between the earth and
+the insulated aerial wire; hence, when the oscillations take place in
+the primary circuit, they induce other oscillations in the aerial
+circuit. But the arrangement is not very effective unless, as is
+shown by Mr. Marconi, the two circuits of the oscillation transformer
+are tuned together.
+
+We shall return presently to the consideration of this form of
+transmitter; meanwhile we may notice that by means of such an
+arrangement it is possible to create in the aerial a far greater
+charging electromotive force than would be the case if the aerial were
+connected directly to one terminal of the secondary circuit of the
+induction coil, the other terminal being to earth, and the two
+terminals connected as usual by spark balls. By the inductive
+arrangement it is possible to create in an aerial electromotive forces
+which are equivalent to a spark of a foot in length, and when the
+length of the aerial is also properly proportioned the potential along
+it will increase all the way up, until at the top or insulated end of
+the aerial it may reach an amount capable of giving sparks several
+feet in length. From the remarks already made on the analogy between
+the closed organ-pipe and the Marconi aerial wire, it will be seen
+that the wave which is radiated from the aerial must have a wave
+length four times that of the aerial if the aerial is vibrating in its
+fundamental manner. It is also possible to create electrical
+oscillations in a vertical wire which are the harmonics of the
+fundamental.
+
+All musicians are aware that in the case of an organ-pipe if the pipe
+is blown gently it sounds a note which is called the fundamental of
+the pipe. The celebrated mathematician, Daniel Bernouilli, discovered
+that an organ-pipe can be made to yield a succession of musical notes
+by properly varying the pressure of the current of air blown into it.
+If the pipe is an open pipe, and if we call the frequency of the
+primary note obtained when the pipe is gently blown, unity, then when
+we blow more strongly the pipe yields notes which are the harmonics of
+the fundamental one; that is to say, notes which have frequencies
+represented by the numbers 2, 3, 4, 5, &c. If, however, the pipe is
+closed at the top, then over-blowing the pipe makes it yield the odd
+harmonics or the tones which are related to the primary tone in the
+ratio of 3, 5, 7, &c., to unity. Accordingly, if a stopped pipe gives
+as its fundamental the note C, its first overtone will be the fifth
+above the octave or G'.
+
+[Illustration: FIG. 10.--SEIBT'S APPARATUS FOR SHOWING STATIONARY
+WAVES IN LONG SOLENOID A. I, induction coil; S, spark gap; L,
+inductance coil; C_{1}C_{2}, Leyden jars; E, earth wire.]
+
+As already remarked, the aerial wire or radiator as used in Marconi
+telegraphy may be looked upon as a kind of ether organ-pipe or siren
+tube, and its electrical phenomena are in every respect similar to the
+acoustic phenomena of the ordinary closed organ-pipe. When the aerial
+is sounding its fundamental ether note, the conditions which pertain
+are that there is a current flowing into the aerial at the lower end,
+but at that point the variation in potential is very small, whereas at
+the upper end there is no current, but the variations of potential are
+very large. Accordingly, we say that at the upper end of the aerial
+there is an antinode of potential and a node of current, and at the
+bottom an antinode of current and a node of potential. By altering the
+frequency of the electrical impulses we can create in the aerial an
+arrangement of nodes of current or potential corresponding to the
+overtones of a closed organ-pipe. But whatever may be the arrangement
+the conditions must always hold that there is a node of current at
+the upper end and an antidote of current at the lower end. In other
+words, there are large variations of current at the place where the
+aerial terminates on the spark-gap and no current at the upper end.
+The first harmonic is formed where there is a node of potential at
+one-third of the length of the aerial from the top. In this case we
+have a node of potential not only at the lower end of the wire, but at
+two-thirds of the way up. In the same way we can create in the closed
+organ-pipe, by properly overblowing the pipe, a region about
+two-thirds of the way up the pipe, where the pressure changes in the
+air are practically no greater than they are at the mouthpiece. We can
+make evident visually in a beautiful manner the existence of similar
+stationary electrical waves in an aerial by means of an ingenious
+arrangement devised by Dr. Georg Seibt, of Berlin. It consists of a
+very long silk-covered copper wire, A (see Fig. 10), wound in a close
+spiral of single layer round a wooden rod six feet long and about two
+inches in diameter. This rod is insulated, and at the lower end the
+wire is connected to a Leyden jar circuit, consisting of a Leyden jar
+or jars and an inductance coil, L, the inductance of which can be
+varied. Oscillations are set up in this jar circuit by means of an
+induction-coil discharge, and the lower end of the long spiral wire is
+attached to one point on the jar circuit. In this manner we can
+communicate to the bottom end of the long spiral wire a series of
+electric impulses, the time period of which depends upon the capacity
+of the jar and the inductance of the discharge circuit. We can,
+moreover, vary this frequency over wide limits. Parallel to the long
+spiral wire is suspended another copper wire, E (see Fig. 10), and
+between this wire and the silk-covered copper wire discharges take
+place due to the potential difference between each part of the wire
+and this long aerial wire. If we arrange matters so that the impulses
+communicated to the bottom end of the long spiral wire correspond to
+its fundamental note or periodic time, then in a darkened room we
+shall see a luminous glow or discharge between the vertical wire and
+the spiral wire, which increases in intensity all the way up to the
+top of the spiral wire. The luminosity of this brush discharge at any
+point is evidence of the potential of the spiral wire at that point,
+and its distribution clearly demonstrates that the difference of
+potential between the spiral wire and the aerial increases all the way
+up from the bottom to the top of the spiral wire. In the next place,
+by making a little adjustment and by varying the inductance of the jar
+circuit, we can increase the frequency of the impulses which are
+falling upon the spiral wire; and then it will be noticed that the
+distribution of the brush discharge or luminosity is altered, and that
+there is a maximum now at about one-third of the height of the spiral
+wire, and a dark place at about two-thirds of the height, and another
+bright place at the top, thus showing that we have a node of potential
+at about two-thirds the way up the wire (see Fig. 11), and we have
+therefore set up in the spiral wire electrical oscillations
+corresponding to the first overtone. It is possible to show in the
+same way the existence of the second harmonic in the coil, but the
+luminosity then becomes too faint to be seen at a distance.
+
+[Illustration: FIG. 11.--HARMONIC OSCILLATIONS IN LONG SOLENOID SHOWN
+WITH SEIBT'S APPARATUS.]
+
+An interesting form of aerial devised by Professor Slaby, of Berlin,
+depends for its action entirely on the fact that the electrical
+oscillations set up in it which radiate are harmonics of the
+fundamental tone.
+
+[Illustration: FIG. 12.--NON-RADIATIVE CLOSED LOOP AERIAL.]
+
+[Illustration: FIG. 13.--SLABY'S LOOP RADIATOR.]
+
+A closed vertical loop, A_{1}A_{2} (see Fig. 12), is formed by
+erecting two parallel insulated wires vertically a few feet apart and
+joining them together at the top. At the bottom these wires are
+connected, with the secondary terminals of an induction coil, a
+condenser, C, or Leyden jar, being bridged across the terminals and a
+pair of spark balls, S, inserted in one side of the loop. It will
+readily be seen that on setting the coil in action, oscillations will
+take place in these vertical wires, but that if the oscillations are
+simply the fundamental note of the system, then at any moment
+corresponding to a current going up one side of the loop of wire there
+must be a current coming down the other. Accordingly, an arrangement
+of this kind, forming what is called a closed circuit, will not
+radiate or radiates but very feebly. Professor Slaby found, however,
+that it might be converted into a powerful radiator if we give the two
+sides of the loop unequal capacity or inductance and at the same time
+earth one of the lower ends of the loop, as shown in Fig. 13. By this
+means it is possible to set up in the loop electrical overtones or
+harmonics of the fundamental oscillation, and if we cause the system
+to vibrate so as to produce its first odd harmonic, there is a
+potential node at the lower end of both vertical sides of the loop, a
+potential node on both vertical sides at two-thirds of the way up, and
+a potential antinode at the summit of the loop; then, under these
+circumstances, the closed loop of wire is in the same electrical
+condition as if two simple Marconi aerials, both emitting their first
+odd harmonic oscillation, were placed side by side and joined together
+at the top.
+
+It is a little difficult without the employment of mathematical
+analysis to explain precisely the manner in which earthing one side of
+the loop or making the loop unsymmetrical as regards inductance has
+the effect of creating overtones in it. The following rough
+illustration may, however, be of some assistance. Imagine a long
+spiral metallic spring supported horizontally by threads. Let this
+represent a conductor, and let any movement to or fro of a part of the
+spring represent a current in that conductor. Suppose we take hold of
+the spring at one end, we can move it bodily to and fro as a whole. In
+this case, every part of the spring is moving one way or the other in
+the same manner at the same time. This corresponds with the case in
+which the discharge of the condenser through the uniform loop
+conductor is a flow of electricity, all in one direction one way or
+the other. The current is in the same direction in all parts of the
+loop at the same time, and, therefore, if the current is going up one
+side of the loop it is at the same time coming down the other side.
+Hence the two sides of the loop are always in exact opposition as
+regards the effect of the current in them on the external space, and
+the loop does not radiate. Returning again to the case of the spring.
+Supposing that we add a weight to one end of the spring by attaching
+to it a metal ball, and then move the other end to and fro with
+certain periodic motion, it will be found quite easy to set up in the
+spring a pulsatory motion resembling the movement of the air in an
+open organ-pipe. Under these circumstances both ends of the spring
+will be moving inwards or outwards at the same time, and the central
+portions of the spring, although being pressed and expanded slightly,
+are moving to and fro very little. This corresponds in the case of the
+looped aerial with a current flowing up or down both sides at the same
+time; in other words, when this mode of electrical oscillation is
+established in the loop, its electrical condition is just that of two
+simple Marconi aerials joined together at the top and vibrating in
+their fundamental manner. Accordingly, if one side of the double loop
+is earthed, we then have an arrangement which radiates waves.
+Professor Slaby found that by giving one side of the loop less
+inductance than the other, and at the same time earthing the side
+having greater inductance at the bottom, he was able to make an
+arrangement which radiated, not in virtue of the normal oscillations
+of the condenser, but in virtue of the harmonic oscillations set up in
+the conductor itself. The mathematical theory of this radiator has
+been very fully developed by Dr. Georg Seibt.
+
+It will be seen, therefore, that there are several ways in which we
+may start into existence oscillations in an aerial. First, the aerial
+may be insulated, and we may charge it to a high potential and allow
+this charge suddenly to rush out. Although this process gives rise to
+a disturbance in the ether, as already explained, it is analogous to a
+pop or explosion in the air, rather than to a sustained musical note.
+The exact acoustic analogue would be obtained if we imagine a long
+pipe pumped full of air and then suddenly opened at one end. The air
+would rush out, and, communicating a blow to the outer air, would
+create an atmospheric disturbance appreciated as a noise or small
+explosion. This is what happens when we cut the string and let the
+cork fly out from a bottle of champagne. At the same time, the
+inertia of the air rushing out of the tube would cause it to overshoot
+the mark, and a short time after opening the valve the tube, so far
+from containing compressed air, would contain air slightly rarefied
+near its mouth, and this rarefication would travel back up the tube in
+the form of wave motion, and, being reflected as condensation at the
+closed end, travel down again; and so after being reflected once or
+twice at the open or closed end, become damped out very rapidly in
+virtue of both air friction and the radiation of the energy. In the
+case, however, of the ordinary organ-pipe, we do not depend merely
+upon a store of compressed air put into the pipe, but we have a store
+of energy to draw upon in the form of the large amount of compressed
+air contained in a wind chest, which is being continually supplied by
+the bellows. This store of compressed air is fed into the organ-pipe,
+with the result that we obtain a continuous radiation of sound waves.
+The first case, in which the only store of energy is the compressed
+air originally contained in the pipe, illustrates the operation of the
+simple Marconi aerial. The second case, in which there is a larger
+store of energy to draw upon, the organ-pipe being connected to a wind
+chest, illustrates the Marconi-Braun method, in which an aerial is
+employed to radiate a store of electric energy contained in a
+condenser, gradually liberated by the aerial in the form of a series
+of electrical oscillations and waves. In this arrangement the
+condenser corresponds to the wind chest, and it is continually kept
+full of electrical energy by means of the induction coil or
+transformer, which answers to the bellows of the organ. From the
+condenser, electrical energy is discharged each time the spark
+discharge passes at a spark gap in the form of electrical oscillations
+set up in the primary circuit of an oscillation transformer. The
+secondary circuit of this transformer is connected in between the
+earth and the aerial, and therefore may be considered as part of it,
+and, accordingly, the energy which is radiated from the aerial is not
+simply that which is stored up in it in virtue of its own small
+capacity, but that which is stored up in the much larger capacity
+represented by the primary condenser or, as it may be called, the
+electrical wind chest. By the second arrangement we have therefore the
+means of radiating more or less continuous trains of electric waves,
+corresponding with each spark discharge. To create powerful
+oscillations in the aerial, one condition of success is that there
+shall be an identity in time-period between the circuit of the aerial
+and that of the primary condenser. The aerial is an open circuit which
+has capacity with respect to the earth, and it has also inductance,
+partly due to the wire of the aerial and partly due to the secondary
+circuit of the oscillation transformer in series with it. The primary
+circuit or spark circuit has capacity--viz., the capacity of the
+energy-storing condenser--and it has also inductance--viz., the
+inductance of the primary circuit of the oscillation transformer. We
+shall consider at a later stage more particularly the details of
+syntonising arrangements, but meanwhile it may be said that one
+condition for setting up powerful waves by means of the above
+arrangement is that the electrical time-period of both the two
+circuits mentioned shall be the same. This involves adjusting the
+inductance and capacity so that the product of conductance and
+capacity for each of these two circuits is numerically the same.
+Instead of employing an oscillation transformer between the condenser
+circuit and the aerial, the aerial may be connected directly to some
+point on the condenser circuit at which the potential oscillations are
+large, and we have then another arrangement devised by Professor Braun
+(see Fig. 14). In this case, in order to accumulate large potential
+oscillations at the top of the aerial, it is, as we have seen,
+necessary that the length of the aerial shall be one quarter the
+length of the wave. If, therefore, the electrical oscillations in the
+condenser circuit are at the rate of N per second, in other words,
+have a frequency N, the wave-length correponding to this frequency is
+given by the expression,
+
+    3x10^{10}/N cms.
+
+The number 3x10^{10} is the value in centimetres per second of the
+velocity of the electromagnetic wave, and is identical with that of
+light. The corresponding resonant length of the aerial is therefore
+one-fourth of this wave-length, or 3x10^{10}/4N. Generally speaking,
+however, it will be found that with any length of aerial which is
+practicable, say, 200 feet or 6,000 cms., this proportion necessitates
+rather a high frequency in the primary oscillation circuit. In the
+case considered--viz., for an aerial 200 feet in height--the
+oscillations in the primary circuit must have a frequency of one and a
+quarter million. This high frequency can only be obtained either by
+greatly reducing the inductance of the primary discharge circuit, or
+reducing the capacity. If we reduce the capacity, we thereby greatly
+reduce the storage of energy, and it is not practicable to reduce the
+inductance below a certain amount.
+
+[Illustration: FIG. 14.--BRAUN'S RADIATOR. B, battery; I, induction
+coil; K, key; S, spark-gap; L, inductance coil; C, condenser; A,
+aerial.]
+
+Summing up, it may be said that there are three, and, as far as the
+writer is aware, at present only three, modes of exciting the
+electrical oscillations in an aerial wire. First, the aerial may
+itself be used as an electrical reservoir and charged to a high
+potential and suddenly discharged to the earth. This is the original
+Marconi method. The second method, due to Braun, consist of attaching
+the aerial to some point on an oscillation circuit consisting of a
+condenser, an inductance coil and a spark gap, in series with one
+another, and charging and discharging the condenser across the spark
+gap so as to create alterations of potential at some point on the
+oscillation circuit. The length of the aerial must then be so
+proportioned as above described that it is resonant to this frequency.
+Thirdly, we may employ the arrangement involving an oscillation
+transformer, in which the oscillations in the primary condenser
+circuit are made to induce others in the aerial circuit, the
+time-period of the two circuits being the same. This method may be
+called the Braun-Marconi method. Professor Slaby has combined together
+in a certain way the original Marconi simple aerial with the resonant
+quarter-wave-length wire of Braun. He constructs what he calls a
+_multiplicator_, which is really a wire wound into a loose spiral
+connected at one point to an oscillation circuit consisting of a
+condenser inductance, the length of this wire being proportioned so
+that there is a great resonance or multiplication of tension or
+potential at its free end. This free end is then attached to the lower
+end of an ordinary Marconi aerial, and serves to charge it with a
+higher potential than could be obtained by the use of the induction
+coil directly attached to it.
+
+       *       *       *       *       *
+
+We have next to consider the appliances for creating the necessary
+charging electromotive force, and for storing and releasing this
+charge at pleasure, so as to generate the required electrical
+oscillations in the aerial.
+
+It is essential that this generator should be able to create not only
+large potential difference, but also a certain minimum electric
+current. Accordingly, we are limited at the present moment to one of
+two appliances--viz., the induction coil or the alternating current
+transformer.
+
+It will not be necessary to enter into an explanation of the action of
+the induction coil. The coil generally employed for wireless
+telegraphy is technically known as a ten-inch coil--_i.e._, a coil
+which is capable of giving a ten-inch spark between pointed conductors
+in air at ordinary pressure. The construction of a large coil of this
+description is a matter requiring great technical skill, and is not to
+be attempted without considerable previous experience in the
+manufacture of smaller coils. The secondary circuit of a ten-inch coil
+is formed of double silk-covered copper wire; generally speaking, the
+gauge called No. 36, or else No. 34 S.W.G. is used, and a length of
+ten to seventeen miles of wire is employed on the secondary circuit,
+according to the gauge of wire selected. For the precautions necessary
+in constructing the secondary coil, practical manuals must be
+consulted.[8]
+
+Very great care is required in the insulation of the secondary circuit
+of an induction coil to be used in Hertzian wave telegraphy, because
+the secondary circuit is then subjected to impulsive electromotive
+forces lasting for a short time, having a much higher electromotive
+force than that which the coil itself normally produces.
+
+The primary circuit of a ten-inch coil generally consists of a length
+of 300 or 400 feet of thick insulated copper wire. In such a coil the
+secondary circuit would require about ten miles of No. 34 H.C. copper
+wire, making 50,000 turns round the core. It would have a resistance
+at ordinary temperatures of 6,600 ohms, and an inductance of 460
+henrys. The primary circuit, if formed of 360 turns of No. 12 H.C.
+copper wire, would have a resistance of 0.36 of an ohm, and an
+inductance of 0.02 of a henry.
+
+An important matter in connection with an induction coil to be used
+for wireless telegraphy is the resistance of the secondary circuit.
+The purpose for which we employ the coil is to charge a condenser of
+some kind. If a constant electromotive force (V) is applied to the
+terminals of a condenser having a capacity C, then the difference of
+potential (_v_) of the terminals of the condenser at any time that the
+contact is made is given by the expression:
+
+    v = V(1 - e^{-t/RC}).
+
+In the above equation, the letter e stands for the number 2.71828, the
+base of the Napierian logarithms, and R is the resistance in series
+with the condenser, of which the capacity is C, to which the
+electromotive force is applied. This equation can easily be deduced
+from first principles,[9] and it shows that the potential difference
+_v_ of the terminals of the condenser does not instantly attain a
+value equal to the impressed electromotive force V, but rises up
+gradually. Thus, for instance, suppose that a condenser of one
+microfarad is being charged through a resistance of one megohm by an
+impressed voltage of 100 volts, the equation shows that at the end of
+the first second after contact, the terminal potential difference of
+the condenser will be only 63 volts, at the end of the second second,
+86 volts, and so on.
+
+Since _e_^{-10} is an exceedingly small number, it follows that in 10
+seconds the condenser would be practically charged with a voltage
+equal to 100 volts. The product CR in the above equation is called the
+_time-constant_ of the condenser, and we may say that the condenser is
+practically charged after an interval of time equal to ten times the
+time-constant, counting from the moment of first contact between the
+condenser and the source of constant voltage. The time-constant is to
+be reckoned as the product of the capacity (C) in microfarads, by the
+resistance of the charging circuit (R) in megohms. To take another
+illustration. Supposing we are charging a condenser having a capacity
+of one-hundreth of a microfarad, through a resistance of ten thousand
+ohms. Since ten thousand ohms is equal to one-hundredth of a megohm,
+the time-constant would be equal to one-ten-thousandth of a second,
+and ten times this time-constant would be equal to a thousandth of a
+second. Hence, in order to charge the above capacity through the above
+resistance, it is necessary that the contact between the source of
+voltage and the condenser should be maintained for at least
+one-thousandth part of a second.
+
+In discussing the methods of interrupting the circuit, we shall return
+to this matter, but, meanwhile, it may be said that in order to secure
+a small time-constant for the charging circuit, it is desirable that
+the secondary circuit of the induction coil should have as low a
+resistance as possible. This, of course, involves winding the
+secondary circuit with a rather thick wire. If, however, we employ a
+wire larger in size than No. 34, or at the most No. 32, the bulk and
+the cost of the induction coil began to rise very rapidly. Hence, as
+in all other departments of electrical construction, the details of
+the design are more or less a matter of compromise. Generally
+speaking, however, it may be said that the larger the capacity which
+is to be charged, the lower should be the resistance of the secondary
+circuit of the induction coil.
+
+In the practical construction of induction coils for wireless
+telegraphy, manufacturers have departed from the stock designs. We are
+all familiar with the appearance of the instrument maker's induction
+coil; its polished mahogany base, its lacquered brass fittings, and
+its secondary bobbin constructed of and covered with ebonite. But such
+a coil, although it may look very pretty on the lecture table, is yet
+very unsuited to positions in which it may be used in connection with
+Hertzian wave telegraphy.
+
+Three important adjuncts of the induction coil are the primary
+condenser, the interrupter and the primary key. The interrupter is the
+arrangement for intermitting the primary current. We have in some way
+or other to rapidly interrupt the primary current, and the torrent of
+sparks that then appears between the secondary terminals of the coil
+is due to the electromotive force set up in the secondary circuit at
+each break or interruption of the primary circuit. We may divide
+interrupters into five classes.
+
+We have first the well-known hammer interrupter which Continental
+writers generally attribute to Neef or Wagner.[10] In this
+interrupter, the magnetisation of the iron core of the coil is caused
+to attract a soft-iron block fixed at the top of a brass spring, and
+by so doing to interrupt the primary circuit between two platinum
+contacts. Mr. Apps, of London, added an arrangement for pressing back
+the spring against the back contact, and the form of hammer that is
+now generally employed is therefore called an Apps break.
+
+As the ten-inch coil takes a primary current of ten amperes at sixteen
+volts when in operation, it requires very substantial platinum
+contacts to withstand the interruption of this current continuously
+without damage. The small platinum contacts that are generally put on
+these coils by instrument makers are very soon worn out in practical
+wireless telegraph work. If a hammer break is used at all, it is
+essential to make the contacts of very stout pieces of platinum, and
+from time to time, as they get burnt away or roughened, they must be
+smoothed up with a fine file. It does not require much skill to keep
+the hammer contacts in good order and prevent them from sticking
+together and becoming damaged by the break spark.
+
+By regulating the pressure of the spring against the back contact, by
+means of an adjusting screw, the rate at which the break vibrates can
+be regulated, but as a rule it is not possible, with a hammer break,
+to obtain more than about 800 interruptions per minute, or, say,
+twelve a second. The hammer break is usually operated by the magnetism
+of the iron core of the coil, but for some reasons it is better to
+separate the break from the coil altogether, and to work it by an
+independent electromagnet, which, however, may be excited by a current
+from the same battery supplying the induction coil. For coils up to
+the ten-inch size the hammer break can be used when very rapid
+interruptions are not required. It is not in general practicable to
+work coils larger than the ten-inch size with a platinum contact
+hammer break, as such a butt contact becomes overheated and sticks if
+more than ten amperes is passed through it. In the case of larger
+coils, we have to employ some form of interrupter in which mercury or
+a conducting liquid forms one of the contact surfaces.
+
+The next class of interrupter is the vibrating or hand-worked mercury
+break, in which a platinum or steel pin is made to vibrate in and out
+of mercury. This movement may be effected by the attraction of an iron
+armature by an electromagnet, or by the varying magnetism of the core
+of the coil, or it may be effected more slowly by hand.
+
+The mercury surface must be covered with water, alcohol, paraffin or
+creosote oil to prevent oxidation and to extinguish the break spark.
+The interruption of the primary current obtained by the mercury break
+is more sudden than that obtained by the platinum contact in air, in
+consequence of the more rapid extinction of the spark; hence the
+sparks obtained from coils fitted with mercury interrupters are
+generally from twenty to thirty per cent. longer than those obtained
+from the same coil under the same conditions, with platinum contact
+interrupters. The mercury breaks will not, however, work well unless
+cleaned at regular intervals by emptying off the oil and rinsing well
+with clean water, and hence they require rather more attention than
+platinum interrupters. It is not generally possible to obtain so many
+interruptions per minute with the simple vibrating mercury interrupter
+as with the ordinary hammer interrupter. The mercury interrupter has,
+however, the advantage that the contact time during which the circuit
+is kept closed may be made longer than is the case with the hammer
+break. Also, if fresh water is allowed to flow continuously over the
+mercury surface, it can be kept clean, and the break will then operate
+for considerable periods of time without attention. The mercury
+interrupter may be worked by a separate electromagnet or by the
+magnetism of the core of the induction coil.
+
+The third class of interrupter may be called the motor interrupter, of
+which a large number have been invented in recent years. In this
+interrupter some form of a continuously-rotating electromotor is
+employed to make and break a mercury or other liquid contact. In one
+simple form the motor shaft carries an eccentric, which simply dips a
+platinum point into mercury, or else a platinum horseshoe into two
+mercury surfaces, making in this manner an interruption of the primary
+circuit at one or two places. As a small motor can easily be run at
+twelve hundred revolutions per minute, or twenty per second, it is
+possible to secure easily in this manner a uniform rate of
+interruption of the primary current at the rate of about twenty per
+second. If, however, much higher speeds are employed, then the time of
+contact becomes abbreviated, and the ability of the coil to charge a
+capacity is diminished.
+
+Professor J. Trowbridge has described an effective form of motor break
+for large coils, in which the interruption is caused by withdrawing a
+stout platinum wire from a dilute solution of sulphuric acid, and by
+this means he increased the spark given by a coil provided with hammer
+break and condenser from fifteen inches to thirty inches when using
+the liquid break and no condenser.[11]
+
+A good form of motor-interrupter, due to Dr. Mackenzie Davidson,
+consists of a slate disc bearing pin contacts fixed on the prolonged
+steel axle of a motor placed in an inclined position; the disc and the
+lower part of the axle lie in a vessel filled one-third with mercury
+and two-thirds with paraffin oil. The circuit is made and broken by
+the revolution of the disc causing the pins to enter and leave the
+mercury. The speed of the motor can be regulated by a small
+resistance, and can be adapted to the electromotive force used in the
+primary circuit. When the motor is running slowly the interrupter can
+be used with a low electromotive force, that is to say, something
+between twelve and twenty volts, but with a higher speed a large
+electromotive force can be used without danger of overheating the
+primary coil, and with an electromotive force of about fifty volts,
+the interruptions may be so rapid that an unbroken arc of flame,
+resembling an alternating-current arc, springs between the secondary
+terminals of the coil.
+
+Mr. Tesla has devised numerous forms of rotating mercury break. In
+one, a star-shaped metal disc revolves in a box so that its points dip
+into mercury covered with oil, and make and break contact. In another
+form, a jet of mercury plays against a similar shaped rotating wheel.
+For details, the reader must consult the fuller descriptions in _The
+Electrical World_ of New York, Vol. XXXII., p. 111, 1898; also Vol.
+XXXIII., p. 247; or _Science Abstracts_, Vol. II., pp. 46 and 47,
+1898.
+
+The fourth class of interrupter is called a turbine interrupter. In
+this appliance, a jet of mercury is forced out of a small aperture by
+means of a centrifugal pump, and is made to squirt against a metal
+plate, and interrupted intermittently by a toothed wheel made of
+insulating material rotated by the motor which drives the pump. The
+current supplying the coil passes through or along this jet of
+mercury, and is therefore rendered intermittent when the wheel
+revolves. In the case of this interrupter, the duration of the
+contacts, as well as the number of interruptions per second, is under
+control, and for this reason better results are probably obtained with
+it than with any other form of break.
+
+A description of a turbine mercury break devised by M. Max Levy was
+given in the _Elektrotechnische Zeitschrift_, Vol. XX., p. 717,
+October 12, 1899 (see also _Science Abstracts_, Vol. III., p. 63,
+abstract No. 165) as follows:--
+
+A toothed wheel made of insulating material carries from 6 to 24
+teeth, and can be made to rotate from 300 to 1,000 times per minute,
+the interruptions being thus regulated between 5 and 400 per second.
+By raising or lowering the position of the jet of mercury and that of
+the plate against which it strikes, the duration of the contact can
+be varied, so that it is possible to regulate this period without
+disturbing the number of interruptions per second.
+
+The sparks obtained from a coil worked with a turbine interrupter have
+more quantity than the sparks obtained with any other interrupter
+under similar conditions, and the coil can be worked with a far higher
+voltage than is possible when using the hammer break. In this manner,
+the appearance of the secondary sparks can be varied from the thin
+snappy sparks given by the hammer break to the thick flame-like arc
+sparks given by the electrolytic break. This break can be adapted for
+any voltage from twelve to two hundred and fifty volts, and the
+primary circuit cannot be closed before the interrupter is acting. The
+mercury in the break is generally covered with alcohol or paraffin oil
+to reduce oxidation, and the appliance is nearly noiseless when in
+operation. The mercury has to be cleaned at intervals, if the
+interrupter is much used. If alcohol is used to cover the mercury, the
+cleaning is very simple; the break requires only to be rinsed under a
+water tap. When paraffin oil is used, the cleaning is generally
+effected with the help of a few ounces of sulphuric acid in a very few
+minutes. It is best, however, to clean the mercury continuously by
+allowing the water to flow over it.
+
+The motor driving the centrifugal pump and the fan can be wound for
+any voltage, and it is best to have it so arranged that this motor
+works on the same battery which supplies the primary circuit of the
+coil, the two circuits working parallel together. A rheostat can be
+added to the motor circuit to regulate the speed.
+
+The turbine break driven by an independent motor, which is kept always
+running, has another advantage over the hammer break in practical
+wireless telegraphy, viz., that a useful secondary spark can be
+secured with a shorter time of closure of the primary circuit, since
+there is no inertia to overcome as in the case of the hammer break.
+This latter form has only continued in use because of its simplicity
+and ease of management by ordinary operators.
+
+The mercury turbine interrupter has been extensively adopted both in
+the German and British navies in connection with induction coils used
+for wireless telegraphy.
+
+Lastly we have the electrolytic interrupters, the first of which was
+introduced by Dr. Wehnelt, of Charlottenburg, in the year 1899, and
+modified by subsequent inventors. In its original form, a glass vessel
+filled with dilute sulphuric acid (one of acid to five or else ten
+parts of water) contains two electrodes of very different sizes; one
+is a large lead electrode formed of a piece of sheet lead laid round
+the interior of the vessel, and the other is a short piece of platinum
+wire projecting from the end of a glass or porcelain tube. The smaller
+of these electrodes is made the positive, and the large one the
+negative. If this electrolytic cell is connected in series with the
+primary circuit of the induction coil (the condenser being cut out)
+and supplied with an electromotive force from forty to eighty volts,
+an electrolytic action takes place which interrupts the current
+periodically.[12] An enormous number of interruptions can, by suitable
+adjustment, be produced per second, and the appearance of a discharge
+from the secondary terminals of the coil, while using the Wehnelt
+break, more resembles an alternate-current arc than the usual
+disruptive spark.
+
+At the time when the Wehnelt break was first introduced, great
+interest was excited in it, and the technical journals in 1899 were
+full of discussions as to the theory of its operation.[13] The general
+facts concerning the Wehnelt break are that the electrolyte must be
+dilute sulphuric acid in the proportion of one of acid to five or ten
+of water. The large lead plate must be the cathode or negative pole,
+and the anode or positive pole must be a platinum wire, about a
+millimetre in diameter, and projecting one or two millimetres from the
+pointed end of a porcelain, glass or other acid-proof insulating tube.
+The aperture through which the platinum wire works must be so tight
+that acid cannot enter, yet it is desirable that the platinum wire
+should be capable of being projected more or less from the aperture by
+means of an adjusting screw. The glass vessel which contains these two
+electrodes should be of considerable size, holding, say, a quart of
+fluid, and it is better to include this vessel in a larger one in
+which water can be placed to cool the electrolyte, as the latter gets
+very warm when the break is used continuously. If such an electrolytic
+cell has a continuous electromotive force applied to it tending to
+force a current through the electrolyte from the platinum wire to the
+lead plate, we can distinguish three stages in its operation, which
+are determined by the electromotive force and the inductance in the
+circuit. First, if the electromotive force is below sixteen or twenty
+volts, then ordinary and silent electrolysis of the liquid proceeds,
+bubbles of oxygen being liberated from the platinum wire and hydrogen
+set free against the lead plate. If the electromotive force is raised
+above twenty-five volts, then if there is no inductance in the
+circuit, the continuous flow of current proceeds, but if the circuit
+of the electrolyte possesses a certain minimum inductance, the
+character of the current flow changes, and it becomes intermittent,
+and the cell acts as an interrupter, the current being interrupted
+from 100 to 2,000 times per second, according to the electromotive
+force and the inductance of the circuit. Under these conditions, the
+cell produces a rattling noise and a luminous glow appears round the
+tip of the platinum wire. Thus, in a particular case, with an
+inductance of 0.004 millihenry in the circuit of a Wehnelt break, no
+interruption of the circuit took place, but with one millihenry of
+inductance in the circuit, and with an electromotive force of 48
+volts, the current became intermittent at the rate of 930 per second,
+and by increasing the voltage to 120 volts, the intermittency rose to
+1,850 a second.
+
+The Wehnelt break acts best as an interrupter with an electromotive
+force from 40 to 80 volts. At higher voltages a third stage sets in:
+the luminous glow round the platinum wire disappears, and it becomes
+surrounded with a layer of vapour, as observed by MM. Violle and
+Chassagny; the interruptions of current cease, and the platinum wire
+becomes red hot. If there is no inductance in the circuit, the
+interrupter stage never sets in at all, but the first stage passes
+directly into the third stage. In the first stage bubbles of oxygen
+rise steadily from the platinum wire, and in the interrupted stage
+they rise at longer intervals, but regularly. The cell will not,
+however, act as a break at all unless some inductance exists in the
+circuit.
+
+In applying the Wehnelt break to an induction coil, the condenser is
+discarded and also the ordinary hammer break, and the Wehnelt break is
+placed in circuit with the primary coil. In some cases, the inductance
+of the primary coil alone is sufficient to start the break in
+operation, but with voltages above 50 or 60, it is generally necessary
+to supplement the inductance of the primary coil by another inductive
+coil. The best form of Wehnelt break for operating induction coils is
+the one with multiple anodes (see Dr. Marchant, _The Electrician_,
+Vol. XLII., p. 841, 1899), and when it has to be used for long
+periods, the cathode may advantageously be formed of a spiral of lead
+pipe, through which cold water is made to circulate.
+
+Another form of electrolytic break was introduced by Mr. Caldwell. In
+this, a vessel containing dilute sulphuric acid is divided into two
+parts. In the partition is a small hole, and in the two compartments
+are electrodes of sheet lead. The small hole causes an intermittency
+in the current which converts the arrangement into a break. Mr.
+Campbell Swinton modified the above arrangement by making the
+partition to consist of a sort of porcelain test-tube with a hole in
+the bottom. This hole can be more or less plugged up by a glass rod
+drawn out to a point, and this is used to more or less close the hole.
+This porcelain vessel contains dilute acid and stands in a larger
+vessel of acid, and lead electrodes are placed in both compartments.
+The current and intermittency can be regulated by more or less closing
+the aperture between the two regions.
+
+When the Wehnelt break is applied to an ordinary ten-inch induction
+coil, and the inductance of the primary circuit and the electromotive
+force varied until the break interrupts the current regularly and with
+the frequency of some hundred a second, the character of the secondary
+discharge is entirely different from its appearance with the ordinary
+hammer break. The thin blue lightning-like sparks are then replaced by a
+thicker mobile flaming discharge, which resembles an alternating-current
+arc, and, when carefully examined or photographed, is found to consist
+of a number of separate discharges superimposed upon one another in
+slightly different positions.
+
+Many theories have been adopted as to the action of the break, but
+time will not permit us to examine these. Professor S. P. Thompson and
+Dr. Marchant have suggested a theory of resonance.[14] One difficulty
+in explaining the action of the break is created by the fact that it
+will not work if the platinum wire is made a cathode.
+
+Although the Wehnelt break has some advantages in connection with the
+use of the induction coil for Roentgen ray work, its utility as far as
+regards Hertzian wave telegraphy is not by any means so marked. It has
+already been explained that, in order to charge a condenser of a given
+capacity at a constant voltage, the electromotive force must be
+applied for a certain minimum time, which is determined by the value
+of the capacity and the resistance of the secondary circuit of the
+induction coil. If the coil is a ten-inch coil and has a secondary
+resistance of, say, 6,000 ohms, and if the capacity to be charged has
+a value, say, of one-thirtieth of a microfarad, then the time-constant
+of the circuit is 1/5,000 of a second. Therefore, the contact with the
+condenser must be maintained for at least 1/500 of a second, during
+the time that the secondary electromotive force of the coil is at its
+maximum, so that the condenser may become charged to a voltage which
+the coil is then capable of producing.
+
+In the induction coil, the electromotive force generated in the
+secondary coil at the "break" of the primary current is higher than
+that at the "make," and this electromotive force, other things being
+equal, depends upon the rate at which the magnetism of the iron core
+dies away, and its duration is shorter in proportion as the whole time
+occupied in the disappearance of the magnetism is less. The Wehnelt
+break does not increase the actual secondary electromotive force, nor
+apparently its duration, but it greatly increases the number of times
+per second this electromotive force makes it appearance. Hence this
+break increases the current, but not the electromotive force in the
+secondary coil. It, therefore, does not assist us in the direction
+required--viz., in prolonging the duration of the secondary
+electromotive force to enable larger capacities to be charged.
+
+The important point in connection with the working of a coil used for
+charging a condenser is not the length of spark which the coil can
+give alone, but the length of spark which can be obtained between
+small balls attached to the secondary terminals, when these terminals
+are also connected to the two surfaces of the condenser. Thus, a coil
+may give a ten-inch spark if worked alone, but on a capacity of
+one-thirtieth of a microfarad it may not be able to give more than a
+five-millimetre spark. Hence, in describing the value of a coil for
+wireless telegraph purposes, it is not the least use to state the
+length of spark which the coil will give between the pointed
+conductors in air, but we must know the spark length which it will
+give between brass balls, say, 1 centimetre in diameter, connected to
+the secondary terminals, when these terminals are also short-circuited
+by a stated capacity, the spark not exceeding that length at which it
+becomes non-oscillatory.
+
+A good way of describing the value of an induction coil for wireless
+telegraph purposes is to state the length of oscillatory spark which
+can be produced between balls one centimetre in diameter connected to
+the secondary terminals, when these balls are short-circuited by a
+condenser having a capacity, say, of one-hundredth of a microfarad,
+and also one-tenth of a microfarad.
+
+If a hammer or motor interrupter is employed with the coil, then a
+primary condenser must be connected across the points between which
+the primary circuit is broken. This condenser generally consists of
+sheets of tinfoil alternated with sheets of paraffin paper, and for a
+ten-inch coil may have a capacity of about 0.4 or 0.5 of a
+microfarad.[15]
+
+Lord Rayleigh discovered that if the interruption of the primary
+circuit is sufficiently sudden and complete, as when the primary
+circuit is severed by a bullet from a gun, the primary condenser can
+be removed and yet the sparks obtained from the secondary circuit are
+actually longer than those obtained with the condenser and the
+ordinary break.[16]
+
+In the use, however, of the coil for Hertzian wave telegraphy, with
+all interrupters except the Wehnelt break a condenser of suitable
+capacity must be joined across the break points.
+
+Turning in the next place to the primary key, or signalling
+interrupter, it is necessary to be able to control the torrent of
+sparks between the secondary terminals of the coil, and to cut them up
+into long and short periods in accordance with the letters of the
+Morse alphabet. This is done by means of the primary key. The primary
+key generally consists of an ordinary massive single contact key with
+heavy platinum contacts. As the current to be interrupted amounts to
+about ten amperes and is flowing in a highly inductive circuit, the
+spark at break is considerable. If the attempt is made to extinguish
+this spark by making the contacts move rapidly away from one another
+through a long distance, in other words, by using a key with a wide
+movement, then the speed at which the signals can be set is greatly
+diminished. The speed of sending greatly depends upon the time taken
+to move the key up and down between sending two dots, and hence a
+short range key sends quicker than a long range key. If it is desired
+to use a short range key, then some method must be employed to
+extinguish the spark at the contacts. This is done in one of three
+ways: Either by using a high resistance coil to short-circuit these
+contacts, or by a condenser, or by a magnetic blow-out, as in the case
+of an electric tramcar circuit controller. Of these, the magnetic
+blow-out is probably the best.
+
+Mr. Marconi has designed a signalling key which performs the function
+not only of interrupting the primary circuit, but at the same time
+breaks connection between the receiving appliance and the aerial.
+
+The author has designed for signalling purposes a multiple contact key
+which interrupts the circuit simultaneously in ten or twelve different
+places. The particular point about this break is the means which are
+taken to make the twelve interruptions absolutely simultaneous. If
+these interruptions are not simultaneous, the spark always takes place
+at the contact which is broken first, but if the circuit is
+interrupted in a dozen places quite simultaneously, then the spark is
+cut up into a dozen different portions, and the spark at each contact
+is very much diminished. By this break, voltages up to two thousand
+volts may be quite easily dealt with.
+
+Various forms of break have been devised in which the circuit is
+broken under oil or insulating fluids, but, generally speaking, these
+devices are not very portable, and a dry contact between platinum
+surfaces with appropriate means for cutting up the spark and blowing
+it out so that the mechanical movement of the switch may be small is
+the best thing to use.
+
+The signalling key is really a very important part of the transmitting
+arrangement, because whatever may be the improvements in receiving
+instruments, it is not possible to receive faster than we can send. A
+great many statements have appeared in the daily papers as to the
+possibility of receiving hundreds of words a minute by Hertzian wave
+telegraphy, but the fact remains that whatever may be the sensibility
+of the receiving appliance, the rate at which telegraphy of any kind
+can be conducted is essentially dependent upon the rate at which the
+signals can be sent, and this in turn is largely dependent upon the
+mechanical movement which the key has to make to interrupt the primary
+circuit, and so interrupt the secondary discharge.
+
+In order to make the separation of the contact points of the switch as
+small as possible, and yet prevent an arc being established, various
+blow-out devices have been employed. The simplest arrangement for this
+purpose is a powerful permanent magnet so placed that its inter-polar
+field embraces the contact points and is at right angles to them.
+
+As already explained, the applicability of the induction coil in
+wireless telegraphy is limited by the fact of the high resistance of
+the secondary circuit and the small current that can be supplied from
+it. Data are yet wanting to show what is the precise efficiency of the
+induction coil, as used in Hertzian wave telegraphy, but there are
+reasons for believing that it does not exceed 50 or 60 per cent.
+
+Where large condensers have to be charged--in other words, where we
+have to deal with larger powers--we are obliged to discard the
+induction coil and to employ the alternating-current transformer. But
+this introduces us to a new class of difficulties. If an
+alternating-current transformer wound for a secondary voltage, say, of
+20,000 or 30,000 volts, has its primary circuit connected to an
+alternator, then if the secondary terminals, to which are connected
+two spark balls, are gradually brought within striking distance of one
+another, the moment we do this an alternating-current arc starts
+between these balls. If the transformer is a small one, there is no
+difficulty in extinguishing this arc by withdrawing the secondary
+terminals, but if the transformer is a large one, say, of ten or
+twenty kilowatts, dangerous effects are apt to ensue when such an
+experiment is tried. The short circuiting of the secondary circuit
+almost entirely annuls the inductance of the primary circuit. There
+is, therefore, a rush of current into the transformer, and if it is
+connected to an alternator of low armature resistance the fuses are
+generally blown and other damage done.
+
+Let us supppse, then, that the secondary terminals of the transformer
+are also connected to a condenser. On bringing together the spark
+balls connected with the secondary terminals we may have one or more
+oscillatory discharges, but the process will not be continuous,
+because the moment that the alternating-current arc starts between the
+spark balls it reduces their difference of potential to a
+comparatively low value, and hence the charge taken by the condenser
+is very small, and, moreover, the circuit is not interrupted
+periodically so as to re-start a train of oscillations.
+
+When, therefore, we desire to employ an alternating-current
+transformer as a source of electromotive force, although it may have
+the advantage that the resistance of the secondary circuit of the
+transformer is generally small compared with that of the secondary
+circuit of an induction coil, yet, nevertheless, we are confronted
+with two practical difficulties: (1) How to control the primary
+current flowing into the transformer, and (2) how to destroy the
+alternating-current arc between the spark balls and reduce the
+discharge entirely to the disruptive or oscillatory discharge of the
+condenser.
+
+The control over the current can be obtained, in accordance with a
+plan suggested by the author, by inserting in the primary circuit of
+the transformer two variable choking coils. The form in which it is
+preferred to construct these is that of a cylindrical bobbin standing
+upon a laminated cross-piece of iron. These bobbins can have let down
+into them an =E=-shaped piece of laminated iron, so as to complete the
+magnetic circuit, and thus raise the inductance of the bobbin. By
+placing two of these variable choking coils in series with the primary
+circuit, the current is under perfect control. We can fix a minimum
+value below which the current shall not fall, by adjusting the
+position of the cores of these two choking coils, and we can then
+cause that current to be increased up to a certain limit which it
+cannot exceed, by short-circuiting one of these choking coils by an
+appropriate switch. Several ways have been suggested for extinguishing
+the alternating current arc which forms between the spark balls
+connected to the secondary terminals when these are brought within a
+certain distance of one another. One of these is due to Mr. Tesla. He
+places a strong electromagnet so that its lines of magnetic flux pass
+transversely between the spark balls. When the discharge takes place
+the electric arc is blown out, but if the balls are short-circuited by
+a condenser the oscillatory discharge of the condenser still takes
+place across the spark gap. Professor Elihu Thomson achieves the same
+result by employing a blast of air thrown on the spark gap. This has
+the effect of destroying the alternating-current arc, but still leaves
+the oscillating discharge of the condenser. The action is somewhat
+tedious to explain in words, but it can easily be understood that the
+blast of air, by continually breaking down the alternating-current arc
+which tends to form, allows the condenser connected to the spark balls
+to become charged with the potential of the secondary circuit of the
+transformer, and that this condenser then discharges across the spark
+gap, producing an oscillatory discharge in the usual manner. The
+author has found that, without the use of any air blast or
+electromagnet, simple adjustment of the double choking coil in the
+primary circuit of the transformer, as above described, is sufficient
+to bring about the desired result, when the capacity of the condenser
+is adjusted to be in resonance.
+
+Another method, which has been adopted by M. d'Arsonval, is to cause
+the spark to pass between two balls placed at the extremities of
+metal rods, which are in rapid rotation like the spokes of a wheel. In
+this case, the draught of air produced by the passage of the spark
+balls blows out the arc and performs the same function as the blast of
+air in Professor Elihu Thomson's method. When these adjustments are
+properly made, it is possible, by means of a condenser and an
+alternating-current transformer supplied with current from an
+alternator, to create a rapidly intermittent oscillatory discharge,
+the sparks of which succeed one another so quickly that it appears
+almost continuous. When using a large transformer and condenser, the
+noise and brilliancy of these sparks are almost unbearable, and the
+eyes may be injured by looking at this spark for more than a moment.
+In the construction of transformers intended to be used in this
+manner, very special precautions have to be taken in the insulation of
+the primary and secondary circuits, and the insulation of these from
+the core.
+
+It may be remarked in passing that experimenting with large
+high-tension transformers coupled to condensers of large capacity is
+exceedingly dangerous work, and the greatest precautions are necessary
+to avoid accident. In the light, however, of sufficient experience
+there is no difficulty in employing high-tension transformers in the
+above-described manner, and in obtaining electromotive forces of
+upwards of a hundred thousand volts supplied through transformers
+capable of yielding any required amount of current.
+
+On occasions where continuous current alone is available, a motor
+generator has to be employed converting the continuous current into an
+alternating current. This is best achieved by the employment of a
+small alternator directly coupled to a continuous-current motor; or by
+providing the shaft of a continuous-current motor with two rings
+connected to two opposite portions of its armature, so that when
+continuous current is supplied to the brushes pressing against the
+commutator, an alternating current can be drawn off from two other
+brushes touching the above-mentioned insulated rings.
+
+The next element of importance in the transmitting arrangement is the
+spark gap. In the case of those transmitters employing an ordinary
+induction coil, the secondary spark, or the discharge of any condenser
+connected to the secondary terminals can be taken between the brass
+balls about half an inch or one inch in diameter, with which the
+terminals of the secondary coil are usually furnished; and it is
+generally the custom to allow this spark discharge to take place in
+air at ordinary pressure. In the very early days of his work Mr.
+Marconi adopted the discharger devised by Professor Rhigi, in which
+the spark takes place between two brass balls placed in vaseline or
+other highly insulating oil.[17] But whatever advantage may accrue
+from using oil as the dielectric in which the spark discharge takes
+place, when carrying out simple laboratory experiments on Hertzian
+waves, there is no advantage in the case of wireless telegraphy. The
+Rhigi discharger was, therefore, soon discarded. If discharges having
+large quantity are passed through oil, it is rapidly decomposed or
+charred, and ceases to retain the special insulating and
+self-restoring character which is necessary in the medium in which an
+oscillating spark is formed. The conditions when the discharges of
+large condensers are passed between spark balls are entirely different
+from those when the quantity of the spark, or to put it in more exact
+language, the current passing, is very small. In the case of Hertzian
+experiments it is necessary, as shown by Hertz, to maintain a high
+state of polish on the spark balls when they are employed for the
+production of short waves of small energy, but when we are dealing
+with large quantities of energy at each discharge, those methods which
+succeed for laboratory experiments are perfectly impracticable. The
+conditions necessary to be fulfilled by a discharger for use in
+Hertzian wave telegraphy are that the surfaces shall maintain a
+constant condition and not be fused or eaten away by the spark, and,
+next, that the medium in which the discharge takes place shall not be
+decomposed by the passage of the spark, but shall maintain the
+property of giving way suddenly when a certain critical pressure is
+reached, and passing instantly from a condition in which it is a very
+perfect insulator to one in which it is a very good conductor; and,
+thirdly, that on the cessation of the discharge, the medium shall
+immediately restore itself to its original condition.
+
+When using the ordinary ten-inch induction coil, and when the capacity
+charged by it does not exceed a small fraction of a microfarad, it is
+quite sufficient to employ brass or steel balls separated by a certain
+distance in air, at the ordinary pressure, as the arrangement of the
+discharger. When, however, we come to deal with the discharges of very
+large condensers, at high electromotive forces, then it is necessary
+to have special arrangements to prevent the destruction of the
+surfaces between which the spark passes, or their continual
+alteration, and many devices have been invented for this purpose. The
+author has devised an arrangement which fulfils the above conditions
+very perfectly for use in large power stations, but the details of
+this cannot be made public at the present time.
+
+       *       *       *       *       *
+
+We have to consider in connection with this part of the subject the
+dielectric strength of air under different pressures and for different
+thicknesses. It was shown by Lord Kelvin, in 1860, that the dielectric
+strength of very thin layers of air is greater than that of thick
+layers.[18] The electric force, reckoned in volts per centimetre,
+required to pierce a thickness of air from two to ten millimetres in
+thickness, at atmospheric pressure, may be taken at 30,000 volts per
+centimetre. The same force in electrostatic units is represented by
+the number 100, since a gradient of 300 volts per centimetre
+corresponds to a force of one electrostatic unit. It appears also that
+for air and other gases there is a certain minimum voltage
+(approximately 400 volts) below which no discharge takes place,
+however near the conducting surfaces may be approximated. In this
+particular practical application, however, we are only concerned with
+spark lengths which are measured in millimetres or centimetres, lying,
+say, between one or two millimetres and five or six centimetres. Over
+this range of spark length we shall not generally be wrong in
+reckoning the voltage required to produce a spark between metal balls
+in air at the ordinary pressure to be given by the rule:
+
+    _Disruptive voltage_ = 3,000 x _spark-gap length in millimetres_.
+
+If, however, the air pressure is increased above the normal by
+including the spark balls in a vessel in which air can be compressed,
+then the spark length, corresponding to a given potential difference,
+very rapidly decreases. Mr. F. J. Jervis-Smith[19] found that by
+increasing the air pressure from one atmosphere to two atmospheres
+round a pair of spark balls he reduced the spark length given by a
+certain voltage from 2.5 to 0.75 centimetre.
+
+Professor R. A. Fessenden has also made some interesting observations
+on the effect of using compressed air round spark gaps. He found that
+if a certain voltage between metal surfaces would yield a spark four
+inches in length, at the ordinary pressure of the air, if the spark
+balls were enclosed in a cylinder, the air round them compressed at
+50lb. per square inch, the spark length for the same potential
+difference of the balls was only one quarter of an inch, or
+one-sixteenth of its former value.
+
+The writer has also made experiments with an apparatus designed to
+study the effect of compressed air round the spark gap. The
+experimental arrangements are as follows: A ten-inch induction coil
+has one of its terminals connected to the internal coating of a
+battery of Leyden jars. The external coating is connected through the
+primary coil of an oscillation transformer with the other secondary
+terminal of the coil, and these secondary terminals are also connected
+to a spark gap consisting of two brass balls enclosed in a glass
+vessel into which air can be forced by a pump, the air pressure being
+measured by a gauge. The balls in the glass vessel are set at a
+distance of about three millimetres apart. The secondary circuit of
+the oscillation transformer is connected to another pair of spark
+balls, the distance of which can be varied.
+
+Suppose we begin with the air in the glass vessel containing the balls
+connected to the secondary terminals of the induction coil, which may
+be called the secondary balls, at atmospheric pressure, and create
+oscillatory discharges in the primary coil of the oscillation
+transformer, we have a spark between the balls, which may be called
+the tertiary balls, connected to the secondary terminals of the
+oscillation transformer. If the secondary balls are placed, say, three
+millimetres apart, the air in the glass vessel enclosing them being at
+the ordinary atmospheric pressure, then with one particular
+arrangement of jars used, a spark twenty-five or twenty-six
+millimetres long between the tertiary balls will take place. Suppose,
+then, we increase the pressure of the air round the secondary balls,
+pumping it by degrees to 10, 20, 30, 40 and 50lb. per square inch
+above the atmospheric pressure. We find that the spark between the
+tertiary balls will gradually leap a greater and greater distance, and
+when the pressure of the air is 50lb. per square inch, we can obtain a
+fifty-millimetre spark between the tertiary balls, whereas when the
+air in the glass vessel is at atmospheric pressure, we can only obtain
+a spark between the tertiary balls of half that length.
+
+This experiment demonstrates that the effect of compressing the air
+round the secondary terminals of the induction coil is to greatly
+increase the difference of potential between these balls before the
+spark passes. In fact, it requires about double the voltage to force a
+spark of the same length through air compressed at 50lb. on the square
+inch that it does to make a spark of identical length between the same
+balls in air at normal pressure. This shows that there is a very great
+advantage in taking the discharge spark in compressed air. A better
+effect can be produced by substituting dry gaseous hydrochloric acid
+for air at ordinary pressures.
+
+One other incidental advantage is that the noise of the spark is very
+much reduced. The continual crackle, of the discharge spark of the
+induction coil in connection with wireless telegraphy is very annoying
+to sensitive ears, but in this manner we can render it perfectly
+silent.
+
+Professor Fessenden also states that when the spark balls are
+surrounded by compressed air, and if one of the balls is connected
+with a radiator, the compression of the air, although it shortens the
+spark-gap corresponding to a given voltage, does not in any way
+increase the radiation. When, however, the air in the spark-ball
+vessel is compressed to 60lb. in the square inch, there is a marked
+increase in the effective radiation, and at 80lb. per square inch the
+energy emitted in the form of waves is nearly three and a-half times
+greater than at 50lb., the potential difference between the balls
+remaining the same.
+
+This effect is no doubt connected with the fact that the production of
+a wave, whether in ether or in any other material, is not so much
+dependent upon the absolute force applied as upon the suddenness of
+its application. To translate it into the language of the electronic
+theory, we may say that the electron radiates only whilst it is being
+accelerated, and that its radiating power, therefore, depends not so
+much upon its motion as upon the rate at which its motion is changing.
+
+The advantage in using compressed air round the spark gap is that we
+can increase the effective potential difference between the balls
+without rendering the spark non-oscillatory. In air of the ordinary
+pressure there is a certain well-defined limit of spark length for
+each voltage, beyond which the discharge becomes non-oscillatory, but
+by the employment of spark balls in compressed air, we can increase
+the potential difference between the balls corresponding to a given
+distance apart before a discharge takes place, or employ higher
+potentials with the same length of spark gap. In addition to this, we
+have, perhaps, the production of a more effective radiation, as
+asserted by Fessenden, when the air pressure exceeds a certain
+critical value.
+
+The next element which we have to consider in the transmitting
+arrangements is a condenser of some kind for storing the energy which
+is radiated at intervals. Where a condenser other than the aerial is
+employed for storing the electric energy which is to be radiated by
+the aerial, some form of it must be constructed which will withstand
+high potentials. As the dielectric for such a condenser, only two
+materials seem to be of any practical use, viz., glass and micanite.
+Glass condensers in the form of Leyden jars have been extensively
+employed, but they have the disadvantage that they are very bulky in
+proportion to their electrical capacity. The instrument maker's quart
+Leyden jar has a capacity of about one-five hundredth of a microfarad,
+but it occupies about 150 cubic inches or more. Professor Braun has
+employed in his transmitting arrangements condensers consisting of
+small glass tubes like test tubes, lined on the inside and outside
+with tinfoil, which are more economical in space. The author has found
+that condensers for this purpose are best made of sheet glass about
+one-eighth or one-tenth of an inch in thickness, coated to within one
+inch of their edge on both sides with tinfoil, and arranged in a
+vessel containing resin or linseed oil, like the plates of a storage
+battery. M. d'Arsonval has employed micanite, but although this
+material has a considerably higher dielectric strength than glass, it
+is much more expensive to obtain a given capacity by means of micanite
+than by glass, although the bulk of the condenser for a given capacity
+is less.
+
+To store up a certain amount of electric energy in a condenser, we
+require a certain definite volume of dielectric, no matter how we may
+arrange it, and the volume required per unit of energy is determined
+by the dielectric strength of the material. Thus, for instance,
+ordinary sheet glass cannot be safely employed with a greater electric
+force than is represented by 20,000 volts for one-tenth of an inch in
+thickness, or, say, a potential gradient of 160,000 volts per
+centimetre. This is equivalent to an electric force of about 500
+electrostatic units. This may be called the safe-working force. The
+electrostatic capacity of a condenser formed of two metal surfaces a
+foot square separated by glass three millimetres in thickness is
+between 1/360 and 1/400 of a microfarad. If this condenser is charged
+to 20,000 volts, we have stored up in it half a joule of electric
+energy, and the volume of the dielectric is 270 cubic centimetres.
+Hence, to store up in a glass condenser electric energy represented by
+one joule at a pressure of 20,000 volts, we require 500 cubic
+centimetres of glass, and it will be found that if we double the
+pressure and double the thickness of the glass, we still require the
+same volume.[20] Hence, in the construction of high-tension condensers
+to store up a given amount of energy, the economical problem is how to
+obtain the greatest energy-storing capacity for the least money.
+Glass fulfils this condition better than any other material. Although
+some materials may have very high dielectric strength, such as paper
+saturated with various oils, or resins, yet they cannot be used for
+the purpose of making condensers to yield oscillatory discharges,
+because the oscillations are damped out of existence too soon by the
+dielectric.
+
+In arranging condensers to attain a given capacity, regard has to be
+taken of the fact that for a given potential difference there must be
+a certain total thickness of dielectric, and that if condensers of
+equal size are being arranged in parallel it adds to their capacity,
+whilst joining them in series divides their capacity. If N equal
+condensers or Leyden jars have each a capacity represented by C, and
+if they are joined _n_ in series and _m_ in parallel, the joint
+capacity of the whole number is _m_C/_n_, where the product _mn_ = N.
+
+Passing on next to the consideration of oscillation transformers of
+various kinds--these are appliances of the nature of induction coils
+for transforming the current or electromotive force of electrical
+oscillations in a required ratio. These coils are, however, destitute
+of any iron core, and they generally consist of coils of wire wound on
+a fibre, wooden or ebonite frame, and must be immersed in a vat of oil
+to preserve the necessary insulation. No dry insulation of the nature
+of indiarubber or gutta-percha will withstand the high pressures that
+are brought to bear upon the circuits of an oscillation transformer.
+In constructing these transformers we have to set aside all previous
+notions gathered from the design of low-frequency iron-core
+transformers. The chief difficulty we have to contend against in the
+construction of an effective oscillation transformer is the inductance
+of the primary circuit and the magnetic leakage that takes place. In
+other words, the failure of the whole of the flux generated by the
+primary circuit to pass through or be linked with the secondary
+circuit. Mr. Marconi has employed an excellent form of oscillation
+transformer, in the design of which he was guided by a large amount of
+experience. In this transformer the two circuits are wound round a
+square wooden frame. The primary circuit consists of a number of
+strands of thick insulated cable laid on in parallel, so that it
+consists of only one turn of a stranded conductor. The secondary
+circuit consists of a number of turns, say, ten to twenty, of thinner
+insulated wire laid over the primary circuit and close to it, so that
+the transformer has the transformation ratio of one to ten or one to
+twenty. In the arrangements devised and patented by Mr. Marconi, these
+two circuits, with their respective capacities in series with them,
+are tuned to one another, so that the time-period of each circuit is
+exactly the same, and without this tuning the device becomes
+ineffective as a transformer.[21] There is no advantage in putting a
+number of turns on the primary circuit, because such multiplication
+simply increases the inductance, and, therefore, diminishes the
+primary current in the same ratio which it multiplies the turns, and
+hence the magnetic field due to the primary circuit remains the same.
+Where it is desired to put a number of turns upon a coil, and yet at
+the same time keep the inductance down, the writer has adopted the
+device of winding a silk or hemp rope well paraffined between the
+turns of the circuit, so as to keep them further apart from one
+another, and as the inductance depends on the turns per centimetre,
+this has the effect of reducing the inductance.
+
+The next and most important element in any transmitting station is the
+aerial or radiator, and it was the introduction of this element by Mr.
+Marconi which laid the foundation for Hertzian wave telegraphy as
+opposed to mere experiments with the Hertzian waves. We may consider
+the different varieties of aerial which have been evolved from the
+fundamental idea. The simple single Marconi aerial consists of a bare
+or insulated wire, generally about 100ft. or 150ft. in length,
+suspended from a sprit attached to a tall mast. As these masts have
+generally to be erected in exposed positions, considerable care has to
+be taken in erecting them with a large margin of strength. To the end
+of a sprit is attached an insulator of some kind, which may be a
+simple ebonite rod, or sometimes a more elaborate arrangement of oil
+insulators, and to the lower end of this insulator is attached the
+aerial wire. As at the top of the aerial we have to deal with
+potentials capable sometimes of giving sparks several feet in length,
+the insulation of the upper end of the aerial is an important matter.
+
+In the original Marconi system, the lower end of the aerial was simply
+attached to one spark ball connected to one terminal of the induction
+coil, and the other terminal and spark ball were connected to the
+earth. In this arrangement, the aerial acted not only as radiator, but
+as energy-storing capacity, and as already explained, its radiating
+power was on that account limited. The earth connection is an
+important matter. For long distance work, a good earth is essential.
+This earth must be made by embedding a metal plate in the soil, and
+many persons are under the impression that the efficiency of the earth
+plate depends upon its area, but this is not the fact. It depends much
+more upon its shape, and principally upon the amount of its "edge." It
+has been shown by Professor A. Tanakadate, of Japan, that if a metal
+plate of negligible resistance is embedded in an infinite medium
+having a resistivity _r_, the electrical conductance of this plate is
+equal to 4pi/_r_ times the electrostatic capacity of the same plate
+placed in a dielectric of infinite extent. Hence in designing an
+earth plate, we have to consider not how to give it the utmost amount
+of surface, but how to give it the greatest electrostatic capacity,
+and for this purpose it is far better to divide a given amount of
+metal into long strips radiating out in different directions, rather
+than to employ it in the form of one big square or circular plate. The
+importance of the "good earth" will have been seen from our discussion
+on the mode of formation of electric waves. There must be a perfectly
+free access for the electrons to pass into and out of the aerial.
+Hence, if the soil is dry, or badly conductive in the neighbourhood,
+we have to go down to a level at which we get a good moist earth. In
+fact, the precautions which have to be taken in making a good earth
+for Hertzian wave telegraphy are exactly those which should be taken
+in making a good earth for a lightning conductor.
+
+Whilst on the subject of aerials, a word may be said on the
+localisation of wireless telegraph stations on the Marconi system. For
+reasons which were explained previously, the transmission of signals
+is effected more easily over water than over dry land, and it is
+hindered if the soil in the neighbourhood of the sending station is a
+poor conductor. Hence, all active Hertzian wave telegraph stations,
+like all active volcanoes, are generally found near the sea. In those
+cases in which a multiple aerial has to be put up consisting of many
+wires, one mast may be insufficient to support the structure, and
+several masts arranged in the form of a square or a circle have to be
+employed. The illustrated papers have reproduced numerous pictures of
+the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova
+Scotia, and Cape Cod in the United States. In these stations, after
+preliminary failures to obtain the necessary structural strength with
+ordinary masts, tall lattice girder wooden towers have been built,
+about 215 feet in height, well stayed against wind pressure, and which
+so far have proved themselves capable of withstanding any storm of
+wind which has come against them.
+
+An important question in connection with the sending power of an
+aerial is that of the relation of its height to the distance covered.
+Some time ago Mr. Marconi enunciated a law, as the result of his
+experiments, connecting these two quantities, which may be called
+Marconi's Law. He stated that the height of the aerial to cover a
+given distance, other things remaining the same, varies as the square
+root of the distance. Let D be the distance and let L be the length of
+the aerial, then if both the transmitting and receiving aerial are the
+same height, we may say that D varies as L^{2}. This relation may be
+theoretically deduced as follows:--Any given receiving apparatus for
+Hertzian wave telegraphy requires a certain minimum energy to be
+imparted to it to make it yield a signal. If the resistance and the
+capacity of the receiver is taken as constant, this minimum working
+energy is proportional to the square of the electromotive force set up
+in the receiving aerial by the impact on it of the electric waves.
+This electromotive force varies as the length of the receiving aerial
+and as the magnetic force due to the wave cutting across it, and the
+magnetic force varies as the current in the transmitting aerial, and
+therefore, for any given voltage varies as the capacity, and therefore
+as the length of the transmitting aerial. If, therefore, the
+transmitting and receiving aerial have the same length, the minimum
+energy varies as the square of the electromotive force in the
+receiving aerial, and therefore as the fourth power of the length of
+either aerial, since the electromotive force varies as the product of
+the lengths of the aerials. Hence, when the distance between the
+aerials is constant, the minimum working energy varies as the fourth
+power of the height of either aerial, but when the lengths of the
+aerials are constant, the energy caught up by the receiving aerial
+must vary inversely as the square of the distance D between the
+aerials. Hence, if we call _e_ this minimum working energy, _e_ must
+vary as 1/D^{2} when L is constant, or as L^4 when D is constant, and
+since _e_ is a constant quantity for any given arrangements of
+receiver and transmitter, it follows that when the height of aerial
+and distance vary, the ratio L^4/D^2 is constant, or, in other words,
+D^2 varies as L^4 or D varies as L^2--_i.e._, distance varies as the
+square of the height of the aerial, which is Marconi's Law. The curve,
+therefore, connecting height of aerial with sending distance for given
+arrangements is a portion of a parabola.
+
+Otherwise, the law may be stated in the form L = _a_[\sq]{D}, where _a_
+is a numerical coefficient. If L and D are both measured in metres,
+then, for recent Marconi apparatus as used on ships, _a_ = 0.15 roughly.
+(See a report on experiments made for the Italian Navy, 1900-1901, by
+Captain Quintino Bonomo--"Telegrafia senza fili," Rome, 1902.)
+
+This law, however, must not be used without discretion. After Mr.
+Marconi had transmitted signals across the British Channel, some
+people, forgetting that a little knowledge is a dangerous thing,
+predicted that aerials a thousand feet in height would be required to
+signal across the Atlantic, but Mr. Marconi has made such improvements
+of late years in the receiving arrangements that he has been able to
+receive signals over three thousand miles in 1903 with aerials only
+thirty-three per cent. longer than those which, in 1899, he employed
+to cover twenty miles across the English Channel.
+
+[Illustration: FIG. 15.--ALTERNATING-CURRENT DOUBLE-TRANSFORMATION
+POWER PLANT FOR GENERATING ELECTRIC WAVES (Fleming). _a_, alternator;
+H_{1}H_{2}, choking coil; K, signalling key; T, step-up transformer;
+S_{1}S_{2} spark-gap; C_{1}C_{2} condensers; T_{1}T_{2}, oscillation
+transformers; A, aerial; E, earthplate.]
+
+We turn, in the next place, to the consideration of those devices for
+putting more power into the aerial than can be achieved when the
+aerial itself is simply employed as the reservoir of energy. Professor
+Braun, of Strassburg, in 1899, described a method for doing this by
+inducing oscillations in the aerial by means of an oscillation
+transformer, these oscillations being set up by the discharges from a
+Leyden jar or battery of Leyden jars, which formed the reservoir of
+energy. The induction coil is employed to produce a rapidly
+intermittent series of electrical oscillations in the primary coil of
+an oscillation transformer by the discharge through it of a Leyden
+jar. Mr. Marconi immensely improved this arrangement, as described by
+him in a lecture given before the Society of Arts on May 17, 1901, by
+syntonising the two circuits and making the circuit, consisting of the
+capacity of the aerial and the inductance of the secondary circuit of
+the oscillation transformer, have the same time-period as the circuit
+consisting of the Leyden jars, or energy-storing condenser, and the
+primary circuit of the oscillation transformer, and by so doing
+immensely added to the power and range of the apparatus.
+
+Starting from these inventions of Braun and Marconi, the author
+devised a double transmission system in which the oscillations are
+twice transformed before being generated in the aerial, each time with
+a multiplication of electromotive force and a multiplication of the
+number of groups of oscillations per second. This arrangement can best
+be understood from the diagram (see Fig. 15).
+
+In this case a transformer, T, or transformers receive alternating
+low-frequency current from an alternator, _a_, being regulated by
+passing through two variable choking coils, H_{1} and H_{2}, so as to
+control it. This alternating current is transformed up from a
+potential of two thousand to twenty, forty or a hundred thousand, and
+is employed to charge a large condenser, C_{1}, which discharges
+across a primary spark-gap, S_{1}, through the primary coil of an
+oscillation transformer, T_{1}. The secondary circuit of the
+oscillation transformer is connected to a second pair of spark balls,
+S_{2}, which in turn are connected by a secondary condenser, C_{2},
+and the primary circuit of a third transformer, T_{2} and the
+secondary circuit of this last transformer are inserted between a
+Marconi aerial, A, and the earth E. When all these circuits are tuned
+to resonance by Mr. Marconi's methods, we have an enormously powerful
+arrangement for creating electric waves, or rather trains of electric
+waves, sent out from the aerial, and the oscillations are controlled
+and the signals made by short-circuiting one of the choking coils.
+
+Another transmitting arrangement, which involves a slightly different
+principle, and employs no oscillation transformer, is one due also to
+Professor Braun. In this case, a condenser and inductance are connected
+in series to the spark balls of an induction coil, and oscillations are
+set up in this circuit. Accordingly, there are rapid fluctuations of
+potential at one terminal of the condenser. If to this we connect a long
+aerial, the length of which has been adjusted to be one quarter of the
+length of wave corresponding to the frequency, in other words, to make
+it a quarter-wave resonator, then powerful oscillations will be
+accumulated in this rod. The relation between the height (H) of the
+aerial and the frequency is given by the equation 3 x 10^{10} = 4_n_H,
+where _n_ is the frequency of the oscillations and H the height of the
+aerial in centimetres. The frequency of the oscillations is determined
+by the capacity (C) and inductance (L) of the condenser circuit, and can
+be calculated from the formula
+
+    n = (5,000,000) / ([\sq]{C (in mfds.) x L (in cms.)}).
+
+That is, the frequency is obtained by dividing into the number
+5,000,000, the square root of the product of the capacity in
+microfarads, and inductance in centimetres, of the condenser circuit.
+It will be found, on applying these rules, that it is impossible to
+unite together any aerial of a length obtainable in practice with a
+condenser circuit of more than a very moderate capacity. It has been
+shown that for an aerial two hundred feet in height the corresponding
+resonating frequency is about one and a quarter million.[22] As we are
+limited in the amount to which we can reduce the inductance of a
+discharge circuit, probably to something like a thousand centimetres,
+a simple calculation shows that the largest capacity we can employ is
+about a sixtieth of a microfarad. This capacity, even if charged at
+60,000 volts, would only contain thirty joules of energy, or about
+22.5 foot-pounds, which is a small storage compared to that which can
+be achieved when we are employing the above-described methods, which
+involve the use of an oscillation transformer. In such a case,
+however, it is an advantage to employ a spark-gap in compressed air,
+because we can then raise the voltage to a much higher value than in
+air of ordinary pressure without lengthening the spark so much as to
+render it non-oscillatory.
+
+When employing methods involving the use of an oscillation
+transformer, it is possible to use multiple aerials having large
+capacity, and hence to store up a very large amount of energy in the
+aerial, which is liberated at each discharge. The most effective
+arrangement is one in which the radiator draws off gradually a large
+supply of energy from a non-radiating circuit, and so sends out a true
+train of waves, and not mere impulses, into the ether, and as we shall
+see later on, it is only when the radiation takes place in the form of
+true wave trains that anything like syntony can be obtained.
+
+There are a number of variants of the above methods of arranging the
+radiator and associated energy-storing in circuit. Descriptions of
+these arrangements will be found in patents by Mr. Marconi, Professor
+Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor
+Popoff, Professor Fessenden and others. In all cases, however, they
+are variations of the three simple forms of radiator already
+described.
+
+Returning to the analogy with the air or steam siren suggested at the
+commencement of this article, the reader will see in the light of the
+explanations already given, that all parts of the air-wave producing
+apparatus have their analogues in the electrical radiator as used in
+Hertzian wave telegraphy. The object in the one case is to produce
+rapid oscillations of air particles in a tube, which result in the
+production of an air wave in external space; in the other case, the
+arrangement serves to produce oscillations of electrons or electrical
+particles in a wire, the movements of which create a disturbance in
+the ether called an electrical wave. Comparing together, item by item,
+it will be seen, therefore, that the induction coil or transformer
+used in connection with electric-wave apparatus is analogous to the
+air pump in the siren plant. In the electrical apparatus, this
+electron pump is employed to put an electrical charge into a
+condenser; in the air wave apparatus, the air pump is employed to
+charge an air vessel with high pressure air. From the electrical
+condenser the charge is released in the form of a series of electrical
+oscillations, and in the air wave producing appliance, the compressed
+air is released in the form of a series of intermittent puffs or
+blasts. In the electrical wave producing apparatus, these electrical
+oscillations in the condenser circuit are finally made to produce
+other oscillations in an air wire or open circuit, just as the puffs
+of air finally expend themselves in producing aerial oscillations in
+the siren tube. Finally, in the one case we have a series of air waves
+and in the other case, a series of electrical waves. These trains of
+electric waves or air waves, as the case may be, are intermitted into
+long and short groups, in accordance with the signals of the Morse
+alphabet, and, therefore, the Hertzian wave transmitter, in whatever
+form it may be employed, when operated by means of a Marconi aerial,
+is in fact an electrical siren apparatus, the function of which is to
+create periodic disturbances in the universal ether of the same
+character as those which the siren produces in atmospheric air.
+
+       *       *       *       *       *
+
+We have to consider in the next place the arrangements of the
+receiving station and the various forms of receivers that have been
+devised for effecting telegraphy by Hertzian waves. Just as the
+transmitting station consists essentially of two parts, viz., a part
+for creating electrical oscillations and a part for throwing out or
+radiating electric waves, so the receiving-station appliances may be
+divided into two portions; the function of one being to catch up a
+portion of the energy of the passing wave, and that of the other to
+make a record or intelligible signal in some manner in the form of an
+audible or visible sign.
+
+Accordingly, there must be at the receiving station an arrangement
+called a receiving aerial, which in general takes the form of a long
+vertical wire or wires, similar in form to the transmitting aerial,
+There is, however, a distinct difference in the function of the
+transmitting aerial and the receiving aerial. The function of the
+first is effective radiation, and for this purpose the aerial must
+have associated with it a store of energy to be released as wave
+energy; but the function of the receiving aerial is to be the seat of
+an electromotive force which is created by the electric force and the
+magnetic force of the incident electric wave.
+
+In tracing out the mode of operation of the transmitting aerial, it
+was pointed out that the electric waves emitted consisted of
+alternations of electric force in a direction which is perpendicular
+to the surface of the earth, and magnetic force parallel to the
+surface of the earth. These two quantities, the electric force and the
+magnetic force, are called the _wave vectors_, and they both lie in a
+plane perpendicular to the direction in which the wave is travelling
+and at right angles to one another, the electric force being
+perpendicular to the surface of the earth. In optical language, the
+wave sent out by the aerial would be called a plane polarised wave,
+the plane of polarisation being parallel to the magnetic force. Hence,
+if at any point in the path of the wave we erect a vertical conductor,
+as the wave passes over it, it is cut transversely by the magnetic
+force of the wave and longitudinally by the electric force. Both of
+these operations result in the creation of an alternating
+electromotive force in the receiving aerial wire.
+
+As in all other cases of oscillatory motion, the principle of
+resonance may here be brought into play to increase immensely the
+amplitude of the current oscillations thereby set up in the receiving
+aerial. As already explained, any vertical insulated wire placed with
+its lower end near the earth has capacity with respect to the earth,
+and it has also inductance, the value of these factors depending on
+its shape and height. Accordingly, it has a natural electrical
+time-period of its own, and if the periodic electromotive impulses
+which are set up in it by the passage of the waves over it agree in
+period with its own natural time-period, then the amplitude of the
+current vibrations in it may become enormously greater than when there
+is a disagreement between these two periods. Before concluding these
+articles we shall return to this subject of electric resonance and
+syntony, and discuss it with reference to what is called the tuning of
+Hertzian wave stations. Meanwhile, it may be said that for the sake of
+obtaining, at any rate in an approximate degree, this coincidence of
+time-period, it is generally usual to make the receiving aerial as far
+as possible identical with the transmitting aerial. If the receiving
+aerial is not insulated, but is connected to the earth at its lower
+end through the primary coil of an oscillation transformer, we can
+still set up in it electrical oscillations by the impact on it of an
+electric wave of proper period; and if the oscillation transformer is
+properly constructed we can draw from its secondary circuit electric
+oscillations in a similar period.
+
+One problem in connection with the design of a receiving aerial is
+that of increasing its effective length and capacity so as to increase
+correspondingly the electromotive force or current oscillations in it.
+It is clear that if we put a number of receiving wires in parallel so
+that each one of them is operated upon by the wave separately,
+although we can increase in this way the magnitude of the alternating
+current which can be drawn off from the aerial, we cannot increase the
+electromotive force in it except by increasing the actual height of
+the wires. Unfortunately, there is a limit to the height of the
+receiving aerial. It has to be suspended, like the transmitting
+aerial, from a mast or tower, and the engineering problem of
+constructing such a permanent supporting structure higher than, say,
+two hundred feet is a difficult one.
+
+Since any one station has to send as well as receive, it is usual to
+make one and the same aerial wire or wires do double duty. It is
+switched over from the transmitting to the receiving apparatus, as
+required. This, however, is a concession to convenience and cost. In
+some respects it would be better to have two separate aerials at each
+station, the one of the best form for sending, and the other of the
+best form for receiving.
+
+In Mr. Marconi's early arrangements, the so-called coherer or
+sensitive wave-detecting appliance, to be described more in detail
+presently, was inserted between the base of the insulated receiving
+aerial and the earth, but it was subsequently found by him to be a
+great improvement to act upon the receiving device, not directly by
+the electromotive force set up in the aerial, but by the induced
+electromotive force of a special form of step-up oscillation
+transformer he calls a "jigger," the primary circuit of which was
+inserted in between the receiving aerial and the earth plate, and the
+secondary circuit was connected to the sensitive organ of the
+telegraphic receiving arrangements.[23] A suggestion to employ
+transformed oscillations in affecting a coherer, had also been
+described in a patent specification by Sir Oliver Lodge, in 1897, but
+the essence of success in the use of this device is not merely the
+employment of a transformer, but of a transformer constructed
+specially to transform electrical oscillations.
+
+Turning, then, to the consideration of the relation existing between
+the transmitting and receiving aerials, we note that in their simplest
+form these consist of two similar tall rods of metal placed upright,
+with their feet in good connection with the earth at two places. We
+may think of them as two identical lightning conductors, well earthed
+at the bottom, and supported by non-conducting masts or towers. These
+rods must be in good connection with the earth, and therefore with it
+form, as it were, one conductor. If, as usual, these aerials are
+separated by the sea, the intermediate portion of this circuit is an
+electrolyte. The operations which take place when a signal is sent are
+as follows:--
+
+At the transmitting station, we set up in the transmitting aerial
+electric oscillations, of which the frequency may be of the order of a
+million, _i.e._, the oscillations as long as they last are at the rate
+of a million a second. Each spark discharge at the transmitter
+results, however, only in the production of a train of a dozen or two
+oscillations, and these trains succeed each other at a rate depending
+upon the transmitting arrangements used. Each oscillation in the
+transmitting aerial is accompanied by the detachment from it of
+semi-loops of electric strain, as already explained. The alterations
+of electric strain directed perpendicularly to the earth, and of the
+associated magnetic force parallel to the earth, constitute an
+electric wave in the ether, just as the alternations of pressure and
+motion of air molecules constitute an air wave. Associated with these
+physical actions above ground, there is a propagation through the
+earth of electric action, which may consist in a motion or atomic
+exchange of electrons. Each change or movement of a semi-loop of
+electric strain above ground has its equivalent below ground in
+inter-atomic exchanges or movements of the electrons, on which the
+ends of these semi-loops of electric strain terminate. The earth must
+play, therefore, a very important part in so-called "wireless
+telegraphy," and we might also say the earth does as much as the ether
+in its production.
+
+The function of the receiving aerial is to bring about a union between
+these two operations above and below ground. When the electric waves
+fall upon it, they give rise to electromotive force in the receiving
+aerial, and, therefore, produce oscillations in it which, in fact,
+are electric currents flowing into and out of the receiving aerial. We
+may say that the transmitting aerial, the receiving aerial and the
+earth form one gigantic Hertz oscillator. In one part of this system,
+electric oscillations of a certain period are set up by the discharge
+of a condenser and are propagated to the other part. In the earth,
+there is a propagation of electric oscillations; in the space above
+and between the aerials, there is a propagation of electric waves. The
+receiving aerial _feels_, therefore, what is happening at the distant
+aerial and can be made to record it.[24]
+
+We have next to consider the question of the wave-detecting devices
+which enable us to appreciate and record the impact of a wave or wave
+train against the aerial. At the very outset it will be necessary to
+coin a new word to apply generally to these appliances. Most readers
+are probably familiar with the term "coherer," which was applied by
+Sir Oliver Lodge, in the first instance, to an electric wave-detecting
+device of one particular kind--viz., that in which a metal point was
+lightly pressed against another metal surface and caused to stick to
+it when an electric wave fell upon it. As our knowledge increased, it
+was found that there were many cases in which the effect of the
+electric radiation was to cause a severance and not a coherence, and
+hence such clumsy phrases as "anticoherer" and "self-decohering
+coherer" have come into use. Moreover, we have now many kinds of
+electric wave detectors based on quite different physical principles.
+At the risk of incurring reprobation for adding to scientific
+nomenclature, the author ventures to think that the time has arrived
+when a simple and inclusive term will be found useful to describe all
+the devices, whatever their nature, which are employed for detecting
+the presence of an electric wave. For this purpose the term
+_kumascope_, from the Greek [Greek: kuma] (a wave), is suggested. The
+scientific study of waves has already been called _kumatology_, and in
+view of our familiarity with such terms as _microscope_,
+_electroscope_ and _hygroscope_, there does not seem to be any
+objection to enlarging our vocabulary by calling a wave-detecting
+appliance a _kumascope_. We are then able to look at the subject
+broadly and to classify kumascopes of different kinds.
+
+We may, in the first place, arrange them according to the principle on
+which they act. Thus, we may have electric, magnetic, thermal,
+chemical and physiological operations involved; and finally, we may
+divide them into those which are self-restoring, in the sense that
+after the passage or action of a wave upon them they return to their
+original sensitive condition; and those which are non-restoring, in
+that they must be subjected to some treatment to bring them back again
+to a condition in which they are fit to respond again to the action of
+a wave.
+
+We have no space to refer to the whole of the steps of discovery which
+led up to the invention of all the various forms of the modern
+electric kumascope or wave detector. Suffice it to say that the
+researches of Hertz in 1887 threw a flood of light upon many
+previously obscure phenomena, and enabled us to see that an electric
+spark, and especially an oscillatory spark, creates a disturbance in
+the ether, which has a resemblance in Nature to the expanding ripples
+produced by a stone hurled into water. Scientific investigation then
+returned with fresh interest to previously incomprehensible effects,
+and a new meaning was read into many old observations. Again and again
+it had been noticed that loose metallic contacts, loose aggregations
+of metallic filings or fragments, had a mysterious way of altering
+their conductivity under the action of electric sparks, lightning
+discharges and high electromotive forces.
+
+As far back as 1852, Mr. Varley had noticed that masses of powdered
+metals had a very small conductivity, which increased in a remarkable
+way during thunderstorms;[25] and in 1866, C. and S. A. Varley
+patented a device for protecting telegraphic instruments from
+lightning, which consisted of a small box of powdered carbon in which
+were buried two nearly touching metal points, and they stated that
+"powdered conducting matter offers a great resistance to a current of
+moderate tension, but offers but little resistance to currents of high
+tension."[26]
+
+We then pass over a long interval and find that the next published
+account of similar observations was due to Professor T.
+Calzecchi-Onesti, who described in an Italian journal, _Il Nuovo
+Cimento_ (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and
+1885 his observations on the decrease in resistance of metal powders
+when the spark from an induction coil was sent through them.[27] These
+observations did not attract much attention until Professor E. Branly,
+of Paris, in 1890 and 1891, repeated them on an extended scale and
+with great variations, making the important observation that an
+electric spark _at a distance_ had a similar effect in increasing the
+conductivity of metallic powders.[28] Branly, however, noticed that in
+some cases of conductors in powder, such as the peroxide of lead or
+antimony, the effect of the spark was to cause a decrease of
+conductivity.
+
+To Professor E. Branly unquestionably belongs the honour of giving to
+science a new weapon in the shape of a tube containing metallic
+filings or powder rather loosely packed between metal plugs, and of
+showing that when the pressure on the powder was adjusted such a tube
+may be a conductor of very high resistance, but that the electrical
+conductivity is enormously increased if an electric spark is made in
+its neighbourhood. He also proved that the same effect occurred in the
+case of two slightly oxidised steel or copper wires laid across one
+another with light pressure, and that this loose or imperfect contact
+was extraordinarily sensitive to an electric spark, dropping in
+resistance from thousands of ohms to a few ohms when a spark was made
+many yards away.
+
+It is curious to notice how long some important researches take to
+become generally known. Branly's work did not attract much attention
+in England until 1892, when Dr. Dawson Turner described his own
+repetition of Branly's experiments with the metallic filings tube at a
+meeting of the British Association in Edinburgh. In the discussion
+which followed, Professor George Forbes made an important remark. He
+asked whether it was possible that the decrease in resistance could be
+brought about by Hertz waves.[29]
+
+This question shows that even in 1892 the idea that the effect of the
+spark on the Branly tube was really due to Hertzian waves was only
+just beginning to arise. The following year, however, Mr. W. B. Croft
+repeated Branly's experiment with copper filings before the Physical
+Society of London, and entitled his short Paper "Electric Radiation on
+Copper Filings."[30] He exhibited a tube containing copper filings
+loosely held between two copper plugs and joined in series with a
+galvanometer and cell. The effect of an electric spark at a distance,
+in causing increase of conductivity, was shown, and the return of the
+tube to its non-conducting state when tapped was also noticed.
+
+In the discussion which followed the reading of this Paper, Professor
+Minchin described the effects of electric radiation on his impulsion
+cells. He followed up this by reading a Paper to the Physical Society
+on November 24, 1893, on the action of Hertzian radiation on films
+containing metallic powders, and expressed the opinion that the change
+in resistance of the Branly tube was due to electric radiation.[31]
+
+Thus, at the end of 1893, a few physicists clearly recognised that a
+new means had been given to us for detecting those invisible ether
+waves, the chief properties of which Hertz had unravelled with
+surpassing skill six years before, by means of a detector consisting
+of a ring of wire having a small spark-gap in it.
+
+In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal
+Institution, entitled "The Work of Hertz," and at this lecture use was
+made of the Branly tube as a Hertz wave detector. The chief object of
+the lecture was to describe the properties of Hertzian waves and their
+reflection, absorption and transmission, and many brilliant
+quasi-optical experiments were exhibited. Although a Branly tube, or
+imperfect metallic contact, then named by him a _coherer_, was
+employed by Sir Oliver Lodge to detect an electric wave generated in
+another room, there was no mention in this lecture of any use of the
+instrument for telegraphic purposes.[32]
+
+As we are here concerned only with the applications in telegraphy, we
+shall not spend any more time discussing the purely scientific work
+done with laboratory forms of this wave detector.
+
+Without attempting to touch the very delicate question as to the
+precise point at which laboratory research passed into technical
+application, we shall briefly describe the forms of kumascope which
+have been devised with special reference to Hertzian wave telegraphic
+work. A very exact classification is at present impossible, but we may
+say that telegraphic kumascopes may be roughly divided into six
+classes. The first class includes all those that depend for their
+action on the "coherer principle" or the reduction of the resistance
+of a metallic microphone by the action of electromotive force. As they
+depend upon an imperfect contact, they may be called _contact
+kumascopes_. This class is furthermore subdivided into the
+self-restoring and the non-self-restoring varieties. The second class
+comprises the _magnetic kumascopes_ which depend upon the action of an
+electrical oscillation as a magnetising or demagnetising agency. The
+third class comprises the _electrolytic responders_, in which the
+action of electric oscillations either promotes or destroys the
+results of electrolysis. The fourth class consists of the
+_electrothermal detectors_, in which the power of an electrical
+oscillation as a high frequency electric current to heat a conductor
+is utilised. The fifth class comprises the _electromagnetic_ or
+_electro-dynamic_ instruments, which are virtually very sensitive
+alternating-current ammeters, adapted for immensely high frequency.
+The sixth class must be made to contain all those which cannot be well
+fitted at present into any of the others, such as the sensitive
+responder of Schaefer, the action of which is not very clearly made
+out.
+
+We may proceed briefly to describe the construction of the principal
+forms of kumascope coming under the above headings. In the first
+place, let us consider those which are commonly called the "coherers"
+or, as the writer prefers to call them, the _contact kumascopes_. The
+simplest of these is the crossed needle or single contact, which
+originated with Professor E. Branly.[33] The pressure of the point of
+a steel needle against an aluminium plate was subsequently found by
+Sir Oliver Lodge to be a very sensitive arrangement when so adjusted
+that a single cell sends little or no current through the contact.[34]
+When an electric wave passes over it, good conducting contact ensues.
+The point is, in fact, welded to the plate, and can only be detached
+by giving the plate or needle a light shock or vibration. A variation
+of the above form is a pair of crossed needles, one resting on the
+other.
+
+Professor Branly found, in 1891, that if a pair of slightly-oxidised
+copper wires rest across one another the contact-resistance may fall
+from 8,000 to 7 ohms by the impact of an electric wave. He has
+recently devised a tripod arrangement, in which a light metal stool
+with three slightly-oxidised legs stands on a polished plate of steel.
+The contact points must be oxidised, but not too heavily, and the
+stool makes a bad electrical contact until a wave falls upon it.[35]
+The decoherence is effected by giving the stool a tilt by means of an
+electromagnet.
+
+These single or multiple-point kumascopes labour under the
+disadvantage that only a very small current can be passed through the
+variable contact when used as a relay arrangement, without welding
+them together so much that a considerable mechanical shock is required
+to break the contact and reset the trap.
+
+The logical development of the single contact is, therefore, the
+infinite number of contacts existing in the tube of metallic filings,
+which has been the form of kumascope most used for many years. In its
+typical form it consists of a tube of insulating material with
+metallic plugs at each end, and between them a mass of metallic
+powder, filings, borings, granules or small spheres, lightly touching
+one another. Imperfect contact must be arranged by light pressure, and
+in the majority of cases the resistance is very large until an
+electric wave falls upon the tube, when it drops suddenly to a small
+value and remains there until the tube is given a slight shake or the
+granules disturbed in any way, when the resistance suddenly rises
+again. This type of responder is a non-restoring kumascope, and
+requires the continual operation of some external agency to keep it in
+a condition in which it is receptive or sensitive to electric waves.
+
+Much discussion and considerable research have taken place in
+connection with the action and improvement of these metallic powder
+kumascopes. As regards materials, the magnetic metals, nickel, iron
+and cobalt, in the order named, appear to give the best results. The
+noble metals, gold, silver and platinum, are too sensitive, and the
+very oxidisable metals too insensitive, for telegraphic work, but an
+admixture may be advantageously made.
+
+Omitting the intermediate developments of invention, it may be said
+that Mr. Marconi was the first to recognise that to secure great
+sensibility in an electric wave detector of this type the following
+conditions must be fulfilled: An exceedingly small mass of metallic
+filings must be placed in a very narrow gap between two plugs, the
+whole being contained in a vessel which is wholly or partly exhausted
+of its air. Mr. Marconi devoted himself with great success to the
+development of this instrument, and in a very short time succeeded in
+transforming it from an uncertain laboratory appliance, capable of
+yielding results only in very skilled hands, into an instrument
+certain and simple in its operations as an ordinary telegraphic relay.
+He did this, partly by reducing its size, and partly by a most
+judicious selection of materials for its construction. As made at
+present, the Marconi metallic filings tube consists of a small glass
+tube, the interior diameter of which is not much more than one-eighth
+of an inch, which has in it two silver plugs which are bevelled off
+obliquely. These are placed opposite to each other, so as to form a
+wedge-shaped gap, about a millimetre in width at the bottom and two,
+or at most three, millimetres in width at the top (see Fig. 16). The
+silver plugs exactly fill the aperture of the tube, and are connected
+to platinum wires sealed through the glass. The tube has a lateral
+glass tube fused into it, by which the exhaustion is made, which is
+afterwards sealed off, and this tube projects on the side of the wider
+portion of the gap between the silver plugs. The sensitive material
+consists of a mixture of metallic filings, five per cent. silver and
+ninety-five per cent. nickel, being carefully mixed and sifted to a
+certain standard fineness. In the manufacture of these tubes, great
+care is taken to make them as far as possible absolutely identical.
+Each tube when finished is exhausted, but not to a very high vacuum.
+The tube so finished is attached to a bone holder, by which it can be
+held in a horizontal position. The object of bevelling off the plugs
+in the Marconi tube is to enable the sensitiveness of the tube to be
+varied by turning it round, so that the small quantity of filings lie
+in between a wider or narrower part of the gap.[36]
+
+[Illustration: FIG. 16.--MARCONI SENSITIVE TUBE OR METALLIC FILINGS
+KUMASCOPE. PP, silver plugs; TT, platinum wires; F, nickel and silver
+filings.]
+
+Other ways of adjusting the quantity of the filings to the width of
+the gap have been devised. Sometimes one of the plugs is made movable.
+In other cases, such as the tubes devised by M. Blondel and Sir Oliver
+Lodge, there is a pocket in the glass receptacle to hold square
+filings, from which more or less can be shaken into the gap.
+
+An interesting question, which we have not time to discuss in full, is
+the cause of the initial coherence of the metallic filings in a Branly
+tube. It does not seem to be a simple welding action due to heat, and
+it certainly takes place with a difference of potential, which is very
+far indeed below that which we know is required to produce a spark. On
+the other hand, it seems to be proved that in a Banly tube, when acted
+upon by electric waves, chains of metallic particles are produced. The
+effect is not peculiar to electric waves. It can be accomplished by
+the application of any high electromotive force. Thus Branly found
+that coherence may be produced by the application of an electromotive
+force of twenty or thirty volts, operating through a very high water
+resistance, and thus precluding the passage of any but an excessively
+small current. Again, the coherence seems to take place in some cases
+when metallic particles are immersed in a liquid, or even in a solid,
+insulator. Processor Branly has, therefore, preferred to speak of
+masses of metallic granules as _radio-conductors_, and Professor Bose
+has divided substances into positive and negative, according as the
+operation of electromotive force is to increase the coherence of the
+particles or to decrease it.
+
+It has been asserted that for every particular Branly tube, there is a
+critical electromotive force, in the neighbourhood of two or three
+volts which causes the tube to break down and pass instantly from a
+non-conductive to a conductive condition, and that this critical
+electromotive force may become a measure of the utility of the tube
+for telegraphic purposes. Thus, C. Kinsley (_Physical Review_, Vol.
+XII., p. 177, 1901) has made measurements of this supposed critical
+potential for different "coherers," and subsequently tested the same
+as receivers at a wireless telegraph station of the U.S.A. Signal
+Corps. The average of twenty-four experiments gave in one case 2.2
+volts as the breaking down potential of one of these coherers or
+Branly tubes, 3.8 volts for a second and 5.5 volts for the third.
+These same instruments, tested as telegraphic kumascopes, showed that
+the first of the three was most sensitive.
+
+On the other hand, W. H. Eccles (_Electrician_, Vol. XLVII., pp. 682
+and 715, 1901) has made experiments with Marconi nickel-silver
+sensitive tubes, using a liquid potentiometer made with copper
+sulphate, to apply the potential so that infinitesimal spark contacts
+might be avoided and the changes in potential made without any
+abruptness. He states that if the coherer tube is continuously tapped,
+say at the rate of fifty vibrations per second, whilst at the same
+time an increasing potential is applied to its terminals and the
+current passing through it measured on a galvanometer, there is no
+abrupt change in current at any point. He found that when the current
+and voltage were plotted against each other, a regular curve was
+obtained, which after a time becomes linear. A decided change occurs
+in the conductivity of the mass of metallic filings when treated in
+this manner at voltages lower than the critical voltage obtained by
+previous methods. He ascertained that there was a complete
+correspondence between the sensitiveness of the tubes used as
+telegraphic instruments and the form of the characteristic curve of
+current and voltage drawn by the above-described method.
+
+In the same manner, K. E. Guthe and A. Trowbridge (_Physical Review_,
+Vol. II., p. 22, 1900) investigated the action of a simple ball
+coherer formed of half a dozen steel, lead or phosphor-bronze balls in
+slight contact. They measured the current _i_ passing through the
+series under the action of a difference of potential _v_ between the
+ends, and found a relation which could be expressed in the form
+
+    v = V(1 - e^{ki}),
+
+where V and _k_ are constants.
+
+The current through this ball coherer is, therefore, a logarithmic
+function of the potential difference between its ends, of the form
+
+    i = log(v - V)
+
+and exhibits no discontinuity.
+
+The inference was drawn that the "resistance" is due to films of water
+adhering to the metallic particles through which electrolytic action
+occurs.
+
+A good metallic filings tube for use as a receiver in Hertzian wave
+telegraphy should exhibit a constancy of action and should cohere and
+decohere, to use the common terms, sharply, at the smallest possible
+tap. It should not have a current passed through it by the external
+cell of more than a fraction of a milliampere, or else it becomes
+wounded and unsensitive.
+
+The investigations which have already taken place seem to show pretty
+clearly that the agency causing the masses of filings to pass from a
+non-conductive to a conductive condition is electromotive force, and
+that, therefore, it is the electromotive force set up in the aerial by
+the incident waves which is the effective agent in causing the change in
+the metallic filings tube, when this is used as a telegraphic kumascope.
+This transformation of the tube from a non-conductor to a conductor is
+made to act as a circuit-closer, completing the circuit, by means of
+which a single cell of a local battery is made to send current through
+an ordinary telegraph relay, and so by the aid of a second battery
+operate a telegraphic printer or recorder of any kind. Hence it is clear
+that after one impact, the metallic filings tube has to be brought back
+to its non-conductive condition, and this may be achieved in several
+ways. (1) By the administration of carefully-regulated taps or shocks or
+by rotating the tube on its axis; (2) by the aid of an alternating
+current; (3) in those cases where filings of magnetic metals are
+employed, by magnetism.
+
+The decoherence by taps was discovered by Branly,[37] and Popoff,
+following the example of Sir Oliver Lodge, employed an electric bell
+arrangement for this purpose.[38]
+
+Mr. Marconi, in his original receiving instruments, placed an
+electromagnet under the coherer tube with a vibrating armature like an
+electric bell.[39] This armature carries a small hammer or tapper,
+which, when set in action, hits the tube on the under side, and
+various adjusting screws are arranged for regulating exactly the force
+and amplitude of the blows. This tapper is actuated by the same
+current as the Morse printer, or other telegraphic recorder, so that
+when the signal is received and the metallic filings tube passes into
+the conductive condition and closes the relay circuit, this latter in
+turn closes the circuit of the Morse printer or other recorder, and at
+the same time, a current passes through the electromagnet of the
+tapper and the tube is tapped back. This sequence of operations
+requires a certain time which limits the speed of receiving. The
+tapper has to be so arranged that it is possible to receive and to
+record not only the _dot_ but a _dash_ on the Morse system. The _dash_
+is really a series of closely adjacent dots, which run together in
+virtue of the inertia and inductance of the different parts of the
+whole receiving apparatus. The adjustment has so to be made that,
+whilst the _dash_ is being recorded and a continuous tapping is kept
+up, yet, nevertheless the continued electromotive force in the aerial,
+due to the continually arriving trains of waves, is able to act
+against the tapping and to keep the filings in the tube in the
+conductive condition. Hence, the successful operation of the
+arrangement requires attention to a number of adjustments, but these
+are not more difficult, or even as difficult, as those involved in the
+use of many telegraphic receivers employed in ordinary telegraphy with
+wires.
+
+Mr. Marconi also introduced devices for preventing the sparks at the
+contacts of the electromagnetic hammer from directly affecting the
+tube, and also to prevent the electric oscillations which are set up
+in the aerial from being partly shunted through other circuits than
+that of the sensitive tube. We pass on to notice the remaining devices
+for restoring the metallic filings tube to a condition of
+sensitiveness or receptiveness.
+
+A method for doing this by alternating currents is due to Mr. S. G.
+Brown.[40] The pole pieces of the coherer tube are made of iron, and
+they are enveloped in magnetising coils traversed by an alternating
+electric current. Between these pole pieces is placed a small quantity
+of nickel or iron filings, and under the action of the electromotive
+force, due to an electric wave acting on them, may be made to cohere
+in the usual fashion; but the moment that the wave ceases, the
+alternating magnetism of the electrodes causes the filings to drop
+apart or decohere. In place of the alternating current, Mr. Brown
+finds that a revolving permanent magnet can be used to produce the
+alternating magnetisation of the pole pieces of the sensitive tube or
+coherer.
+
+The third method of causing the decoherence of the filings is that due
+to T. Tommasina. He found that when a Branly tube is made with filings
+of a magnetic metal, such as iron, nickel and cobalt, the decoherence
+of the filings can be produced by means of an electromagnet placed in
+a suitable position under the tube.[41] The explanation of this fact
+seems to be that, when an electric wave falls upon the tube or when
+any other source of electromotive force acts upon it, chains of
+metallic particles are formed, stretching from one electrode to the
+other. Tommasina contends that he has proved the existence of these
+chains of particles by experiments made with iron filings; and R.
+Malagoli,[42] in referring to Tommasina's assertion, states that it
+can be witnessed in the case of brass filings placed between two
+plates of metal and immersed in vaseline oil, when a difference of
+potential is made between the plates.
+
+T. Sundorph[43] says he has confirmed Tommasina's discovery of the
+formation of these chains of metallic particles in the coherer. The
+filings do not all cling together, but certain chains are formed which
+afford a conducting path for the current subsequently passed through
+the coherer from an external source. Accordingly, Tommasina's method
+of causing decoherence in the case of filings of magnetic metals is to
+pull them apart by an external magnetic field; and he stated that
+decoherence can be effected more easily and regularly in this way than
+by tapping. Whilst on this point, it may be mentioned that C.
+Tissot[44] says that he has found that the sensitiveness of a coherer
+formed of nickel and iron filings can be increased by placing it in
+the magnetic field, the lines of which are parallel to the axis of
+the tube. According to MM. A. Blondel and G. Dobkevitch, this is
+merely the result of an increased coherence of the particles.
+
+       *       *       *       *       *
+
+Quite recently, Sir Oliver Lodge and Dr. Muirhead have employed as a
+self-restoring coherer or kumascope a steel disc revolved by
+clockwork, the edge of which just touches a globule of mercury covered
+with a thin film of paraffin oil. The contact is made between the
+mercury and the steel by the electric wave generating an electromotive
+force in the aerial, sufficient to break through the thin film of oil.
+When the wave stops, the circuit is again interrupted automatically.
+
+This device is used without a relay to actuate directly a syphon
+recorder as used in submarine telegraphy. The working battery employed
+with it must only have an electromotive force of about a tenth of a
+volt. It may be used also with a telephone in circuit and can,
+therefore, be employed either for telegraphic or telephonic
+reception.[45]
+
+[Illustration: FIG. 17.--ITALIAN NAVY SELF-RESTORING KUMASCOPE. C,
+carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery;
+T, telephone; S, adjusting screw.]
+
+One of the most sensitive of these self-restoring kumascopes is the
+carbon-steel-mercury coherer, the invention of which has been
+attributed to Castelli, a signalman in the Italian Navy,[46] but also
+stated on good authority to have been the invention of officers in the
+Royal Italian Navy, and has, therefore, been called the Italian Navy
+coherer.[47] This instrument has been arranged in several forms, but
+in the simplest of these it consists of a glass tube, having in it a
+plug of iron and a plug of arc-lamp carbon, or two plugs of iron with
+a plug of carbon between them. The plugs of iron, or of iron and
+carbon, are separated by an exceedingly small globule of mercury, the
+size of which should be between one and a-half and three millimetres.
+The plugs closing the tube must be capable of movement, one of them by
+means of a screw, as shown in the diagram (Fig. 17), taken from a
+patent specification communicated to Mr. Marconi by the Marchese Luigi
+Solari, of the Royal Italian Navy. One of the plugs of this tube is
+connected to the aerial and the other to the earth, and they are also
+connected through another circuit composed of a single dry cell and a
+telephone. The arrangement then forms an extremely sensitive detector
+of electric waves or of small electromotive forces, or, if a wave
+falls on the aerial, the electromotive force at once improves the
+contact between the mercury and the plugs, and therefore causes a
+sudden increase in the current through the telephone, giving rise to a
+sound; but when the wave ceases, or the electromotive force is
+withdrawn, the resistance falls back again to its origin value, and
+the arrangement is, therefore, self-acting, requiring no tapping or
+other device for restoring it to receptivity.
+
+A very ingenious form of combined telephone and coherer has been
+devised by T. Tommasina.[48] In this instrument the diaphragm of an
+ordinary Bell telephone carries upon it a very small carbon or
+metallic coherer. This coherer is connected in between the aerial and
+the earth, and is also in circuit with a battery and the electromagnet
+of a telegraphic relay. When this relay operates it closes the circuit
+of another battery which is placed in series with the telephone coil.
+The moment the current passes through the telephone coil it attracts,
+and therefore vibrates, the diaphragm and shakes up the metallic
+filings. If an observer, therefore, places the telephone to his ear,
+he hears a sound corresponding to every train of waves incident upon
+the aerial. With this arrangement, one can obtain two different kinds
+of results, according to the nature of the cohering powder placed in
+the cavity in the diaphragm. First, if the powder consists of a
+non-magnetic metal, gold, silver, platinum or the like, the receiver
+will be very sensitive; and at the same time the current passing
+through it when it is cohered will be sufficient to work a sensive
+recording apparatus in series with the telephone coil. Secondly, if
+the metallic powder placed in the cavity is a magnetic metal, the
+receiver will be somewhat less sensitive, but will work with more
+precision, because of the magnetic action of the magnet of the
+telephone upon the cohering powder. If no recording apparatus is used,
+the observer must write down the signals as heard in the telephone,
+since corresponding to a short spark at the transmitting station, a
+single tick or short sound is heard at the telephone, and
+corresponding to a series of rapidly successive sparks, a more
+prolonged sound is heard in the telephone. These two sounds, as
+already explained, constitute the dot and the dash of the Morse
+signals.
+
+We may, in the next place, refer to that form of kumascope in which
+the action of the wave or of electromotive force is not to decrease
+the resistance of a contact, but to increase that of an imperfect
+contact. As already mentioned, Professor Branly discovered long ago
+that peroxide of lead acts in an opposite manner to metallic filings,
+in that when placed in a Branly tube it increases its resistance under
+the action of an electric spark, instead of decreasing it. Again,
+Professor Bose has found that fragments of metallic potassium in
+kerosene oil behave in a similar manner, and that certain varieties of
+silver, antimony and of arsenic, and a few other metals, have a
+similar property. Branly tubes, therefore, made with these materials,
+or any arrangements which act in a similar manner, have been called
+"anti-coherers." The most interesting arragement which has been called
+by this name is that of Schaefer.[49] Schaefer's kumascope is made in
+the following manner: A very thin film of silver is deposited upon
+glass, and a strip of this silver is scratched across with a diamond,
+making a fine transverse cut or gap. If the resistance of this divided
+strip of silver is measured, it will be found not to be infinite, but
+may have a resistance as low as forty or fifty ohms if the strip is
+thirty millimetres wide. On examining the cut in the strip with a
+microscope, it will be found that the edges are ragged and that there
+are little particles of silver lying about in the gap. If, then, an
+electromotive force of three volts or more is put on the two separated
+parts of the strip, these little particles of silver fly to and fro
+like the pith balls in a familiar electrical experiment, and they
+convey electricity across from side to side. Hence a current passes
+having a magnitude of a few milliamperes. If, however, the strip is
+employed as a kumascope and connected at one end to the earth and at
+the other end to an aerial, when electric waves fall upon the aerial,
+the electrical oscillations thereby excited seem to have the property
+of stopping this dance of silver particles and the resistance of the
+gap is increased several times, but falls again when the wave ceases.
+If, therefore, a telephone and battery are connected between two
+portions of the strip, the variation of this battery current will
+affect the telephone in accordance with the waves which fall upon the
+aerial, and the arrangement becomes therefore a wave-detecting device.
+It is said to have been used in wireless telegraph experiments in
+Germany up to a distance of ninety-five kilometres.
+
+We must next direct attention to those wave-detecting devices which
+depend upon magnetisation of iron, and here we are able to record
+recent and most interesting developments. More than seventy years ago
+Joseph Henry, in the United States, noticed the effect of an electric
+spark at a distance upon magnetised needles.[50] Of recent times, the
+subject came back into notice through the researches of Professor E.
+Rutherford,[51] who carried out at Cambridge, England, in 1896, a
+valuable series of experiments on this subject. He found that if a
+magnetised steel needle or a very small bundle of extremely thin iron
+wires is magnetised and placed in the interior of a small coil, the
+ends of which are connected to two long collecting wires, then an
+electric wave started from a Hertz oscillator at a distance causes an
+immediate demagnetisation of the iron. This demagnetisation he
+detected by means of the movement of the needle of a magnetometer
+placed near one end of the iron wire. Although Rutherford's wave
+detector has been much used in scientific research, it was not, in the
+form in which he used it, a telegraphic instrument, and could not
+record alphabetic signals.
+
+Not long ago Mr. Marconi invented, however, a telegraphic instrument
+based upon his discovery that the magnetic hysteresis of iron can be
+annulled by electric oscillations. In one form, Mr. Marconi's magnetic
+receiver is constructed as follows[52] (see Fig. 18): An endless band
+of thin iron wire composed of several iron wires about No. 36 gauge,
+arranged in parallel, is made to move slowly round on two pulleys,
+like the driving belt of a machine. In one part of its path the wire
+passes through a glass tube, on which are found two coils of wire, one
+a rather short, thick coil, and the other a very fine, long one. The
+fine, long coil is connected with a telephone, and the shorter coil is
+connected at one end to the earth and the other to the aerial. Two
+permanent horse-shoe magnets are placed as shown n Fig. 18, with their
+similar poles together, and, as the iron band passes through their
+field, a certain length of it is magnetised, and owing to the
+hysteresis of the material, it retains this magnetism for a short time
+after it has passed out of the centre of the field. If then an
+electric oscillation, coming down from the aerial, is passed through
+the shorter coil, it changes the position of the magnetised portion of
+the iron and, so to speak, brings the magnetised portion of iron back
+into the position it would have occupied if the iron had had no
+hysteresis. This action, by varying the magnetic flux through the
+secondary coil, creates in it an electromotive force which causes a
+sound to be heard in the telephone connected to it. If at a distant
+place a single wave or train of waves is started and received by the
+aerial, this will express itself by making an audible tick in the
+telephone, and if several groups of closely adjacent wave trains are
+sent, these will indicate themselves by producing a rapid series of
+ticks in the telephone, heard as a short continuous noise and taken as
+equivalent to the Morse _dash_.
+
+[Illustration: FIG. 18.--MARCONI MAGNETIC RECEIVER. W_{1}W_{2}, wheels;
+I, iron wire band; P, primary coil; S, secondary coil; T, telephone;
+A, aerial; E, earthplate.]
+
+It was by means of this remarkably ingenious instrument that Mr.
+Marconi was able, in the summer of 1902, to detect the waves sent out
+from Poldhu on the coast of Cornwall, and receive messages as far as
+Cronstadt in the Baltic, in one direction, and as far as Spezzia in
+the Mediterranean in another direction, and also to receive messages
+across the Atlantic from the power stations situated in Glace Bay,
+Nova Scotia, and from one at Cape Cod in Massachusetts, U.S.A., in
+December, 1902.
+
+There can be no question that this magnetic detector of Mr. Marconi's,
+used in connection with a good telephone and an acute human ear, is
+the most sensitive device yet invented for the detection of electric
+waves and their utilisation in telegraphy without continuous wires. It
+is marvellously simple, ingenious and yet effective, as a Hertzian
+wave telegraphic receiver.
+
+Whilst on the subject of magnetic wave detectors, the author may
+describe experiments that he has been recently making to construct a
+Hertzian wave detector on the Rutherford principle, which shall be
+strictly quantitative. All the receivers of the coherer type and
+electrolytic type give no indications that are at all proportional to
+the energy of the incident wave. Their indications are more or less
+accidental and depend upon the manner in which the receiver was last
+left. There is a great need for a quantitative wave detector, the
+indications of which shall give us a measure of the energy of the
+arriving wave. It is only by the possession of such an instrument that
+we can hope to study properly the sending powers of various
+transmitters or the efficiency of different forms of aerial or devices
+by which the wave is produced. This magnetic receiver is constructed
+as follows:
+
+A coil of fine wire is constructed in sections like the secondary coil
+of an induction coil, and in the instrument already made, this coil
+contains thirty or forty thousand turns of wire. In the interior of
+this coil are placed a number of little bundles of fine iron wire
+wound round with two coils, a fine wire coil which is a magnetising
+coil, and a thicker wire coil which is a demagnetising coil. These
+sets of coils are joined up, respectively, in series or in parallel.
+Then, associated with this form of induction coil is a commutator of a
+peculiar kind, which performs the following functions when a battery
+is connected to it and when it is made to revolve by a motor or by
+clockwork. First, during part of the revolution, the commutator closes
+the battery circuit and magnetises the iron cores, and whilst this is
+taking place the secondary circuit of the induction coil is
+short-circuited and the galvanometer is disconnected from it.
+Secondly, the magnetising current is stopped, and soon after that the
+secondary coil is unshort-circuited and connected to the galvanometer,
+and remains in this condition during the remainder of the revolution.
+This cycle of operations is repeated at every revolution. If then an
+electrical oscillation is sent into the demagnetising coils, and if it
+continues longer than one revolution of the commutator, it will
+demagnetise the iron core during that period of time in which the
+battery is disconnected and the galvanometer connected. The
+demagnetisation of the iron which ensues produces an electromotive
+force in the secondary coil and causes a deflection of the
+galvanometer, and this deflection will continue and remain steady if
+the oscillation persists. Moreover, since this deflection is due to
+the passage through the galvanometer of a rapid series of discharges,
+it is large when the oscillations continue for a long time and are
+powerful, and small when they continue for a short time or are weak.
+We can, therefore, with this arrangement, receive on the galvanometer,
+just as on the mirror galvanometer used in submarine cable work, a dot
+or dash, and, moreover, the magnitude of these deflections is a
+measure of the energy of the wave.
+
+It is probable that when this arrangement is perfected it will become
+exceedingly useful for making all kinds of tests and measurements in
+connection with Hertzian telegraphy, even if it is not sensitive
+enough to use as a long distance receiver.
+
+Of late years a variety of wave-detecting devices have been brought
+forward which depend upon electrolysis. One of the best known of these
+is that by De Forest and Smythe.[53] In this arrangement, a tube
+contains two small electrodes like plugs, which may be made of tin,
+silver or nickel, or other metal. The ends of these plugs are flat and
+separated from each other by about one two-hundredth of an inch.
+Sometimes the end of one of these plugs is made cup shaped and the cup
+or recess is filled with a mass of peroxide of lead and glycerine. In
+the interval between the electrodes is placed an electrolyzable
+mixture, which consists of glycerine or vaseline mixed with water or
+alcohol, and a small quantity of litharge and metallic filings. These
+metallic filings act as secondary electrodes. When a small
+electromotive force is applied between the terminals of the electrodes
+of this tube through a very high resistance of twenty or thirty
+thousand ohms, an exceedingly small current passes through this
+mixture, and it causes an electrolytic action which results in the
+production of chains of metallic particles connecting the two
+electrodes together. If, in addition to this, one terminal or
+electrode of the arrangement is connected to an aerial wire and the
+other terminal to the earth, then on the arrival of an electric wave
+creating oscillations in the wire, these oscillations pass down into
+the electrolytic cell, where they break up the chains of metallic
+particles and thus interrupt the current passing through the telephone
+quite suddenly, which is heard as a slight tick by an ear applied to
+it. As soon as the wave ceases, the chain of metallic particles is
+re-established, so that the appliance is always in a condition to be
+affected by a wave. It is said that this breaking up and reformation
+of the chains of metallic particles is so rapid that a short spark
+made at the transmitting station is heard as a tick in the telephone,
+but a rapid succession of oscillatory sparks is heard as a short
+continuous sound; hence the two signals necessary for alphabetical
+conversation can be transmitted.
+
+Another receiver which has some resemblance to the above, although
+different in principle, is that of Neugschwender.[54] In this
+arrangement, which to a certain extent resembles the Schaefer detector,
+a glass plate has upon it a deposit of silver in the form of a strip,
+which is cut across at one place, thus interrupting it. If the cut is
+breathed upon or placed in a moist atmosphere, a little dew is
+deposited upon the glass, which bridges over the cut in the metal and
+creates an electric continuity. Hence a small current can be passed
+across the gap and through a telephone by one or two cells of a
+battery. If, however, an electric oscillation passes across the gap on
+its way from an aerial to the earth, then the continuity of the liquid
+film is destroyed, and the current is interrupted and a sound created
+in the telephone.
+
+The opinion has been expressed by Sir Oliver Lodge that in this case
+the interruption of the circuit which occurs is really due to the
+coalescence of minute water particles into larger drops, as when
+vapour is condensed into rain, and hence the continuity of the
+material is interrupted.
+
+We must then make a brief reference to other kumascopes which depend
+upon the heating power of an electrical oscillation, which it
+possesses in common with every other form of electric current.
+Professor R. A. Fessenden[55] has constructed a very ingenious thermal
+receiver in the following manner: An extremely fine platinum wire,
+about 0.003 of an inch in diameter, is embedded in the middle of a
+silver wire about one tenth of an inch in diameter, like the wick of
+a candle. This compound wire is then drawn down until the diameter of
+the silver wire is only 0.002 of an inch, and hence the platinum wire
+in its interior, being reduced in the same ratio, will have been drawn
+to a diameter of 0.00006 of an inch. A short piece of this drawn wire
+is then bent into a loop and the ends fixed to wires. The tip of the
+loop is then immersed in nitric acid and dissolved in the silver,
+leaving an exquisitely fine platinum wire a few hundredths of an inch
+in length and having a resistance of about thirty ohms. This little
+loop is sealed into a glass bulb like a very small incandescent lamp,
+or it may be enclosed in a small silver bulb and the air may be
+exhausted. If an electrical oscillation is sent through this
+exceedingly fine platinum wire it heats it and rapidly increases its
+resistance. The electrical oscillations produced in an aerial are sent
+through a number of these loops arranged in parallel, and the loops
+are short-circuited by a telephone, joined in series with a source of
+very small electromotive force produced by shunting a single cell or
+opposing to one another two cells of nearly equal electromotive force.
+Any variation of resistance of the little platinum loops due to the
+heat produced by the oscillations, by suddenly altering the current
+flowing through the telephone, will cause a sound to be heard in it.
+The electrical oscillations when passing through the loops are
+therefore detected by the heat which they generate in these
+exquisitely fine platinum wires.
+
+Finally, one word must be said on the subject of electrodynamic
+receivers, due to the same inventor. An exceedingly small silver ring
+is suspended by a quartz fibre and has a mirror attached to it in the
+manner of a galvanometer. This ring is suspended between two coils
+joined in series, which are placed either in the circuit of the aerial
+or in the secondary circuit of the small air core transformer inserted
+between the aerial and the earth. When electrical oscillations travel
+down the aerial they induce other electrical oscillations in the
+silver ring, and if the ring is so placed that its normal position is
+with its plane inclined at an angle of forty-five degrees to the plane
+of the fixed coils, then the ring will be slightly deflected every
+time an oscillation occurs in the aerial.
+
+Omitting further mention of the details of the kumascopes in use and
+the receiving aerial, we must next proceed to consider the receiving
+arrangements taken as a whole.
+
+In the original Marconi system, the sensitive tube or coherer was
+inserted between the bottom of the receiving aerial and the earth.[56]
+Accordingly, when the incident electric wave strikes the receiving
+aerial and creates in it an oscillatory electromotive force, this last
+will, if of sufficient amplitude, cause the particles of the coherer
+to cohere and become conductive. This sudden change from a nearly
+perfect non-conductivity to a conductive condition is made to act as a
+switch or relay, closing or completing the circuit of a single cell,
+and so sending a current through an ordinary telegraphic relay,
+closing or completing the circuit of a single cell, which may in turn
+actuate another recording telegraphic instrument, such as a Morse
+printer. To prevent the oscillations from passing into the relay
+circuit, small choking or inductance coils are inserted between the
+ends of the sensitive tube and the relay and cell and serve to confine
+the oscillations to the tube.
+
+It has already been pointed out that in the transmitting aerial the
+amplitude at the potential vibrations increases from the bottom to the
+top, and when vibrating in its fundamental manner there is a potential
+node at the earth connection and a potential loop or antinode at the
+top. The same is true of the receiving aerial. Hence, if the kumascope
+employed is a Branly metallic filings tube and is inserted near the
+base of the aerial, the difference of potential between its two ends
+will be small.
+
+It has also been mentioned that a receiver of this type acts in virtue
+of electromotive force or potential difference, and hence the proper
+place to insert the coherer is not at the base of the aerial, but
+between the top of the aerial and the earth. This, however, could not
+be done by running up another wire from the earth, as that would
+amount to putting the coherer between the tops of two identical
+aerials, and between its ends there would be no difference of
+potential. Professor Slaby, in conjunction with Count von Arco, has
+given an ingenious solution of this problem. If we take two equal
+lengths of wire, bent at right angles, and connect the point of
+intersection with the earth, placing one of these wires vertically and
+the other horizontally, we then have an arrangement which responds to
+the impact of electric waves, and has electrical oscillations set up
+in it in such fashion that the common point of the two wires has a
+very small amplitude of potential, but the two extremities have equal
+and large variations. If, then, we insert a coherer tube between the
+earth and the outer extremity of the horizontal wire, it is influenced
+in the same manner as it would be by the potential variations at the
+top of the vertical wire. In other words, it is acted upon by a large
+difference of potential instead of a small one. It is not found
+necessary to stretch the horizontal wire out straight; it may be
+coiled into a spiral with open turns, and the slight decrease in
+capacity and increase in inductance resulting from this can be
+compensated by cutting off a short piece of it.
+
+[Illustration: FIG. 19.--SLABY RECEIVER. A, aerial; E, earth plate; F,
+coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth
+plate.]
+
+In this way we have an arrangement (see Fig. 19) in which the outer
+extremity of this open spiral experiences variations or potential which
+exactly correspond with those at the summit of the vertical aerial. The
+receiving arrangements are then completed as in Fig. 19, one end of the
+coherer being attached to the outer end of the spiral and the other end
+through a condenser to the earth, a relay and a voltaic cell being
+arranged as shown in the diagram. The mode of operation of this receiver
+is as follows: When the wave strikes the aerial it sets up in it
+electrical oscillations with a potential antinode at the summit, and at
+the same time a potential antinode is created at the outer end of the
+spiral attached near the base of the aerial, this spiral being called by
+Professor Slaby a _multiplicator_. As long as the coherer tube remains
+non-conductive, the local cell cannot send a current through the relay,
+but, as soon as the resistance is broken down by the impact of a wave,
+the local cell sends a current through the coherer tube which, passing
+down to the earth through the base of the aerial and up through the
+earth connection to the condenser, completes its circuit through the
+relay. Many variations of this arrangement have been made by Slaby and
+Von Arco and by the Allgemeine Elektricitaets Gesellschaft of Berlin.
+
+In 1898, Mr. Marconi made a great advance in the construction of his
+receiving apparatus by the insertion of his "jigger" or oscillation
+transformer in the aerial receiving circuit.[57] In this arrangement,
+the primary coil of an air core transformer wound in a particular way
+is inserted between the receiving aerial and the earth, and the
+secondary circuit is cut in the middle and connected to the two
+surfaces of a condenser, these surfaces being also connected through
+the circuit of an ordinary telegraphic relay and a single cell (see
+Fig. 20). The ends of the secondary circuit of this oscillation
+transformer are also connected to the terminals of the coherer tube,
+and these again are short-circuited by a small condenser.
+
+[Illustration: FIG. 20.--MARCONI RECEIVER. A, aerial; J, jigger; CC,
+condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse
+printer.]
+
+The operation of this receiver is as follows: The oscillations set up
+in the aerial pass through the primary circuit of the jigger, and
+these induce other oscillations in the secondary circuit; the
+electromotive force or difference of potential between the primary
+terminals being transformed up in any desired ratio. It is this
+exalted electromotive force which is made to act on the coherer tube,
+and, inasmuch as the jigger operates in virtue of a current passing
+through its primary circuit and this current is at a maximum at the
+lower end of the aerial, the arrangement is exceedingly effective,
+because it, so to speak, converts current into voltage. At the lower
+end of the aerial, although the amplitude of the potential
+oscillations is a minimum, the amplitude of the current oscillations
+is a maximum, and the jigger transforms these large current
+oscillations into large potential oscillations, _provided it is
+constructed in the right manner_. We can also transform up or increase
+the amplitude of the small potential variations near the bottom of the
+aerial by employing the principle of resonance. Many devices of this
+kind, due to Professor Slaby and others, have been suggested and tried
+but the details are rather too technical to be fully described here.
+
+It will be noticed that the receiving aerial may be arranged in one of
+two ways--it may be either earthed at the lower end or it may be
+insulated. It has been claimed that there is a great advantage in
+earthing the receiving aerial directly in that it eliminates
+atmospheric disturbances.
+
+We shall allude to this point more particularly later on. Meanwhile it
+may be mentioned that the receiving arrangements, as a whole,
+constitute a sensitive arrangement, as shown by Popoff, Tommasina and
+by all the large experience of Mr. Marconi himself for detecting
+changes in the electrical condition of the atmosphere, which are
+doubtless of the nature of electrical oscillations. On the other hand,
+the receiving arrangements may be perfectly insulated and some
+experimentalists have asserted that by this method the greatest
+freedom is secured from atmospheric disturbances. Amongst the
+non-earthed arrangements the system invented by Professor F. Braun, of
+Strassburg, and worked by Messrs. Siemens, of Berlin, may be
+mentioned.[58]
+
+[Illustration: FIG. 21.--BRAUN'S NON-EARTHED RECEIVER. I, induction
+coil; CC, condensers; S, spark gap; J, transmitting jigger; K,
+receiving jigger; F, filings tube; R, relay; B, battery.]
+
+Professor Braun's arrangements are indicated in the diagram in Fig.
+21. In this case an induction coil is used to create a discharge
+between two spark balls, and to these two balls are connected the two
+outer coatings of two condensers, the inner coatings of which are
+connected together through the primary coil of an air core
+transformer. The secondary coil of this transformer is connected to
+two extension wires forming a Hertz resonator, and the length of these
+wires is so adjusted with reference to the time period of the primary
+circuit that they resonate to it, the whole length from end to end of
+the secondary circuit being half a wave-length. The receiver, as shown
+in the diagram, consists of a pair of quarter wave-length receiving
+wires connected through two condensers, which are short-circuited by
+the primary coil of an oscillation transformer. The secondary circuit
+of this last oscillation transformer has two extension wires to it,
+turned in the same manner, to respond to the primary oscillator; and
+in the circuit of one of these extension wires is placed a coherer
+tube, short-circuited by a relay and a local battery.
+
+It will thus be seen that there is an entire abolition of ground
+connection, which, Professor Braun claims, practically avoids all
+atmospheric disturbances.[59] The details of the receiving arrangement
+are as follows:--The coherer tube consists of an ebonite tube
+containing hard steel particles of a uniform size, placed in the
+adjustable space between two polished steel electrodes. It is found
+that with this steel coherer, a small amount of magnetism in the
+particles increases its sensitiveness, and to obtain this, a ring
+magnet is employed in connection with a coherer tube. Receiving
+apparatus arranged on this system is said to have been used for
+telegraphing between Heligoland and Cuxhaven, a distance of thirty-six
+miles.
+
+All the immense experience, however, gained by Mr. Marconi and those
+who have worked with his system, is in favour of using the earth
+connection. There is no doubt that Hertzian wave telegraphy can be
+conducted over short distances by means of totally insulated aerials,
+but for long distances the earth connection is essential, for the
+reasons that have been explained previously.
+
+There are many of the details of the receiving arrangements which
+remain to be considered. If the communication is received by a
+telegraphic instrument like the Morse printer, which requires a
+current of anything like ten milliamperes to work it, then an
+important element in the receiving arrangement is the relay. The relay
+that is generally used is a modified form of the Siemens polarised
+relay, which is so adjusted as to make a single contact. For marine
+work on board ship, it is essential that this relay shall be balanced
+so that variations in position shall not affect it. Sometimes the
+relay is hung in gimbals like a compass, and at other times suspended
+from a support by elastic bands, so as to avoid jolting. In any case,
+the relay must be so adjusted that no change of position will cause it
+to close the circuit of the telegraphic printer or recorder. Its
+sensibility ought to be such that it is actuated by a tenth of a
+milliampere, and, if possible, even by less. The alteration of
+sensibility in the ordinary contact form of relay is the pressure that
+is necessary to bring the platinum points of the circuit closer
+together, so as to pass the minimum current which will work the
+telegraph printer.
+
+The important matter, however, in connection with the use of the relay
+in Hertzian wave telegraphy, is that it should be capable of
+adjustment without extraordinary skill. It is no use to put into the
+hands of an operator a relay which requires abnormal dexterity to make
+it work at all.
+
+       *       *       *       *       *
+
+It remains, then, to consider some of the questions connected with
+practical Hertzian wave telegraphy and the problem of the limitation
+of communication. These matters at the present moment very much occupy
+the public attention, and many conflicting opinions are expressed
+concerning them.
+
+It may be observed at the outset that the difficulty of dealing with
+the subject as freely as many desire is that Hertzian wave telegraphy
+is no longer merely a subject of scientific investigation, but has
+developed into a business and involves, therefore, other interests
+than the simple advancement of scientific knowledge. We can, however,
+discuss in a general manner some of the scientific problems which
+present themselves for solution. The first of these is the
+independence of communication between stations. It is desirable, at
+the outset, to clear up a little misunderstanding. There is a great
+difference between preventing the reception of communication when it
+is not desired by the recipient, and preventing it when it is the
+object of the latter to overhear if he can. It is, therefore,
+necessary to distinguish between isolation and overhearing. We may say
+that a station is _isolated_ when it is not affected by Hertzian waves
+other than those it desires to receive; but that a station _overhears_
+when it can, if it chooses, pick up communications not intended for
+it, or cannot help receiving them against its will.
+
+This distinction is a perfectly fair one. Any telegraph or telephone
+wire can be tapped, if it is desired, but unless there is some fault
+on the line, no station will receive a message against its desires.
+Moreover, it may be noted that there are penalties attached to tapping
+a telegraph wire, and at present there are none connected with the
+misappropriation of an ether wave.
+
+We shall, therefore, consider in the first place the methods so far
+proposed for preventing any given receiver from being affected by
+Hertzian waves sent out from other stations, except that of those from
+which it is desired to receive them. The first method is that which
+has been called the method of _electrical syntony_, and consists in
+adjusting the electrical capacity and inductance of the various open
+and closed circuits of the receiving and transmitting stations to be
+put in communication so that they have the same electrical
+time-period.[60]
+
+In the Cantor Lectures before the Society of Arts in 1900, on
+electrical oscillations and electric waves, the author has discussed
+at length the conditions under which powerful electrical oscillations
+can be set up in a circuit. It was there shown that every electric
+circuit having capacity and inductance has a particular or natural
+time-period of electrical oscillation depending on the product of
+these qualities, and that, to accumulate powerful electrical
+oscillations in it, the electromotive impulses on it must be delivered
+at this rate. Illustrations were drawn from mechanics, such as the
+examples furnished by vibrating pendulums and springs, and from
+acoustics, as illustrated by the phenomena of resonance, to show that
+small or feeble blows or impulses delivered at the proper time
+intervals have a cumulative effect in setting up vibrations in a body
+capable of oscillation. It is a familiar fact that if we time our
+blows, we can achieve that which no single blow, however powerful, can
+accomplish in throwing into vibration a body such as a pendulum, which
+is capable of oscillation under the action of a restoring force.
+Precisely the same is true of an electric circuit. We have already
+seen that the receiving aerial has an alternating electromotive force
+set up in it by the impact of the successive electric waves sent out
+from the transmitter. It must, however, be remembered that the
+transmitter sends out a series of trains of waves, not by any means a
+continuous train, but one cut up into groups of probably ten to fifty
+waves, each separated by intervals of silence, long, compared with the
+duration of a single train of waves.
+
+[Illustration: FIG. 22.--SEIBT'S APPARATUS FOR EXHIBITING ELECTRIC
+RESONANCE. I, induction coil; S, spark gap; CC, condensers; L,
+variable inductance; E, earth plate; WW, wire spirals; VV, vacuum
+tubes.]
+
+If, however, by a suitable adjustment of capacity and inductance, we
+make the natural time-period of oscillation of the receiving aerial
+circuits agree with those of the transmitting aerial, within certain
+limits the former will only be receptive for waves of the frequency
+sent out by the transmitter. It is quite easy to illustrate this
+principle by numerous experiments. It can be done by means of an
+apparatus devised by Dr. Georg Seibt for showing in an interesting
+manner the syntonisation or tuning of two electric circuits. This
+consists of two bobbins, each consisting of one layer of insulated
+wire wound on a wooden rod (see Fig. 22). Each of these bobbins has a
+certain electrical capacity with respect to the earth, when considered
+as an insulated conductor, and it has also a certain inductance. If,
+therefore, electromotive impulses are applied to one end of the bobbin
+at regular intervals, electrical oscillations will be set up in it,
+and, as already explained, if these are timed at a certain rate, the
+bobbin will act like a closed organ-pipe to air impulses and
+oscillations of potential will be accumulated at the opposite end,
+which have much greater amplitude than the impressed oscillations at
+the end at which they are applied. We can make the existence of the
+amplitude oscillations of potential evident by attaching to one end of
+the bobbin a vacuum tube, which will be illuminated thereby, or by
+terminating it by a pointed piece of wire, so that an electrical brush
+can be formed at the point, if the potential variations have
+sufficient amplitude. We arrange also another closed oscillation
+circuit, consisting of two Leyden jars and a variable inductance coil
+and a pair of spark balls which are connected to an induction coil. In
+this manner we can set up oscillations in the discharge circuit of
+these Leyden jars, and we can vary the time period by altering the
+inductance and the capacity. If we denote the capacity of the jars in
+the microfarads by the letter C and the inductance in centimetres of
+the discharge circuit of the jars by the letter L, it can then be
+shown that the number of oscillations per second denoted by _n_ is
+given by the expression--[61]
+
+    n = (5,000,000,000) / ([\sq]{CL}).
+
+If now we adjust the Leyden jar circuit to a particular rate of
+oscillation, we have between the terminals of the jar or condenser an
+alternating difference of potential or electromotive force. If we
+connect one side of the jars to the earth and the other side to the
+foot of one of the spirals or bobbins above described, we shall find
+perhaps that the vacuum tube at the other end is not rendered
+luminous. When, however, we adjust the inductance in the discharge
+circuit of the jar to a certain value to make the frequency of the
+condenser oscillations agree with the natural time period of the
+bobbin terminated by the vacuum tube, this latter at once lights up
+brilliantly. Again, if we connect both these bobbins at the same time
+to the discharge circuit of the Leyden jar, we shall find that we can
+make an adjustment of the inductance of that circuit, such that either
+of the bobbins at pleasure can be made to respond and be set in
+electrical vibration, as shown by the illumination of the vacuum tube
+at its upper end or by an electrical brush being formed at the
+terminal. In making this adjustment of inductance, we are _tuning_, as
+it is called, the Leyden jar discharge circuit to the resonating
+bobbin. A very small variation of the inductance of the jar circuit
+causes the vacuum tube to change in luminosity. If, however, the
+natural time periods of these bobbins do not lie very far apart, then
+a faint luminosity will make its appearance in both the vacuum tubes.
+Supposing, therefore, that we connect to the oscillating circuit of
+the jar a number of bobbins having different time periods of
+oscillation, like organ-pipes, and supply them all with one common
+alternating electromotive force. These bobbins, whose natural time
+period is very different to that of the osciilating circuit or
+impressed electromotive force, will not respond, but those bobbins of
+which the natural time period lies near to, even if not quite exactly
+the same as, that of the impressed electromotive force will give
+evidence of being set in oscillation. A very violent electromotive
+force will cause them all to respond to some slight extent, no matter
+whether the period of that impluse is tuned to their common period
+precisely or not.
+
+At this point questions arise of great practical importance. A matter
+which has been in dispute in connection with practical Hertzian wave
+telegraphy is how far this electrical tuning is a sufficient solution
+of the practical problem of isolation. It is not denied that
+experiments such as those made with Seibt's apparatus can be shown on
+a small scale; and, on a still larger scale, Mr. Marconi gave to the
+author in September, 1900, a demonstration in practical telegraphic
+work of sending two independent Hertzian wave messages and receiving
+them on two independent receivers attached to the same aerial.
+
+Since that date much experience has been gained and large power
+stations erected, and a statement has been frequently made that
+syntony is no protection against interference when one of the stations
+is sending out very powerful waves. The contention has been raised
+that large power stations producing electric waves will therefore play
+havoc with Hertzian wave telegraphy on a smaller scale, such as the
+ship to shore and intermarine communication. Under these
+circumstances, it appeared to the author important to subject the
+matter to a special test, and Mr. Marconi, therefore, offered to give
+a demonstration, with this object, in support of the opinion that he
+has expressed positively that waves from his power stations do not
+interfere with the working of his ship installations. This matter is
+vital to the whole question of practical Hertzian wave telegraphy, for
+the ship to shore communication is of stupendous importance; and if
+Mr. Marconi had done nothing else except to render this possible and
+effective, he would have earned, as he has done, the gratitude of
+humanity for all time. Accordingly, the author embraced the
+opportunity of making some careful tests to settle the question
+whether the powerful waves sent out from a station such as Poldhu did
+or did not affect the exchange of messages between ship and shore
+stations in proximity, equipped with Marconi apparatus of a suitable
+type.
+
+These experiments were carried out on the eighteenth of March last, at
+Poldhu, in Cornwall, and a programme was arranged by the author of the
+following kind. Close to the Poldhu station is an isolated mast, which
+was equipped by Mr. Marconi with a Hertzian wave apparatus, similar to
+that he places on ships. Six miles from Poldhu is the Lizard receiving
+station, with which ships proceeding up or down the English Channel
+communicate. It was arranged that a series of secret messages, some of
+them in cipher, should be delivered simultaneously at certain known
+times, both to the power station at Poldhu and to the small adjacent
+ship station; and it was arranged that these messages should be sent
+off simultaneously, the operators being kept in ignorance up to the
+moment of sending as to the nature of the messages. At the Lizard, Mr.
+Marconi connected two of his receiving instruments to the aerial, one
+of them tuned to the waves proceeding from the power station at
+Poldhu, and the other to those proceeding from the small ship station.
+At the appointed time, these two sets of messages were received
+simultaneously in the presence of the author, each message being
+printed down independently on its own receiver; and Mr. Marconi read
+off and interpreted all these messages perfectly correctly, not having
+known before what was the message that was about to be sent. In
+addition to this, precautions were taken to prove that the power
+station at Poldhu was really emitting waves sufficiently powerful to
+cross the Atlantic and not being made to sing small for the occasion.
+To assist in proving this, the messages sent out from the power
+station were also received at a station at Poole, two hundred miles
+away, and the assistant there was instructed to telegraph back these
+messages by wire as soon as he received them. These messages came back
+perfectly correctly, thus demonstrating that the power station was
+sending out power waves. The whole programme was carried out with the
+greatest care to avoid any mistakes on the part of the assistants, and
+provided an absolute demonstration of the truth of Mr. Marconi's
+assertion that the waves from one of his power stations, such as
+Poldhu, do not in the least degree interfere with the transmission and
+reception of messages between ship and shore, effected by means of
+certain forms of Marconi apparatus for producing and detecting waves
+of a different wave length.[62] This complete independence of
+transmission, however, is entirely due to the employment of a
+receiving circuit of a certain type in Mr. Marconi's receivers. It
+does not at all follow that a receiving circuit of any kind, even a
+Marconi receiver not especially arranged, set up in proximity to a
+power station would not be affected. This, however, is not an
+important matter. Far more important is it to show, as has been shown,
+that practically perfect isolation can be achieved if it is desired.
+
+It must be noted, however, that, although the fact that electric
+circuits have a natural time-period of oscillation of their own is a
+scientific principle which carries us a considerable way towards a
+solution of what is called syntonic Hertzian wave telegraphy, it is
+not in itself alone in every respect an entire solution of the
+practical problem. The degree to which it is a solution depends to a
+considerable extent upon the nature of the detecting device, or
+kumascope, which we are employing. The coherer, or Branly filings
+tube, has the peculiarity that its passage from a non-conductive to a
+conductive condition follows immediately when the difference of
+potential between its ends is made sufficiently great. In other words,
+if the tube is acted upon by a sufficient electromotive force, it is
+not necessary that electromotive force should be repeated at intervals
+to make this particular form of kumascope responsive. Again, if we
+consider the nature of the oscillations which are sent out from any
+transmitting aerial, we find that each group of oscillations
+corresponding to a single spark consists of waves gradually decreasing
+in amplitude. In other words, the first wave of the group is the
+strongest, and the decay in amplitude is often very rapid. Supposing,
+then, we construct a simple receiver consisting of an aerial having
+inserted in its circuit a sensitive Branly filings tube. Such a
+receiver is almost entirely non-syntonic; that is to say, it is
+affected by any wave passing over it which is sufficiently powerful.
+We may look upon it that if the first wave of the series is
+sufficiently powerful to affect the kumascope, the conductive change
+takes place whether or not the first wave is followed by others.
+Accordingly, it is perfectly certain that if a transmitter is sending
+out trains of waves of any period, a simple combination of coherer and
+aerial will be influenced, if it is placed near enough to the
+transmitter. On the other hand, it is possible to combine a kumascope
+of a certain type with a receiving aerial and other circuits in such a
+manner that when the waves that reach it are feeble it shall not
+respond at all unless those waves have very nearly a time period of a
+certain value.
+
+At this stage, it may be perhaps well to explain a little in detail
+what is meant by an easily responsive circuit, and, on the other hand,
+by an irresponsive circuit, or, as we may call it, a _stiff_ circuit.
+Supposing that we consider an aerial consisting of a simple straight
+wire having small capacity and small inductance, such a circuit admits
+of being sent into electrical oscillation, not only by waves of its
+own natural time-period, but by the sudden application of any violent
+electromotive impulse. If, on the other hand, we bestow upon the
+circuit in any way considerable inductance, we then obtain what may be
+called a stiff or irresponsive circuit, which is one in which
+electrical oscillations can be accumulated only by the prolonged
+action of impulses tuned to a particular period.
+
+A mechanical analogue of this difference may be found in considering
+the different behaviour of elastic bodies to mechanical blows. Take,
+for instance, a piece of elastic steel and fix the bottom end in a
+vice. The steel strip may be thrown into vibration by deflecting the
+upper end. It has, however, a very small mass, and therefore any
+violent blow or blows, even although not repeated, will set it in
+oscillation. If, however, we add mass to it by fixing at the other end
+a heavy weight, such as a ball of lead, and at the same time make the
+spring stiffer, we have an arrangement which is capable of being sent
+into considerable oscillation only by the action of a series of
+impulses or blows which are timed at a particular rate.
+
+Returning then to the electrical problem, we see that in order to
+preserve a kumascope or wave detector from being operated on by any
+vagrant wave or waves having a period very different to an assigned
+period, it must be associated with an electrical circuit of the kind
+above called a stiff circuit.
+
+We will now consider the manner in which the problem has been
+practically attacked by Mr. Marconi, Dr. Slaby, Sir Oliver Lodge and
+others, who have invented forms of receiver and transmitter, which are
+syntonic or sympathetic to one another.
+
+Some of the methods which Mr. Marconi has devised for the achievement
+of syntonic wireless telegraphy were fully described by him in a Paper
+read before the Society of Arts on May, 17, 1901.[63]
+
+[Illustration: FIG. 23.--MARCONI TRANSMITTER AND RECEIVER. I,
+induction coil; A, aerial; E, earth plate; HH, choking coils; S, spark
+gap; J, transmitting jigger; K, receiving jigger; R, relay; C,
+condenser; F, filings tube; B, battery. Many practical details are
+omitted.]
+
+On referring to his Paper, it will be seen that in one form his
+transmitter consists of an aerial, near the base of which is inserted
+the secondary circuit of an oscillation transformer or transmitting
+jigger. One end of this secondary circuit is attached to the aerial
+and the other end is connected to the earth through a variable
+inductance coil. The primary circuit of this oscillation transformer
+is connected in series with a condenser, consisting of a battery of
+Leyden jars, and the two together are connected across to the spark
+balls which close the secondary circuit of an induction coil, having
+the usual make and break key in the primary circuit. Mr. Marconi so
+adjusts the induction of the aerial and the capacity of the condenser,
+or battery of Leyden jars, that the two circuits, consisting
+respectively of this battery of Leyden jars and the primary circuit of
+the transformer, and on the other hand of the capacity of the aerial
+and the inductance in series with it, and that of the secondary
+circuit of the transformer have the same time period. In other words,
+these two inductive circuits are tuned together. At the receiving end,
+the aerial is connected in series with a variable inductance and with
+the primary circuit of another oscillation transformer, the second
+terminal of which is connected to the earth. The secondary circuit of
+this last oscillation transformer is cut in the middle and is
+connected to the terminals of a small condenser. The outer terminals
+of this secondary circuit are connected to the metallic filings tube
+or other sensitive receiver and to a small condenser in parallel with
+it (see Fig. 23). The terminals of the condenser which is inserted in
+the middle of the secondary circuit of the oscillation transformer are
+connected through two small inductance coils with a relay and a single
+cell. This relay in turn actuates a Morse printer by means of a local
+battery. The two circuits of the oscillation transformer are tuned or
+syntonised to one another, and also to the similar circuit of the
+transmitting arrangement. When this is the case, the transmitter
+affects the co-resonant receiving arrangement, but will not affect any
+other similar arrangement, unless it is within a certain minimum range
+of distance. Owing to the inductance of the oscillation transformer
+forming part of the receiving arrangements, the receiving circuit is,
+as before stated, very stiff or irresponsive; the sensitive tube is
+therefore not acted upon in virtue merely of the impact of the single
+wave against the aerial, but it needs repeated or accumulated effects
+of a great many waves, coming in proper time, to break down the
+coherer and cause the recording mechanism to act. An inspection of the
+diagram will show that as soon as the secondary electromotive force in
+the small oscillation transformer or jigger of the receiving
+instrument is of sufficient amplitude to break down the resistance of
+the coherer, the local cell in circuit with the relay can send a
+current through it and cause the relay to act and in turn make the
+associated telegraphic instrument record or sound.
+
+Mr. Marconi described in the above-mentioned Paper some other
+arrangements for achieving the same result, but those mentioned all
+depend for their operation upon the construction of a receiving
+circuit on which the time-period of electrical oscillations is
+identical with that of a transmitting arrangement. By this means he
+showed experiments during the reading of his Paper, illustrating the
+fact that two pairs of transmitting and receiving arrangements could
+be so syntonised that each receiver responded only to its particular
+transmitter and not to the other.
+
+With arrangements of substantially the same nature, he made
+experiments in the autumn of 1900 between Niton, in the Isle of Wight,
+and Bournemouth, a distance of about thirty miles, in which
+independent messages were sent and received on the same aerial.
+
+Dr. Slaby and Count von Arco, working in Germany, have followed very
+much on the same lines as Mr. Marconi, though with appliances of a
+somewhat different nature. As constructed by the General Electric
+Company, of Berlin, the Slaby-Arco syntonic system of Hertzian
+telegraphy is arranged in one form as follows:--The transmitter
+consists of a vertical rod like a lightning conductor, say, 100 or 150
+feet in height. At a point six or nine feet above the ground, a
+connection is made to a spark ball (see Fig. 24), and the
+corresponding ball is connected through a variable inductance with one
+terminal of a condenser, the other terminal of which is connected to
+the earth. The two spark balls are connected to an induction coil, or
+alternating current transformer, and by variation of the inductance
+and capacity the frequency is so arranged that the wave-length
+corresponding to it is equal to four times the length of that portion
+of the aerial which is above the spark ball connection. The method by
+which this tuning is achieved is to insert in the portion of the
+aerial below the spark balls, between it and the earth, a hot wire
+ammeter of some form. It has already been shown that in the case of
+such an earthed aerial, when electrical oscillations are set up in it,
+there is a potential node at the earth and a potential anti-node or
+loop at the summit, if it is vibrating in its fundamental manner;
+also, there is a node of current at the summit of the aerial and an
+anti-node at the base. This amounts to saying that the amplitude of
+the potential vibrations is greatest at the top end of the aerial, and
+the amplitude of the current vibrations is greatest at the bottom or
+earthed end. Accordingly, the inductance and capacity of the lateral
+branch of the transmitter is altered until the hot wire ammeter in the
+base of the aerial shows the largest possible current.
+
+[Illustration: FIG. 24.--SLABY-ARCO SYNTONIC TRANSMITTER AND RECEIVER.
+I, induction coil; M, multiplier; B, battery; A, aerial; F, filings
+tube; R, relay; E, earth plate; C, condenser.]
+
+The corresponding receiver is constructed in a very similar manner. A
+lightning conductor or long vertical rod of the same height as the
+transmitting aerial is set up at the receiving station, and at a point
+six or nine feet from the ground a circuit is taken off, consisting of
+a wire loosely coiled in a spiral, the length of which is nearly equal
+to, although a little shorter than, the height of the vertical wire
+above the point of connection. The outer end of this loose spiral is
+connected to one terminal of the coherer tube, and the other terminal
+of the coherer is connected to the earth through a condenser of rather
+large capacity. The terminals of this last condenser are
+short-circuited by a relay and a single cell. When the adjustments are
+properly made, it is claimed that the receiver responds only to waves
+coming from its own syntonised or tuned transmitter. In this case the
+length of the receiving aerial above the point of junction with the
+coherer circuit is one quarter the length of the wave. A variation of
+the above arangements consists in making this lateral circuit equal in
+length to one-half of a wave, and connecting the coherer to its centre
+through a condenser to the earth. The outer end of this lateral
+circuit is also connected to the earth (see Fig. 24).[64]
+
+Dr. Slaby claims that this arrangement is not affected by atmospheric
+electricity, and that the complete and direct earthing of the aerial
+and also in the second arrangement, of the receiver of the outer end
+of the lateral conductor, conduces to preserve the receiver immune
+from any electrical disturbances except those having a period to which
+it is tuned.
+
+[Illustration: FIG. 25.--LODGE-MUIRHEAD SYNTONIC RECEIVER. I,
+induction coil; S, spark gap; A, aerial; CC, condensers; E, earth
+plate; R, relay; L, variable inductance; F, filings tube; B, battery.]
+
+A method has also been arranged by him for receiving on the same
+aerial two messages from different transmitting stations
+simultaneously. In this case, two lateral wires of different lengths
+are connected to the receiving aerial, and to the outer end of each of
+these is connected a coherer tube, the other end of which is earthed
+through a condenser. One of these lateral wires is made equal, or
+nearly equal, in length to the aerial, and the other is made longer to
+fulfil the following condition.[65] If we call H the height of the
+receiving aerial above point of junction of the lateral wires, then
+the length of one lateral wire is made equal to H, and the height of
+the aerial is adjusted to be equal to one-quarter of the wave length
+of one incident wave. The other lateral wire may then be made of a
+length equal to one-third of H, and it will then respond to the first
+odd harmonic of that wave, of which the fundamental is in syntony with
+the vertical wire. By suitably choosing the relation between the
+wave-lengths of the two transmitting stations, it is possible to
+receive in this manner two different messages at the same time on the
+same aerial. Subsequently to the date of the above-mentioned
+demonstration of multiplex wireless telegraphy by Mr. Marconi an
+exhibition of a similar nature was given by Professor Slaby in a
+lecture given in Berlin on December 22, 1900.[66]
+
+Both the above-described syntonic systems of Mr. Marconi and Dr. Slaby
+are "earthed" systems, but arrangements for syntonic telegraphy have
+been devised by Sir Oliver Lodge and Professor Braun which are
+"non-earthed."
+
+Sir Oliver Lodge and Dr. Muirhead have devised also syntonic systems.
+According to their last methods, the systonic transmitting and
+receiving arrangements are as shown in Fig. 25.[67] On examining the
+diagrams it will be seen that the secondary terminals of the induction
+coil are, as usual, connected to a pair of spark balls, and that these
+spark balls are connected by a condenser and by a variable inductance.
+One terminal of the condenser is earthed through another condenser of
+large capacity, and the remaining terminal of the first condenser is
+connected to an aerial. It should, therefore, be borne in mind in
+dealing with electrical oscillations that a condenser of sufficient
+capacity is practically a conductor, and an inductance coil of
+sufficient inductance is practically a non-conductor. Hence the
+insertion of a large capacity in the path of the aerial wire is no
+advantage whatever and makes no essential difference in the
+arrangement. In order to obtain any powerful radiation, the length of
+the aerial, or sky wire, as they call it, must be so adjusted that its
+length is one-quarter the wave-length corresponding to the oscillation
+circuit, consisting of the condenser and variable inductance.
+
+The receiving arrangement consists of a similar sky wire or aerial
+earthed through a condenser of large capacity and having in the
+portion above this last condenser another condenser of similar
+capacity. At the earthed side of this last condenser a connection is
+made to a resonant circuit, consisting of a variable inductance, and
+another condenser and a sensitive metallic filings tube of the Branly
+type; also a portion of this resonant circuit is shunted by another
+consisting of a battery and telegraphic relay, as shown in the
+diagram. The circuit, including the coherer, is tuned to its own
+aerial and also to that of the transmitting circuit, and under these
+circumstances trains of waves thrown off at the transmitting aerial
+will sympathetically affect the receiving aerial.
+
+There is nothing in the arrangement which specially calls for notice.
+It is simply a variation of other known forms of syntonic transmitter
+and receiver, and possesses all the advantages and disadvantages
+attaching to such electrical syntonic methods.
+
+Professor Braun's syntonic system, the receiver and transmitter of
+which have been described, is also in one form a non-earthed system.
+Innumerable other patentees have taken out patents for devices which
+are modifications in small degree of the above arrangements.
+
+It may be well to note at this point the disadvantages that are
+possessed by any form of coherer as a telegraphic kumascope in
+connection with proposed arrangements for the isolation of Hertzian
+wave stations. All the detectors of the coherer type really depend for
+their actuation upon electromotive force; that is to say, upon the
+application to the terminals of the detector of a certain
+electromotive force. Although there may be no sharp and defined
+critical electromotive force, yet, nevertheless, as a matter of fact,
+if the electromotive force applied exceeds a certain value, then the
+detector passes suddenly from one state of conductivity to another. It
+may be of great conductivity, as in the case of the Branly coherer, or
+of lesser conductivity, as in the case of the so-called anti-coherers,
+of which the Schaefer kumascope may be taken as a type. Accordingly,
+when these instruments are subjected to a train of waves, each
+individual group of which is damped, their operation is largely
+governed by the fact that if the first wave or oscillation set up in
+the receiving circuit is powerful enough to break down the coherer,
+then the receiving mechanism acts, no matter whether the first impulse
+is followed by others or not.
+
+In comparison with so-called coherers, those depending upon the
+changes in the magnetisation of iron by electrical oscillations
+certainly have an advantage, because this is a process which requires
+the application of alternating electric currents decreasing in
+strength for a certain time; and it is found, therefore, that the
+magnetic receivers do not require to be associated with such a stiff
+or irresponsive resonant circuit to confine their indications to
+oscillations or waves of one definite period, and that they lend
+themselves much more perfectly to the work of "tuning" or syntonising
+stations than do those kumascopes depending upon the contact or
+coherer principle.
+
+We may then glance at the alternative solutions of the problem offered
+by other investigators. M. Blondel has proposed to effect the
+syntonisation of two stations, not by syntonising the receiver for the
+exceedingly high-frequency oscillations of the individual electric
+waves, but to syntonise it for the much lower frequency, corresponding
+to that of the intervals between the groups of waves. Thus, for
+instance, if an ordinary simple transmitting aerial is set up, the
+production of sparks between the spark balls results in the emission
+of short trains of waves, each of which may consist of half a dozen or
+more individual waves, the time of production of the whole group being
+very small compared with the interval between the groups. M. Blondel
+proposes, however, to syntonise the receiver, not for the
+high-frequency period of the waves themselves, which may be reckoned
+in millions per second, but for the low-frequency period between the
+groups of waves, which is reckoned in hundreds per second. Thus, for
+instance, if sparks are made at the rate of fifty or a hundred per
+second, they can be made to actuate the telephone receiver and so
+produce in the telephone a sound corresponding to a frequency of 50 or
+100; in other words, to make a low musical note or hum. This
+continuous sound can be cut up, by means of a key placed in the
+primary circuit of the transmitting arrangement, into long or short
+periods, and hence the letters of the alphabet signal.
+
+M. Blondel's arrangements comprise a Mercadier's monotone telephone
+and either a coherer or a particular form of vacuum tube as a
+kumascope. On August 16, 1898, M. Blondel deposited with the Academy
+of Sciences in Paris a sealed envelope containing a description of his
+improvements in syntonic wireless telegraphy, which was opened on May
+19, 1900.[68] The arrangement of the receiving apparatus was as
+follows:--A single-battery cell keeps a condenser charged until the
+kumascope is rendered conductive by the oscillations coming down the
+aerial; and under these circumstances the condenser discharges through
+the telephone and causes a tick to be heard in it. If the trains of
+waves are at the rate of 50 or 100 per second, these small sounds run
+together into a musical note, and this continuous hum can be cut up
+into long and short spaces, in accordance with the Morse alphabet
+signals. The telephone must not be an ordinary telephone, capable of
+being influenced by any frequency, but be one which responds only to a
+particular note, and under these conditions the receiving arrangement
+is receptive only when the trains of waves arrive at certain regular
+predetermined intervals, corresponding with the tone to which the
+telephone is sensitive.
+
+       *       *       *       *       *
+
+A number of more or less imperfect arrangements, having the isolation
+of communications for their object, have been devised or patented,
+which are dependent upon the use of several aerials, each supposed to
+be responsive only to a particular frequency; and attempts have been
+made to solve the problem of isolation by MM. Tommasi, Tesla, Jegon,
+Tissot, Ducretet and others.
+
+We may then pass on to notice the attempts that have been made to
+secure isolation by a plan which is not dependent on electrical
+syntony. One of these, which has the appearance of developing into a
+practical solution of the problem, is that due to Anders Bull.[69] In
+the first arrangements proposed by this inventor, a receiver is
+constructed which is not capable of being acted upon merely by a
+single wave or train of waves or even a regularly-spaced train of
+electric waves, but only by a group of wave trains which are separated
+from one another by certain unequal, predetermined intervals of time.
+Thus, for instance, to take a simple instance, the transmitting
+arrangements are so devised as to send out groups of electric waves,
+these wave trains following one another at time intervals which may be
+represented by the numbers 1, 3 and 5; that is to say, the interval
+which elapses between the second and third is three times that between
+the first two, and the interval between the fourth and fifth is five
+times that between the first two. This is achieved by making five
+electric oscillatory sparks with a transmitter of the ordinary kind,
+the intervals between which are settled by the intervals between holes
+punched upon strips of paper, like that used in a Wheatstone automatic
+telegraphic instrument. It will easily be understood that by a device
+of this kind, groups of sparks can be made, say, five sparks rapidly
+succeeding each other, but not at equal intervals of time. One such
+group constitutes the Morse dot, and two or three such groups
+succeeding one another very quickly constitute the Morse dash. These
+waves, on arriving at the receiving station, are caused to actuate a
+punching arrangement by the intermediation of a coherer or other
+kumascope, and to punch upon a uniformly moving strip of paper holes,
+which are at intervals of time corresponding to the intervals between
+the sparks at the transmitting station. This strip of paper then
+passes through another telegraphic instrument, which is so constructed
+that it prints upon another strip a dot or a dash, according to the
+disposition of the holes on the first strip. Accordingly, taken as a
+whole, the receiving arrangement is not capable of being influenced so
+as to print a telegraphic sign except by the operation of a series of
+wave trains succeeding one another at certain assigned intervals of
+time.
+
+An improvement has been lately described by the same inventor,[70] in
+which the apparatus used, although more complicated, performs the same
+functions. At each station two instruments have to be employed; at the
+transmitting station one to effect the conversion of Morse signals
+into the properly arranged series of wave trains, and at the receiving
+station an instrument to effect the re-conversion of the series of
+wave trains into the Morse signals. These are called respectively the
+dispenser and the collector. The details of the arrangements are
+somewhat complicated, and can only be described by the aid of numerous
+detailed drawings, but the inventor states that he has been able to
+carry on Hertzian wave telegraphy by means of these arrangements for
+short distances. Moreover, the method lends itself to an arrangement
+of multiplex telegraphy, by sending out from different transmitters
+signals which are based upon different arrangements of time intervals
+between the electric wave trains. Although this method may succeed in
+preventing a receiving arrangement from being influenced by vagrant
+waves or waves not intended for it, yet an objection which arises is
+that there is nothing to prevent any one from intercepting these wave
+trains, and with a little skill interpreting their meaning. Thus, if
+the record were received in the ordinary way on a simple receiver,
+corresponding to a Morse dot would be printed five dots at unequal
+intervals, and corresponding to a Morse dash would be printed two such
+sets of five dots. A little skill would then enable an operator to
+interpret these arbitrary signals. On the other hand, the inventor
+asserts that he can overcome this difficulty by making intervals of
+time between the impulses in the series so long that the latter become
+longer than the intervals between each of the series of waves which
+are despatched in continuous succession when the key is pressed for a
+dash. In this case, when telegraphing, the series of dots would
+overlap and intermingle with each other in a way which would make the
+record unintelligible if received in the usual manner, but would be
+perfectly legible if received and interpreted by a receiver adapted
+for the purpose.
+
+Another way of obliterating the record, as far as outsiders are
+concerned, is to interpolate between the groups of signals an
+irregular series of dots--_i.e._, of wave trains--which would affect
+an ordinary coherer, and so make an unintelligible record on an
+ordinary receiver, but these dots are not received or picked up by the
+appropriate selecting instrument used in the Anders Bull system.
+
+The matter most interesting to the public at the present time is the
+long-distance telegraphy by Hertzian waves to the accomplishment of
+which Mr. Marconi has devoted himself with so much energy of late
+years. Everyone, except perhaps those whose interests may be
+threatened by his achievements, must accord their hearty admiration of
+the indomitable perseverance and courage which he has shown in
+overcoming the immense difficulties which have presented themselves.
+Five years ago he was engaged in sending signals from Alum Bay, in the
+Isle of Wight, to Bournemouth, a distance of twelve or fourteen miles;
+and to-day he has conquered twice that number of hundred miles and
+succeeded in sending, not merely signals, but long messages of all
+descriptions over three thousand miles across the Atlantic. Critics
+there are in abundance, who declare that the process can never become
+a commercial one, that it will destroy short-distance Hertzian
+telegraphy, or that the multiplication of long-distance stations will
+end in the annihilation of all Hertzian wave telegraphy. No one,
+however, can contemplate the history of any development of applied
+science without seriously taking to heart the lesson that the
+obstacles which arise and which prove serious in any engineering
+undertaking are never those which occur to armchair critics. Sometimes
+the seemingly impossible proves the most easy to accomplish, whilst
+difficulties of a formidable nature often spring up where least
+expected.
+
+The long-distance transmission is a matter of peculiar interest to the
+author of these articles, because he was at an early stage in
+connection with it invited to render Mr. Marconi assistance in the
+matter.[71] The particular work entrusted to him was that of planning
+the electrical engineering arrangements of the first power station
+erected for the production of electric waves for long-distance
+Hertzian wave telegraphy at Poldhu, in Cornwall. When Mr. Marconi
+returned from the United States in the early part of 1900, he had
+arrived at the conclusion that the time had come for a serious attempt
+to accomplish wireless telegraphy across the Atlantic. Up to that date
+the project had been an inventor's dream, much discussed, long
+predicted, but never before practically taken in hand. The only
+appliances, moreover, which had been used for creating Hertzian waves
+were induction coils or small transformers, and the greatest distance
+covered, even by Mr. Marconi himself, had been something like 150
+miles over sea. Accordingly, to grapple with the difficulty of
+creating an electric wave capable of making itself felt at a distance
+of 3,000 miles, even with the delicate receiving appliances invented
+by Mr. Marconi, seemed to require the means of producing at least four
+hundred times the wave-energy that had been previously employed. The
+author was, therefore, requested to prepare plans and specifications
+for an electric generating plant for this purpose, which would enable
+electrical oscillations to be set up in an aerial on a scale never
+before accomplished.
+
+This work involved, not merely the ordinary experience of an
+electrical engineer, but also the careful consideration of many new
+problems and the construction of devices not before used. Every step
+had to be made secure by laboratory experiments before the
+responsibility could be incurred of advising on the nature of the
+machinery and appliances to be ordered. Many months in the year 1901
+were thus occupied by the author in making small-scale experiments in
+London and in superintendence of large-scale experiments at the site
+of the first power station at Poldhu, near Mullion, in Cornwall,
+before the plant was erected and any attempt was made by Mr. Marconi
+to commence actual telegraphic experiments. As this work was of a
+highly confidential nature, it is obviously impossible to enter into
+the details of the arrangements, either as made by the writer in the
+first instance, or as they have been subsequently modified by Mr.
+Marconi. The design of the aerial and of the oscillation transformers
+and many of the details in the working appliances are entirely due to
+Mr. Marconi, but as a final result, a power plant was erected for the
+production of Hertzian waves on a scale never before attempted. The
+utilisation of 50 H.P. or 100 H.P. for electric wave production has
+involved dealing with many difficult problems in electrical
+engineering, not so much in novelty of general arrangement as in
+details. It will easily be understood that Leyden jars, spark balls
+and oscillators, which are quite suitable for use with an induction
+coil, would be destroyed immediately if employed with a large
+alternating-current plant and immensely powerful transformers.
+
+[Illustration: FIG. 26.--WOODEN TOWERS SUPPORTING THE MARCONI AERIAL
+AT POLDHU POWER STATION, CORNWALL, ENGLAND.]
+
+In the initial experiments with this machinery and in its first
+working there was very considerable risk, owing to its novel and
+dangerous nature; but throughout the whole of the work from the very
+beginning, no accident of any kind has taken place, so great have been
+the precautions taken. The only thing in the nature of a mishap was
+the collapse of a ring of tall masts, erected in the first place to
+sustain the aerial wires, but which now have been replaced by four
+substantial timber towers, 215 feet in height, placed at the corners
+of a square, 200 feet in length. These four towers sustain a conical
+arrangement of insulated wires (see Fig. 26) which can be used in
+sections and which constitute the transmitting radiator or receiver,
+as the case may be. Each of these wires is 200 feet in length and
+formed of bare stranded wire.
+
+At the outset, there was much uncertainty as to the effect of the
+curvature of the earth on the propagation of a Hertzian wave over a
+distance of many hundreds of miles. In the case of the Atlantic
+transmission between the station at Poldhu in Cornwall and that at
+Cape Cod in Massachusetts, U.S.A., we have two stations separated by
+about 45 degrees of longitude on a great circle, or one-eighth part of
+the circumference of the world. In this case, the versine of the arc
+or height of the sea at the half-way point above the straight line or
+chord joining the two places is 300 miles.
+
+The question has recently attracted the attention of several eminent
+mathematical physicists. The extent to which a free wave propagated in a
+medium bends round any object or is diffracted depends on the relation
+between the length of the wave and the size of the object. Thus, for
+instance, an object the size of an orange held just in front of the
+mouth does not perceptibly interfere with the propagation of the waves
+produced by the speaking or singing voice, because these are from two to
+six feet in length: but if arrangements are made by means of a Galton
+whistle to produce air waves half an inch in length, then an obstacle
+the size of an orange causes a very distinct acoustic shadow. The same
+thing is true of waves in the ether. The amount of bending of light
+waves round material objects is exceedingly small, because the average
+length of light waves is about one-fifty-thousandth part of an inch. In
+the case of Hertzian wave telegraphy, we are, however, dealing with
+ether waves many hundreds of feet in length, and the waves sent out from
+Poldhu have a wave-length of a thousand feet or more, say, one-fifth to
+one-quarter of a mile. The distance, therefore, between Poldhu and Cape
+Cod is only at most about twelve thousand wave-lengths, and stands in
+the same relation to the length of the Hertzian wave used as does a body
+the diameter of a pea to the wave-length of yellow light. There is
+unquestionably a large amount of diffraction or bending of the electric
+wave round the earth, and, proportionately speaking, it is larger than
+in the case of light waves incident on objects of the same relative
+size.
+
+Quite recently Mr. H. M. Macdonald (see _Proc._ Roy. Soc., London,
+Vol. LXXI., p. 251) has submitted the problem to calculation, and has
+shown that the power required to send given electric waves 3,000 miles
+along a meridian of the earth is greater than would be required to
+send them over the same distance if the sea surface were flat in the
+ratio of 10 to 3. Hence the rotundity of the earth does introduce a
+very important reduction factor, although it does not inhibit the
+transmission. Mr. Macdonald's mathematical argument has, however, been
+criticised by Lord Rayleigh and by M. H. Poincare (see _Proc._ Roy.
+Soc., Vol. LXXII., p. 40, 1903).
+
+The accomplishment of very long distances by Hertzian wave telegraphy
+is, however, not merely a question of power, it is also a question of
+wave-length. Having regard, however, to the possibility that the
+propagation which takes place in Hertzian wave telegraphy is not that
+simply of a free wave in space, but the transmission of a semi-loop of
+electric strain with its feet tethered to the earth, it is quite
+possible that if it were worth while to make the attempt, an ether
+disturbance could be made in England sufficiently powerful to be felt
+in New Zealand.
+
+Leaving, however, these hypothetical questions and matters of pure
+conjecture, we may consider some of the facts which have resulted from
+Mr. Marconi's long-distance experiments. One of the most interesting
+of these is the effect of daylight upon the wave propagation. In one
+of his voyages across the Atlantic, when receiving signals from Poldhu
+on board the S.S. _Philadelphia_, he noticed that the signals were
+received by night when they could not be detected by day.[72] In these
+experiments Mr. Marconi instructed his assistants at Poldhu to send
+signals at a certain rate from 12 to 1 a.m., from 6 to 7 a.m., from 12
+to 1 p.m., and from 6 to 7 p.m., Greenwich mean time, every day for a
+week. He has stated that on board the _Philadelphia_ he did not notice
+any apparent difference between the signals received in the day and
+those received at night until after the vessel had reached a distance
+of 500 statute miles from Poldhu. At distances of over 700 miles, the
+signals transmitted during the day failed entirely, while those sent
+at night remained quite strong up to 1,551 miles, and were clearly
+decipherable up to a distance of 2,099 miles from Poldhu. Mr. Marconi
+also noted that at distances of over 700 miles, the signals at 6 a.m.,
+in the week between February 23 and March 1, were quite clear and
+distinct, whereas by 7 a.m. they had become weak almost to total
+disappearance. This fact led him at first to conclude that the cause
+of the weakening was due to the action of the daylight upon the
+transmitting aerial, and that as the sun rose over Poldhu, so the wave
+energy radiated, diminished, and he suggested as an explanation the
+known fact of the dissipating action of light upon a negative charge.
+
+Although the facts seem to support this view, another explanation may
+be suggested. It has been shown by Professor J. J. Thomson that
+gaseous ions or electrons can absorb the energy of an electric wave,
+if present in a space through which waves are being transmitted.[73]
+If it be a fact, as suggested by Professor J. J. Thomson, that the sun
+is projecting into space streams of electrons, and if these are
+continually falling in a shower upon the earth, in accordance with the
+fascinating hypothesis of Professor Arrhenius, then that portion of
+the earth's atmosphere which is facing the sun will have present in it
+more electrons or gaseous ions than that portion which is turned
+towards the dark space, and it will therefore be less transparent to
+long Hertzian waves.[74] In other words, clear sunlit air, though
+extremely transparent to light waves, acts as if it were a slightly
+turbid medium for long Hertzian waves. The dividing line between that
+portion of the earth's atmosphere which is impregnated with gaseous
+ions or electrons is not sharply delimited from the part not so
+illuminated, and there may be, therefore, a considerable penetration
+of these ions into the regions which I may call the twilight areas.
+Accordingly, as the earth rotates, a district in which Hertzian waves
+are being propagated is brought, towards the time of sunrise, into a
+position in which the atmosphere begins to be ionised, although far
+from as freely as is the case during the hours of bright sunshine.
+
+Mr. Marconi states that he has found a similar effect between inland
+stations, signals having been received by him during the night between
+Poldhu and Poole with an aerial the height of which was not sufficient
+to receive them by day. It has been found, however, that the effect
+simply amounts to this, that rather more power is required by day than
+by night to send signals by Hertzian waves over long distances.
+
+Some interesting observations have also been made by Captain H. B.
+Jackson, R.N.,[75] on the influence of various states of the
+atmosphere upon Hertzian wave telegraphy. These experiments were all
+made between ships of the British Royal Navy, furnished with Hertzian
+wave telegraphy apparatus on the Marconi system. Some of his
+observations concerned the effect of the interpositon of land between
+two ships. He found that the interposition of land containing iron
+ores reduced the signalling distances, compared with the maximum
+distance at open sea, to about 30 per cent. of the latter; whilst hard
+limestone reduced it to nearly 60 per cent. and soft sandstone or
+shale to 70 per cent. These results show that there is a considerable
+absorption effect when waves of certain wave-length pass through or
+over hard rocks containing iron ores. It would be interesting to know,
+however, whether this reduction was in any degree proportional to the
+dryness or moisture of the soil. Earth conductivity is far more
+dependent upon the presence or absence of moisture than upon the
+particular nature of the material which composes it other than water.
+
+The observations of Captain Jackson, however, only confirm the already
+well-known fact that Hertzian waves, as employed in the Marconi system
+of wireless telegraphy, within a certain range of wave-length, are
+considerably weakened by their passage through land, over land or
+round land. In some cases he noticed that quite sharp electric shadows
+were produced by rocky promontories projecting into the line of
+transmission. His attention was also directed (_loc. cit._) to the
+more important matter of the effect of atmospheric electrical
+conditions upon the transmission. The effect of all lightning
+discharges, whether visible or invisible, is to make a record on the
+telegraphic receiver. On the approach of an atmospheric electrical
+disturbance towards the receiving station on a ship, the first visible
+indications generally are the recording of dots at intervals from a
+few minutes to a few seconds on the telegraphic tape. Captain Jackson
+states that the most frequent record is that of three dots, the first
+being separated from the other two by a slight interval like the
+letters E I on the Morse code, and this is the sign most frequently
+recorded by distant lightning. But in addition to this, dashes are
+recorded and irregular signs, which, however, sometimes spell out
+words in the Morse code. He noted that these disturbances are more
+frequent in summer and autumn than in winter and spring, and in the
+neighbourhood of high mountains more than in the open sea. In settled
+weather, if present, they reach their maximum between 8 p.m. and 10
+p.m., and frequently last during the whole of the night, with a
+minimum of disturbance between 9 a.m. and 1 p.m. Another important
+matter noted by Captain Jackson is the shorter distance at which
+signals can usually be received when any electrical disturbances are
+present in the atmosphere, compared with the distance at which they
+can be received when none are present. This reduction in signalling
+distance may vary from 20 to 70 per cent, of that obtainable in fine
+weather. It does not in any way decrease with the number of lightning
+flashes, but rather the reverse, the loss in signalling distance
+generally preceding the first indications on the instrument of the
+approaching electrical disturbance. It is clear that these
+observations fit in very well with the theory outlined above, viz.,
+that the atmosphere when impregnated with free electrons or
+negatively-charged gaseous ions is more opaque to Hertzian waves than
+when they are absent. Captain Jackson gives an instance of ships whose
+normal signalling distance was 65 miles, failing to communicate at 22
+miles when in the neighbourhood of a region of electrical disturbance.
+These effects in the case of wireless telegraphy have their parallel
+in the disturbances caused to telegraphy with wires by earth currents
+and magnetic storms.
+
+Another effect which he states reduces the usual maximum signaling
+distance is the presence of material particles held in suspension by
+the water spherules in moist atmosphere. The effect has been noticed
+in the Mediterranean Sea when the sirocco wind is blowing. This is a
+moist wind conveying dust and salt particles from the African coast. A
+considerable reduction in signalling distance is produced by its
+advent.
+
+Another interesting observation due to Captain Jackson is the
+existence of certain zones of weak signals. Thus, for instance, two
+ships at a certain distance may be communicating well; if their
+distance increases, the signalling falls off, but is improved again at
+a still greater distance. He advances an ingenious theory to show that
+this fact may be due to the interference between two sets of waves
+sent out by the transmitter having different wave-lengths.
+
+Finally, in the Paper referred to, he emphasises the well-known fact
+that long-distance signalling can only be accomplished by the aid of
+an aerial wire and a "good earth." Summing up his results, he
+concludes: (1) That intervening land of any kind reduces the practical
+signalling distance between two ships or stations, compared with that
+which would be obtainable over the open sea, and that this loss in
+distance varies with the height, thickness, contour, and nature of the
+land; (2) material particles, such as dust and salt, held in
+suspension in a moist atmosphere also reduce the signalling distance,
+probably by dissipating and absorbing the waves; (3) that electrical
+disturbances in the atmosphere also act most adversely in addition to
+affecting the receiving instrument and making false signals or
+_strays_, as they are called; (4) that with certain forms of
+transmitting arrangement, interference effects may take place which
+have the result of creating certain areas of silence very similar to
+those which are observed in connection with sound signals from a
+siren.
+
+It is clear, therefore, from all the above observations, that
+Hertzian-wave telegraphy taking place through the terrestrial
+atmosphere is not by any means equivalent to the propagation of a wave
+in free or empty space; and that just as the atmosphere varies in its
+opacity to rays of light, sometimes being clear and sometimes clouded,
+so it varies from time to time in transparency to Hertzian waves, the
+cause of this variation in transparency probably being the presence in
+the atmosphere of negatively-charged corpuscles or electrons. If there
+are present in the atmosphere at certain times "clouds of electrons"
+or "electronic fogs," these may have the effect of producing a certain
+opacity, or rather diminution in transparency to Hertzian waves, just
+as water particles do in the case of sunlight.
+
+We may, therefore, in conclusion, review a few of the outstanding
+problems awaiting solution in connection with Hertzian wave wireless
+telegraphy. In spite of the fact that this new telegraphy has not been
+accorded a very hearty welcome by the representatives of official or
+established telegraphy in Great Britain, it has reached a point,
+unquestionably owing to Mr. Marconi's energy and inventive power, at
+which it is bound to continue its progress. But that progress will not
+be assisted by shutting our eyes to facts. Many problems of great
+importance remain to be solved. We have not yet reached a complete
+solution of all the difficulties connected with isolation of stations.
+In the next place, the question of localising the source of the
+signals and waves is most important. Our kumascopes and receiving
+appliances at present are like the rudimentary eyes of the lower
+organisms, which are probably sensitive to mere differences in light
+and darkness, but which are not able to _see_ or _visualise_, in the
+sense of locating the direction and distance of a radiating or
+luminous body. Just as we have, as little children, to learn to see,
+so a similar process has to be accomplished in connection with
+Hertzian telegraphy, and the accomplishment of this does not seem by
+any means impossible or even distant. We are dealing with
+hemispherical waves of electric and magnetic force, which are sent out
+from a certain radiating centre, and in order to localise that centre
+we have to determine the position of the plane of the wave and also
+the curvature of the surface at the receiving point. Something,
+therefore, equivalent to a range finder in connection with light is
+necessary to enable us to locate the distance and the direction of the
+radiant point.
+
+Lastly, there are important improvements possible in connection with
+the generation of the waves themselves. At the present moment, our
+mode of generating Hertzian waves involves a dissipation of energy in
+the form of the light and heat of the spark. Just as in the case of
+ordinary artificial illuminants, such as lamps of various kinds, we
+have to manufacture a large amount of ether radiation of long wave
+length, which is of no use to us for visual purposes--in fact,
+creating ninety-five per cent, of dark and useless waves for every
+five per cent. of luminous or useful waves--so in connection with
+present methods of generating Hertzian waves, we are bound to
+manufacture by the discharge spark a large amount of light and heat
+rays which are not wanted, in order to create the Hertzian waves we
+desire. It is impossible yet to state precisely what is the
+efficiency, in the ordinary sense of the word, of a Hertzian wave
+radiator; how much of the energy imparted to the aerial falls back
+upon it and contributes to the production of the spark, and how much
+is discharged into the ether in the form of a wave.
+
+Nothing is more remarkable, however, than the small amount of energy
+which, if properly utilised in electric wave making, will suffice to
+influence a sensitive receiver at a distance of even one or two
+hundred miles. Suppose, for instance, that we charge a condenser
+consisting of a battery of Leyden jars, having a capacity of one
+seventy-fifth of a microfarad, to a potential of 15,000 volts; the
+energy stored up in this condenser is then equal to 1.5 joules, or a
+little more than one foot-pound. If this energy is discharged in the
+form of a spark five millimetres in length through the primary coil of
+an oscillation transformer, associated with an aerial 150 feet in
+height, the circuits being properly tuned by Mr. Marconi's method,
+then such an aerial will affect, as he has shown, one of Mr. Marconi's
+receivers, including a nickel silver filings coherer tube, at a
+distance of over two hundred miles over sea. Consider what this means.
+The energy stored up in the Leyden jars cannot all be radiated as wave
+energy by the aerial, probably only half of it is thus radiated. Hence
+the impartation to the ether at any one locality of about half a
+foot-pound of energy in the form of a long Hertzian wave is sufficient
+to affect sensitive receivers situated at any point on the
+circumference of a circle of 200 miles radius described on the open
+sea. Hertzian wave telegraphy is sometimes described as being
+extravagant in power, but, as a matter of fact, the most remarkable
+thing about it is the small amount of power really involved in
+conducting it. On the other hand, Hertzian wave manufacture is not
+altogether a matter of power. It is much more dependent upon the
+manner in which the ether is struck. Just as half an ounce of dynamite
+in exploding may make more noise than a ton of gunpowder, because it
+hits the air more suddenly, so the formation of an effective wave in
+the ether is better achieved by the right application of a small
+energy than by the wrong mode of application of a much larger amount.
+If we translate this fact into the language of electronic theory, it
+amounts simply to this. It is the electron alone which has a grip of
+the ether. To create an ether wave, we have to start or stop crowds of
+electrons very suddenly. If in motion, their motion implies energy,
+but it is not only their energy which is concerned in the wave making,
+but the acceleration, positive or negative--_i.e._, the quickness with
+which they are started or stopped. It is possible we may discover in
+time a way of manufacturing long ether waves without the use of an
+electric spark, but at present we know only one way of doing
+this--viz., by the discharge of a condenser, and in the discharge of
+large condensers of very high potentials it is difficult to secure
+that extreme suddenness of starting the discharge which we can do in
+the case of smaller capacities and voltages.
+
+How strange it is that the discharge of a Leyden jar studied so
+profoundly by Franklin, Henry, Faraday, Maxwell, Kelvin and Lodge
+should have become an electrical engineering appliance of great
+importance!
+
+Whilst there are many matters connected with the commercial aspect of
+Hertzian wave telegraphy with which we are not here concerned, there
+is one on which a word may properly be said. The ability to
+communicate over long distances by Hertzian waves is now demonstrated
+beyond question, and even if all difficulties are not overcome at
+once, it has a field of very practical utility, and may even become of
+national importance. Under these circumstances, we may consider
+whether it is absolutely necessary to place the signalling stations so
+near the coast. The greater facility of transmission over sea has
+already been discussed and explained, but in time of war, the masts
+and towers which are essential at present in connection with
+transmitting stations could be wrecked by shot or shell from an
+enemy's battleship at a distance of five or six miles out at sea, and
+would certainly be done within territorial waters. Should not this
+question receive attention in choosing the location of important
+signalling stations? For if they can, without prejudice to their use,
+be placed inland by a distance sufficient to conceal them from sight,
+their value as a national asset in time of war might be greatly
+increased.
+
+It has been often contended that whilst cables could be cut in time of
+war no one can cut the ether; but wireless telegraph stations in
+exposed situations on high promontories, where they are visible for
+ten to fifteen miles out at sea and undefended by any forts, could
+easily be destroyed. The great towers which are essential to carry
+large aerials are a conspicuous object for ten miles out at sea; and a
+single well-placed shell from a six-inch gun would wreck the place and
+put the station completely out of use for many months. Hence if
+oceanic telegraphy is ever to be conducted in a manner in which the
+communication will be inviolable or, at any rate, not be capable of
+interruption by acts of war, the careful selection of the sites for
+stations is a matter of importance. A small station consisting of a
+single 150-foot mast and a wooden hut can easily be removed or
+replaced, but an expensive power station, the mere aerial of which may
+cost several thousand pounds, is not to be put up in a short time.[76]
+
+Meanwhile, whatever may be the future achievements of this new
+_supermarine_ wireless telegraphy conducted over long distances, there
+can be no question as to its enormous utility and present value for
+intercommunication between ships on the ocean and ships and the shore.
+At the present time, there are some forty or more of the transatlantic
+ocean liners and many other ships equipped with this Hertzian wave
+wireless telegraph apparatus on the Marconi system. Provided with this
+latest weapon of applied science, they are able to chat with one
+another, though a hundred miles apart on the ocean, with the ease of
+guests round a dinner table, to exchange news or make demands for
+assistance.
+
+    Ships that pass in the night, and speak each other in passing--
+    Only a signal shown, and a distant voice in the darkness;
+    So, on the ocean of life, we pass and speak one another,
+    Only a look and a voice, then darkness again, and a silence.
+
+Abundant experience has been gathered to show the inexpressible value
+of this means of communication in case of accident, and it can hardly
+be doubted that before long the possession of this apparatus on board
+every passenger vessel will be demanded by the public, even if not
+made compulsory. Although the privacy of an ocean voyage may have been
+somewhat diminished by this utilisation of ether waves, there is a
+vast compensation in the security that is thereby gained to human life
+and property by this latest application of the great energies of
+nature for the use and benefit of mankind.
+
+GEO. TUEKER, PRINTER, SALISBURY COURT, FLEET STREET, LONDON.
+
+[1] This series of articles is based on the Cantor Lectures delivered
+before the Society of Arts, London, in March, 1903. The lectures were
+attended by many of the leading British scientific men and electrical
+engineers, and attracted wide attention as the most complete and
+authoritative statement hitherto made of wireless telegraphy. In writing
+the articles for the "Popular Science Monthly," the author has omitted
+advanced technicalities in order that the substance may be suitable for
+the general reader.--EDITOR.
+
+[2] For a more detailed account of this hypothesis, the reader is
+referred to an article by the present writer, entitled "The Electronic
+Theory of Electricity," published in the "Popular Science Monthly" for
+May, 1902.
+
+[3] See J. J. Thomson, "Recent Researches in Electricity and Magnetism,"
+chap. I., p. 16.
+
+[4] See O. Heaviside, "Electromagnetic Theory," Vol. I., p. 54.
+
+[5] Wiedemann's _Annalen_, 36, p. 1, 1889; or in his republished Papers,
+"Electric Waves," p. 137, English translation by D. E. Jones.
+
+[6] The fraction 7/22 here denotes a stranded wire formed of seven
+strands, each single wire having a diameter expressed by the number 22
+on the British standard wire gauge.
+
+[7] G. Marconi, "Syntonic Wireless Telegraphy," _Journal_ of the Society
+of Arts, Vol. XLIX., p. 501, 1901.
+
+[8] Instruction for the manufacture of large induction coils may be
+obtained from a "Treatise on the Construction of Large Induction Coils,"
+by A. T. Hare. (Methuen & Co., London.)
+
+Also see Vol. II. of "The Alternate-Current Transformer," by J. A.
+Fleming, chap. I. ("The Electrician" Printing and Publishing Co., 1, 2
+and 3, Salisbury-court, Fleet-street, London, E.C.)
+
+[9] See "The Alternate-Current Transformer," by J. A. Fleming. Vol. I.,
+p. 184.
+
+[10] Du Moncel states that MacGauley of Dublin independently invented
+the form of hammer break as now used. See "The Alternate-Current
+Transformer," Vol. II. chap. I. J. A. Fleming.
+
+[11] See Professor J. Trowbridge, "On the Induction Coil" _Phil. Mag._,
+April, 1902 Vol. III., Series 6, p. 393.
+
+[12] See Dr. Wehnelt's article in the _Elektrotechnische Zeitschrift_,
+January, 1899.
+
+[13] See _The Electrician_, Vol. XLII., 1899, pp. 721, 728, 731, 732 and
+841; communications from Mr. Campbell Swinton, Professor S. P. Thompson,
+Dr. Marchant, the author and others; also p. 864, same volume, for a
+leader on the subject; also p. 870, letters by M. Blondel and Professor
+E. Thomson. See also _The Electrician_, Vol. XLIII., p. 5, 1899,
+extracts from a Paper by P. Barry; _Comptes Rendus_, April, 1899. See
+also the _Electrical Review_, Vol. XLIV., p. 235, 1899, February 17.
+
+[14] See _The Electrician_, Vol. XLII., 1899.
+
+[15] For a discussion of the function of the condenser in an ordinary
+induction coil, see "The Alternate-Current Transformer," by J. A.
+Fleming. Vol. II., p. 51.
+
+[16] See Lord Rayleigh, _Phil. Mag._, December, 1901.
+
+[17] It has sometimes been stated that the spark balls must be _solid_
+metal and no hollow, but this is a fallacy, and has been disproved by
+Mr. C. A. Chant. See "An Experimental Investigation into the Skin Effect
+in Electrical Oscillators," _Phil. Mag._, Vol. III., Sec. 6, p. 425,
+1902.
+
+[18] See _Proc._ Roy. Soc., London, February 23 and April 12, 1860; or
+reprint of Papers on electrostatics and magnetism, p. 247.
+
+[19] See _Phil. Mag._, August, 1902, Vol. IV., p. 224, 6th Series. Mr.
+Jervis-Smith has also described an experiment to show how much the use
+of compressed air round a spark gap is of advantage in working an
+ordinary Tesla coil. In his British specification, No. 12,039 of 1896,
+Mr. Marconi had long previously mentioned the use of compressed air
+round the spark gap.
+
+[20] This energy storage is at the rate of 44 foot-pounds per cubic foot
+of glass. This figure shows what a relatively small amount of energy is
+capable of being stored up in the form of electric strain in glass. In
+the case of an air condenser, it is only stored at the rate of 1
+foot-pound per cubic foot.
+
+[21] See British specification No. 7,777 of 1900.--G. Marconi.
+"Improvements in Apparatus for Wireless Telegraphy."
+
+[22] That this number really does represent the order of this
+oscillation frequency in an aerial has been shown by C. Tissot, _Comptes
+Rendus_, 132, p. 763, March 25, 1901, by photographs taken of the
+oscillatory spark of a Hertzian wave telegraphic transmitter. (See
+_Science Abstracts_, Vol. IV., Abs. 1,518.) He found frequencies from
+0.5 million to 1.6 million.
+
+[23] The term "jigger" is one of those slang terms which contrive to
+effect a permanent attachment to various arts and crafts. Similarly, the
+word "booster" is now used for a step-up or voltage-raising transformer
+or dynamo, inserted in series with an electric supply main. The word
+"boost" is a slang term signifying to raise or lift up. "To give a real
+good boost" is an expression for lending a helping hand. The term
+"jigger," in the same manner, is an adaptation of a seaman's term for
+hoisting tackle or lift.
+
+[24] The "earth" itself probably only conducts electrolytically. All
+such materials as sand, clay, chalk, etc., and most surface soils are
+fairly good insulators when very dry, but conduct in virtue of moisture
+present in them.
+
+[25] _The Electrician_, Vol. XL., p. 86 (leader).
+
+[26] British Patent Specification, C. and S. A. Varley, No. 165, 1866.
+
+[27] See also _Journal de Physique_, Vol. V., p. 573, 1886.
+
+[28] See _Comptes Rendus_, Vol. CXI., p. 785; Vol. CXII., p. 112, 1891;
+or _La Lumiere Electrique_, Vol. XL., pp. 301, 506, 1891; or _The
+Electrician_, Vol. XXVII., 1891, pp. 221, 448.
+
+[29] See _The Electrician_, Vol. XXIX., 1892, pp. 397 and 432.
+
+[30] Mr. W. B. Croft, _Proc._ Phys. Soc., Vol. XII., p. 421. Report of
+meeting on October 27, 1893.
+
+[31] See Professor Minchin, _Proc._ Phys. Soc., November 24, 1893; or
+_The Electrician_, Vol. XXXII., 1893, p. 123. See also Professor
+Minchin, _Phil Mag._, January, 1894, Vol. XXXVII., p. 90, "On the Action
+of Electromagnetic Radiation on Films containing Metallic Powders."
+
+[32] This lecture was afterwards published as a book, the first edition
+bearing the same title as the lecture--viz., "The Work of Hertz and Some
+of His Successors." In the second edition, published in 1898, an
+appendix was added (p. 59) containing "The History of the Coherer
+Principle," and the original title of the work had prefixed to it
+"Signalling Without Wires."
+
+[33] See _The Electrician_, Vol. XXVII., p. 222, 1891. E. Branly,
+"Variations of Conductivity under Electrical Influence."
+
+[34] See _The Electrician_, Vol. XL., p. 90. Sir Oliver Lodge, "The
+History of the Coherer Principle."
+
+[35] See Professor E. Branly, "A Sensitive Coherer," _Comptes Rendus_,
+Vol. CXXXIV., p. 1,187, 1902; or _Science Abstracts_, Vol. V., p. 852,
+1902.
+
+[36] This device of making the inter-electrode gap in a tubular filings
+coherer wedge-shaped has been patented again and again by various
+inventors. See German patent No. 116,113, Class 21a, 1900. It has also
+been claimed by M. Tissot.
+
+[37] See _The Electrician_, Vol. XXVII., 1891, p. 448.
+
+[38] _Journal_ of the Russian Physical and Chemical Society, Vol.
+XXVIII., Division of Physics, Part I., January, 1896.
+
+[39] See British Patent Specification No. 12,039, June 2, 1896.
+
+[40] British Patent Specification No. 19,710 of 1899.
+
+[41] _Comptes Rendus._, Vol. CXXVIII., p. 1,225, 1889; _Science
+Abstracts_, Vol. II., p. 521.
+
+[42] _Il Nuovo Cimento_, Vol. X., p. 279, 1899.
+
+[43] _Wied Ann._, Vol. LXVIII., p. 594, 1899; _Science Abstracts_, Vol.
+II., p. 757.
+
+[44] _Comptes Rendus_, Vol. CXXX., p. 902, 1900; _Science Abstracts_,
+Vol. III., p. 615.
+
+[45] See _Proc._ Roy. Soc., London, Vol. LXXI., p. 402.
+
+[46] See Report by Capt. Quintino Bonomo, "Telegrafia Senza Fili," Rome,
+1902; _L'Elettricista_, Ser. II., Vol. I., pp. 118, 173.
+
+[47] See Royal Institution, Friday evening discourse, by Mr. Marconi,
+June 13, 1902; also _The Electrician_, Vol. XLIX., p. 490; also a letter
+to _The Times_ of July 3, 1902, by the Marchese Luigi Solari.
+
+[48] See U.S.A. Patent Specification No. 700,161, May 24, 1900.
+
+[49] See E. Marx, _Phys. Zeitschrift_, Vol. II., p. 249; _Science
+Abstracts_, Vol. IV., p. 471. See also German Patent Specification No.
+121,663, Class 21a.
+
+[50] See "The Scientific Writings of Professor Joseph Henry."
+
+[51] _Phil. Trans._ Roy. Soc., London, 1897, Vol. CLXXXIX.A, p. 1.
+
+[52] See _Proc._ Roy. Soc., London, June 12, 1902. "Note on a Magnetic
+Detector for Electric Waves which can be employed as a Receiver for
+Space Telegraphy," by G. Marconi.
+
+[53] See U.S.A. Patent Specification No. 716,000, Application of July 5,
+1901.
+
+[54] See the _Electrical Review_, Vol. XLIV., 1899, May 26; _Wied Ann._,
+Vol. LXVIII., p. 92; or German Patent Specification No. 107,843.
+
+[55] U.S.A. Patent Specification No. 706,742, 1902.
+
+[56] See British Patent Specification, G. Marconi, No. 12,039, June 2,
+1896.
+
+[57] See G. Marconi, British Patent Specification No. 12,326, of June 1,
+1898.
+
+[58] See the _Electrical Review_, September 26, 1902, Vol. LI., p. 543.
+
+[59] There is a good deal of contradiction between various inventors on
+this point, some saying that "earthed" aerials obviate atmospheric
+electrical disturbances, and others that insulated aerials are in this
+respect superior. The truth appears to be that, neither form is
+absolutely free from risk of disturbance by this cause.
+
+[60] The capacity of an electrical circuit corresponds to the elastic
+pliability, or what is commonly called the elasticity, of a material
+substance, and the inductance to mass or inertia. Hence capacity and
+inductance are qualities of an electric circuit which are analogous to
+the elasticity and inertia of such a body as a heavy spring.
+
+[61] See Cantor Lectures, on "Electrical Oscillations and Electric
+Waves," delivered before the Society of Arts, London, November 26,
+December 4, 10, 17, 1900. Lecture I., p. 12, of reprint.
+
+[62] A fuller account of these experiments was given by the author in a
+letter to the London _Times_ published on April 14, 1903.
+
+[63] See _Journal_ of the Society of Arts, Vol. XLIX., p. 505. "Syntonic
+Wireless Telegraphy," by G. Marconi.
+
+[64] See German Patent Specifications, Class 21a, No. 7,452 of 1900, and
+also No. 8,087 of 1901.
+
+[65] See German Patent Specification, Class 21a, No. 7,498 of 1900,
+applied for November 9, 1900. The above-mentioned patent is subsequent
+in date to Mr. Marconi's experiments on the same subject.
+
+[66] See _The Electrician_, January 18, 1900, Vol. XLVI., p. 475. Also
+reprint of a Paper of Professor A. Slaby, "Abgestimmte und mehrfache
+Funkentelegraphie."
+
+[67] See British Specification No. 11,348 of 1901.
+
+[68] See _Comptes Rendus_, May 21, 1900; Rapports du Congres
+International d'Electricite, Paris, 1900, p. 341.
+
+[69] See _The Electrician_, Vol. XLVI., p. 573, February 8, 1901.
+
+[70] See _The Electrician_, Vol. L., p. 418, January 2, 1903.
+
+[71] See Mr. Marconi's Friday evening discourse at the Royal
+Institution, June 13, 1902; also _The Electrician_, Vol. XLIX., p. 390.
+
+[72] See _Proc._ Roy. Soc., June 12, 1902. "A Note on the Effect of
+Daylight upon the Propagation of Electromagnetic Impulses over Long
+Distances," by G. Marconi.
+
+[73] See _Phil. Mag._, Vol. IV., p. 253, Series 6, August, 1902. J. J.
+Thomson, "On Some Consequences of the Emission of Negatively-electrified
+Corpuscles by Hot Bodies."
+
+[74] The opinion that ionisation of the air by sunlight is a cause of
+obstruction to Hertzian waves propagated over long distances has also
+been expressed by Mr. J. E. Taylor. See _Proc._ Roy. Soc., Vol. LXXI.,
+p. 225, 1903. "Characteristics of Earth Current Disturbances and their
+Origin."
+
+[75] See _Proc._ Roy. Soc., May 15, 1902. "On Some Phenomena affecting
+the Transmission of Electric Waves over the Surface of the Sea and
+Earth," by Captain H. B. Jackson, R.N., F.R.S.
+
+[76] Mr. Marconi has informed the writer that these strategic questions
+have received attention in selecting the sites for large Marconi power
+stations in Italy.
+
+                  *       *       *       *       *
+
+ [Detailed Transcriber's Notes
+
+ The text has been made to match the original text as much as possible
+ retaining all apparent printer's errors and inconsistencies. The
+ following, detail the apparent printer's errors etc. identified in
+ the original text.
+
+ Variation in spelling, Strasburg and Strassburg for Strasbourg.
+
+ There are a number of inconsistencies in hyphenation present in the
+ original text. Those concerned with the variation between one word or
+ a hyphenated word are detailed below. Those concerned with the
+ variation between multiple words and hyphenated words are too
+ numerous to detail individually.
+
+ Inconsistent hyphenation of word, 'anti-node' and 'antinode' both
+ present in original text.
+
+ Inconsistent hyphenation of word, 'electro-dynamic' and
+ 'electrodynamic' both present in original text.
+
+ Inconsistent hyphenation of word, 'horse-shoe' and 'horseshoe' both
+ present in original text.
+
+ Inconsistent hyphenation of word, 'over-blowing' and 'overblowing'
+ both present in original text.
+
+ Page 5, possible printer's error, a for at, 'consisting when a rest'.
+
+ Page 6, printer's error, comma rather than full stop at end of
+ sentence, 'ether constituting electric radiation,'.
+
+ Page 10, printer's error, millmetre for millimetre, 'three thousand
+ volts per millmetre,'.
+
+ Page 13, possible printer's error, set for sets, 'there are three set
+ of phenomena'.
+
+ Page 13, printer's error, duplicate word, 'detached and and travel
+ away.'.
+
+ Page 13, brackets added to in-line equation to aid clarity, 'F =
+ (3/8)CV^{2}/10^{6}.'.
+
+ Page 13, both equations originally multi-line fraction, rendered into
+ one line for clarity.
+
+ Page 15, both equations originally multi-line fraction, rendered into
+ one line for clarity.
+
+ Page 22, printer's error, correponding for corresponding,
+ 'correponding to this frequency'.
+
+ Page 22, printer's error, consist for consists, 'due to Braun, consist
+ of attaching'.
+
+ Page 24, printer's error, one-hundreth for one-hundredth, 'capacity of
+ one-hundreth of a microfarad,'.
+
+ Page 28, printer's error, missing full stop at end of sentence added,
+ 'in the case of the hammer break.'.
+
+ Page 33, printer's error, supppse for suppose, 'Let us supppse'.
+
+ Page 44, equation originally multi-line fraction, rendered into one
+ line for clarity.
+
+ Page 46, printer's error, comma rather than full stop at end of
+ sentence, 'to the transmitting aerial,'.
+
+ Page 48, possible printer's error, alterations for alternations,
+ 'alterations of electric strain'.
+
+ Page 54, printer's error, Banly for Branly, 'proved that in a Banly
+ tube,'.
+
+ Page 56, variation in spelling, unsensitive for insensitive, 'wounded
+ and unsensitive.'.
+
+ Page 59, possible printer's error, sensive for sensitive 'to work a
+ sensive recording apparatus'.
+
+ Page 59, possible printer's error, arragement for arrangement, 'most
+ interesting arragement'.
+
+ Page 61, printer's error, missing letter i, 'as shown n Fig. 18,'.
+
+ Page 70, equation originally multi-line fraction, rendered into one
+ line for clarity.
+
+ Page 71, printer's error, osciilating for oscillating, 'to that of the
+ osciilating circuit'.
+
+ Page 71, printer's error, impluse for impulse, 'the period of that
+ impluse'.
+
+ Page 74, possible printer's error, extra comma in date, 'on May, 17,
+ 1901.'.
+
+ Page 76, printer's error, arangements for arrangements, 'variation of
+ the above arangements'.
+
+ Page 77, printer's error, systonic for syntonic, 'the systonic
+ transmitting'.
+
+ Page 86, printer's error, interpositon for interposition, 'effect of
+ the interpositon of land'.
+
+ Page 87, printer's error, signaling for signalling, 'the usual
+ maximum signaling'.
+
+ Footnote 17, printer's error, missing letter t, 'must be _solid_
+ metal and no hollow,'.
+
+ Footnote 31, printer's error, missing full stop after abbreviation,
+ '_Phil Mag._'.
+
+ Footnote 41, printer's error, extra full stop after reference,
+ '_Comptes Rendus._'.
+ ]
+
+
+
+
+
+End of the Project Gutenberg EBook of Hertzian Wave Wireless Telegraphy, by
+John Ambrose Fleming
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HERTZIAN WAVE WIRELESS TELEGRAPHY ***
+
+***** This file should be named 38526.txt or 38526.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/3/8/5/2/38526/
+
+Produced by Robert Cicconetti, Tim Madden and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive/American Libraries.)
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+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
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH 1.F.3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need are critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.