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THE
ALEXANDER GRAY MEMORIAL
LIBRARY
ELECTRICAL ENGINEERING
THE GIFT OFMAXWELL’S THEORY
AND
WIRELESS TELEGRAPHY
PART ONE
MAXWELL’S THEORY AND HERTZIAN
OSCILLATIONS
By H. POINCARÉ
TRANSLATED
By FREDERICK K. VREELAND
* PART TWO
THE PRINCIPLES OF WIRELESS TELEGRAPHY
By FREDERICK K. VREELAND
NEW YORK:
McGRAW PÜBI^ISHING COMPANY,
114 Liberty Street.
1904.
TUCopyright, 1904,
BY THE
McGRAW PUBLISHING COMPANY,
New York.CONTENTS.
PART ONE.
MAXWELL'S THEORY AND HERTZIAN OSCILLATIONS,
CHAPTER I.
Generalizations Regarding Electrical Phenomena
SEC.	PAGE.
1.	Attempts at Mechanical Explanation .....	1
2.	Electrostatic Phenomena .......	3
3.	Résistance of Conductors ....... G
4.	Induction ..........	7
5.	Electrodynamic Attraction .......	9
CHAPTER II.
Maxwell's Theory.
1.	Relations Between Light and Electrieity .	.	.	.13
2.	Displacement Currents .......	14
3.	The Nature of Light ........	19
CHAPTER III.
Electrical Oscillations Before Hertz.
1.	Experiments of Feddersen .......	22
2.	Lord Kelvin’s Theory ........	23
3.	Other Analogies .........	26
4.	Damping ..........	27
CHAPTER IV.
Hertz’s Oscillator.
1.	Hertz’s Discoverv	.	.	.	.	.	.31
2.	Principle of the Oscillator	......	32
[iii]iv
CONTENTS.
SEC.	PAGE.
3.	Different Forms of Oscillators .	.	.	.	.33
4.	Function of the Spark ........	35
5.	Influence of Light ........	36
6.	The Use of Oil.............................................37
7.	Value of the Wave-length .......	37
CHAPTER V.
Methods of Observation.
1.	Principle of the Resonator	.	.	.	...	38
2.	Operation of the Resonator .......	39
3.	Other Methods of Using the Spark	.	.	.	.	.42
4. Thermal Methods	........ 43
5. Mechanical Methods	........ 44
6.	Comparison of the Different Methods	.	.	.	.	.45
7.	Coherers ...................................45
CHAPTER VI.
Propagation Along a Wire.
1.	Production of Waves in a Wire .	.	.	.	.	.48
2.	Mode of Propagation ........	49
3. Velocity of Propagation and Diffusion .	.	.	.50
4. Experiments of MM. Fizcau and Gounelle	.	. .	.52
5.	Diffusion of Currents .	.	•	•	.54
6.	Experiments of M. Blondlot .......	55
CHAPTER VII.
Measurement of Wave-length and Multiple Résonance
1.	Stationary Waves ........	59
2.	Multiple Résonance .	.	.	.	•	•	•	.61
3.	Another Explanation ........	62
4.	Experiments of Garbasso and Zehnder	.	.	.	.65
5.	Measurement of the Décrément .	.	.	.	.66
6.	Experiments of Strindberg .......	68
7.	Experiments of Pérot and Jones .	.	.	.	.	•	69
8.	Experiments of Décombe .......	69
CHAPTER VIII.
Propagation 7in Air.
1.	The Experimentum Crucis	.	.	.	.	.	.	.71
2.	Experiments at Karlsruhe	.	.	*	.	.	•	•	.73CONTENTS.
y
SEC.	PAGE.
3.	Expérimenta at Geneva	.	.	.	,	.	.	.	74.
4.	Use of the Small Oscillator	.	.	.	.	.	.	*.	74
5.	Nature of the Radiations	.	.	.	.	.	.	.	76
CHAPTER IX.
Propagation in Dielectbics.
1.	Maxwell’s Relation ........	79
2.	Dynamic Methods	.......	80
3.	Static Methods .........	80
4.	Results .......... si
5.	Conducting Bodies .	.	.	.	.	.	.	.	82
6.	Electrolytes ..........	83
CHAPTER X.
Production of Very Rapid Oscillations.
1.	Very Short Waves	.......	85
2.	Righi’s Oscillator .	.	.	.	.	.	.85
3.	Resonators	.........	87
4.	Bose’s Oscillator	.........	88
5.	Bose’s Detector	.........	89
CHAPTER XI.
Imitation of Optical Phenomena.
1.	Conditions of Imitation .	.	.	.	.	.91
2.	Interférence .	. ■	.	.	.	.	.	.92
3.	Thin Films ..........	94
4.	Secondary Waves ........	94
5.	Diffraction	.........	96
6.	Polarization	.......	97
7.	Polarization by Reflection .......	98
8.	Refraction ..........	98
9.	Total Reflection .........	99
10.	Double Refraction .	.	.	.	.	.	.	, 100
CHAPTER XII.
Synthesis of Light.
1.	Synthesis of Light .	.	.	.	.	.	.	.101
2.	Other Différences ........ 102
3.	Explanation of Secondary Waves .	.	.	.	.	. 103
4.	Miscellaneous Remarks ....... 106VI
CONTENTS.
PART nvo.
THE PRINCIPLES O F IVIRELESS TELEGRAPHY.
CHAPTER I.
General Principles.
.	page.
1.	Methods of Signaling ïhrough Space	.	.	.	.	.111
2.	Method of Electromagnetic Induction	.	.	.	.	.113
3.	Preece’s Apparatus .	.	.	.	.	.	.	.115
4.	Method of Electrostatic Induction	.	.	.	.	.117
5.	Dolbear’s and Edison’s Apparatus	.	.	.	.	iigr
6.	Method of Electromagnetic Waves	.	.	.	.	.121
7.	Comparison of the Three Methods.............................12G
CHAPTER IL
Telegraphy by Hertzian Waves.
1.	Early Experiments .....
2.	Marconi’s Early Apparatus ....
3.	Improved Apparatus .....
4.	The Antenna .	.	.
CHAPTER III.
The Grounded Oscillator.
1. Development of Antenna	.
2.	Other Means of Increasing the Radiation
"3. The Induction Coil .
4. High-Power Generators	.
CHAPTER IV.
Propagation of Grounded Waves.
1.	Three Hypothèses Suggested
2.	The Free-Wave Hypothesis ....
3.	The Alternating-Current Hypothesis
. 130
. 132
. 135
. 133
. 141
. 140
. 140
. 151
. 150
. 157
. 159CONTENTS.
vii
SEC.	PAGE.
4.	Propagation Free Waves ....... 163
5.	Propagation of Guided Waves .	.	.	.	.	.172
6.	Effect of Daylight on Wave Transmission	.	.	.176
CHAPTER V.
The Receiving Apparatus.
1.	Detectors Classified .	.	.	.	.	.	.	.179
2.	Microphonie Detectors	.	.	.	.	.	.	.179
3.	Mechanical Detectors ........ 185
4.	Thermal Detectors .	.	.	.	.	.	.186
5.	Electrolytic Detectors ........ 187
6.	Magnetic Detectors	.	.	.	.	.	.191
7.	Arrangement of Receiving Apparatus	.	.	.	.	.194
8.	Current Multiplying Devices ...... 199
CHAPTER VI.
SELECTIVE SiGNALING.
1.	The Problem .	.	.	.	.	.	.	. 202
2.	Directed Signais ......... 203
3.	Syntonie Signaling ........ 204
4.	Lodge’s Syntonie Cônes and Leyden Jars .... 208
5.	Marconi’s Concentric Cylinders	.	.	.	.	.211
6.	The Closed Oscillating Circuit .	.	.	.	.212
7.	Inductively Interlinked Circuits	.	.	.	.	.215
8.	Tuned Receiving Apparatus ....... 223
9.	Marconi’s Coherer System	.	.	.	.	.	.	. 226
10.	Slaby-Arco System ........ 228
11.	Braun s System ......... 234
12.	Lodge-Muirhead System	.	.	.	.	.	. 235
13.	Limitations of Syntonie Signaling ..... 237
14.	Other Means of Securing Selectivity ..... 242
15.	Conclusion .......... 245PREFACE.
The object of this book is to give a physical treatment of
Maxwell’s theory and its applications to some modem elec-
trical problems,— to set forth the fundamental principles
which nnderlie ail electrical phenomena, according to Max-
well and his followers, to show how these principles explain
the ordinary facts of electricity and optics, and to dérivé
from them a practical nnderstanding of the essentials of
wireless telegraphy.
Mathematics and abstruse reasoning are avoided, for the
purpose is not to establish or defend a theory; but ratherto give
the reader a clear mental picture of what takes place when,
for example, a condenser is charged or a signal is sent around
the earth — not to fight over old battles, but to pick out the
fundamentals that hâve stood the test and are now generally
accepted, and put them in such form that the busy man may
use them or the student may take them as stepping-stones
to the more advanced theory.
Maxwell’s theory without mathematics may seem at first
an incongruity, for has not Hertz himself said that Max-
well’s theory is best defined as MaxwelPs System of équa-
tions ?* Maxwell indeed used many hypothèses and physical
assumptions in building up his theory, but later, when the
mathematical structure was complété, he cast aside the
scaffolding on which it was built, leaving a broad and com-
préhensive System, unencumbered by needless details. His
* Electric Waves, Eng. Trans. p. 21.
[ix]PREFACE.
x.
équations are thus general rather than spécifie ; they express
ail that is necessary and permanent in his theory, while
ignoring that which is hypothetical and spéculative.
A purely physical theory is likely to be imperfect by
reason of its very definiteness ; s to be incomplète because it
is too spécifie. Yet a mathematical theory without a physi-
cal interprétation loses much of its value. Maxwell’s équa-
tions are not an end in themselves — they are rather the
means of expressing physical truths. The équation rep-
resents the fact, but unless we recognize and grasp the fact
the analysis becomes mere mathematical jugglery: and this
is perhaps one reason why Maxwell’s great generalizations
— the very “ Principia ” of modem electrical science —
hâve not received more general attention. They are difficult
to approach in the abstract, but so are the commonest elec-
trical phenomena. It is not easy to conceive, in its essence,
of an electrical current following a wire; we therefore pic-
ture it to ourselves as something that flows like a material
fluid in a pipe, and we find that it obeys similar lawrs.
When we corne to the more complex phenomena of induction
and magnetism the need of a physical concept is even greater,
and we turn for assistance to the traditional Unes or tubes
of force and induction. These physical conventions lack the
précision and elegance of a mathematical expression, but
they are more easily grasped and handled : if we realize their
limitations and take pains to discriminate between that
which is absolute fact and that which is onlv figurative,
they become most useful implements of research, and we
may take them, as Faraday did his “ tubes of force,” as the
équivalents of the mathematical forms for which they stand.
So in dealing with Maxwell’s theory it is not necessary to
resort to mathematics. Although electrical truths are most
readily and most accurately expressed in this universalPREFACE.
xi
shorthand of the sciences, it is none the less possible to trans-
late them, as it were, into the language of every-day life,
and thus cause them to appeal more directly to the un-
derstanding.
That M. Poincaré is, of ail men, qualified to do this, no
one familiar with his classic mathematical works on the
subject will question; and with reference to Part One I need
only say that I hâve endeavored to follow the original with
as literal exactness as is practicable in a translation, striving
to preserve the thought of the author without sacrificing the
vigorous suggestiveness of his style. The illustrations, how-
ever, I hâve prepared especially for this volume, witb the
exception of a few diagrams which appeared in the original,
and which hâve been redrawn.
In Part Two it has been my purpose to take up the thread
where M. Poincaré dropped it, carrying the line of thought
into the practical field of wireless telegraphy, and applying
the principles laid down in Part One to the varions problème
involved; to describe certain typical Systems, to show why
some hâve failed while others succeeded, and to explain their
mode of operation in the light of MaxwelFs ideas.
This is not intended as a treatise on wireless telegraphy —
no attempt is made to describe the myriad forms of apparatus
nor to settle questions of priority and history. Such ma-
terial is readily accessible to those who may desire it. The
object is rather to deal with principles and to trace the de-
velopment of the art in its essential features. Where spécifie
cases are cited they are chosen with reference to their fitness
to illustrate an idea or to serve as milestones on the path of
progress, and they are treated with a view to emphasize that
which is essential and minimize superficial or unimportant
details.PREFACE.
xii
In Chapter IV is discussed the question of wave-propo-
gation over a conducting surface, and various hypothèses are
reviewed and tested in the light of the preceding chapters.
In approaching a conclusion it has seemed advisable to de-
part a little from the purely Maxwellian idea of displace-
ment currents, which does not readily appeal to the imagina-
tion in this connection, and to substitute Faraday’s
conception of moving tubes of induction, which embodies
the same principles in more tangible form. It is hoped that
this figure, so successfully used by J. J. Thomson in explain-
ing other electrical phenomena, may give the reader a clear
understanding of what takes place when an electromagnetie
wave glides over the surface of the earth.
The other chapters are self-explanatory and need not be
considered here.
I desire to acknowledge my indebtedness to M. Poincaré
for his most courteous permission to translate and publish
the work that appears as Part One, and to the manv writers on
whom I hâve drawn for references and historical data. My
thanks are due also to the Macmillan Company for permis-
sion to copy some of the figures illustrating the work of
Hertz, and to the publishers for their hearty coopération in
seeing the work through the press.
Frederick K. Vreeland.
New York, January, 1904.PART ONE.
MAXWELL’S THEORY AND HERTZIAN OSCILLA-
TIONS.PART ONE.
MAXWELL’S THEORY AND HERTZIAN OSCILLA-
TIONS.
CHAPTER I.
GENERALIZATIONS REGARDING ELECTRICAL PHE-
NOMENA.
1. Attempts at Mechanical Explanation.— To give a com-
plété mechanical explanation of electrical phenomena, reduc-
ing the laws of physics to the fundamental principles of
dynamics, is a problem that has attracted many investigatcrs.
But is it not rather a fruitless task, and will not our efforts
be expended in vain ?
If the problem admitted of only one solution, the posses-
sion of this solution, which would be the truth, could not be
bought too dearly. But this is not the case. It is doubtless
possible to devise a mechanism giving a more or less perfect
imitation of electrostatic and electrodynamic phenomena ; but
if we can imagine one such mechanism, we can also imagine
an infinity of others. Among them ail, we do not at présent
find one that appeals to our choice on the score of simplicity,
nor is it évident that any one would enable us, better than
the others, to penetrate the secret of nature. Ail those that
hâve been proposed hâve a savor of artificiality which is ré-
pugnant to the reason.
[1]2
MAXWELL’S THEORY.
One of the most complété of these was developed by Max-
well, at a time when his ideas had not yet taken definite form.
.The complicated structure wThich he attributed to the ether
rendered his System strange and unattractive ; one seemed to
be reading the description of a workshop with gearing, with
rods transmitting motion and bending under the effort, with
wheels, belts and governors.
Whatever may be the taste of the English for conceptions
of this kind, whose concrète appearance appeals to them, Max-
well was the first to abandon his own extraordinary theory,
and it does not appear in his complété works. But we cannot
regret that his mind followed this by-path, since it was thus
led to the most important discoveries.
In following the same course, it seems hardly possible to
obtain a better resuit. But if it is vain to attempt to picture
the mechanism of electrical phenomena in ail its details, it is
nevertheless important to show that these phenomena obey
the general laws of mechanics.
These laws, in fact, are independent of the particular
mechanism to which they apply : they must remain invariable
throughout the diversity of their manifestations. If the
electrical phenomena are exceptions, we must abandon ail
hope of a mechanical explanation; but if they conform to
these laws, the possibility of such explanation is assured, and
we are confronted onlv by the difficulty of choosing ainong
the various solutions wThich the problem admits.
But how can we assure ourselves, without following ail the
complications of mathematical analysis, of the conformity of
the laws of electrostatics and electrodynamics to the general
principles of dynamics ? By a sériés of comparisons. When
we wish to analyze an electrical phenomenon, we shall take
one or two well-known mechanical phenomena and endeavor
to show their perfect parallelism. This parallelism will thusELECTRICAL PHENOMENA.
3
be a sufficient proof of the possibility of a mechanical expla-
nation.
The use of mathematical analysis would serve only to show
that these analogies are not merely rough approximations,
but may be followed into the most minute details. The limits
of this work will not permit us to go thus far, and we must
be content with a comparison, as it were, qualitative.
2. Electrostatic Phenomena.— In charging a condenser
energy is always expended; mechanical work if we turn a
statical machine or a dynamo, Chemical energy if we charge
it with a battery. But the energy thus expended is not lost ;
+
Fig. 1.— Two conductors are charged to different potentials and then
conneeted by a wire. A current flows from one to the other until the
potentials are equalized.
it is stored in the condenser, to be liberated again when the
condenser is discharged. It will be liberated in the form of
heat if the two plates of the condenser are simply joined by a
wire, which is heated by the discharge current ; or it may be
made to take the form of mechanical work by causing the
discharge current to operate a little electric motor.
Similarly, to raise the level of water in a réservoir, work
must be done ; but this work may be given back, if, for exam-
ple, the water in the réservoir be used to turn a water-wheel.
If two conductors be charged to the same potential, and
then conneeted by a wire, the equilibrium will not be dis-4
MAXWELL1 S THEORY.
turbed ; but if the initial potentials be different, a current will
flow through the wire from one conductor to the other until
the equality of potential is established. (Fig. 1.)
Similarly, if the water in two réservoirs stand at different
levels and thje réservoirs be joined by a pipe, water will flow
from one to the other until it stands at the same level in
both. (Fig. 2.)
The parallelism is thus complété: the 'potential of a con-
denser corresponds to the height of the water in a réservoir,
the charge of the condenser to the mass of the water contained
in the réservoir.
+
Fig. 2.— Hydraulic analogue of the charged conductors. The height of
water represents the potential ; the mass of the water, the charge on the
conductor ; the cross-section of the réservoir, the capacity of the con-
ductor.
For example, if the horizontal section of the réservoir be
100 square meters, one cubic meter of water will be re-
quired to raise the level one centimeter. If the section be
twice as great, double the quantity of water will be required.
The horizontal section thus corresponds to what is called the
capacity of the condenser.
How can we interpret in this manner the attractions and
repulsions which are exerted betweçn electrified bodies ?
These mechanical forces tend to diminish the différence of
potential between the bodies. If they be opposed, as when
two mutually attracting bodies are separated, work is done,
electrical energy is stored up, and the différence of potentialELECTRICAL PHEXOMENA.
5
is increased. If, on the other hand, the conductors be left
free to obey their mutual attractions, the stored-np electrical
energy is partly given up in the form of mechanical work, and
the potentials tend to be equalized.
These mechanical forces thus correspond to the pressures
exerted on the walls of the réservoirs by the water which they
contain. Suppose, for example, that our two réservoirs be
joined by a horizontal cylindrical pipe of large cross-section,
in which is fitted a piston. (Fig. 3.) When the piston is
moved in snch a direction as to force water into the réser-
voir where the level is already higher than in the other, work
is expended ; if, on the other hand, the piston be left free to
Fig. 3.— The mechanical force between two oppositely charged bodies
tends to diminish their différence of potential. If this force be opposed,
as when the piston is forced against the pressure, work is done, energy is
stored, the différence of potential is increased.
yield to the pressures on its opposite faces, it will be dis-
placed in such a way that the wàter-levels tend to be equal-
ized, and part of the energy stored in the réservoirs will be
released.
This hvdraulic analogy is the most convenient and most
complété ; but it is not the only one possible. For instance,
we might compare the work done in charging a condenser to
that required to raise a weight or compress a spring. The
energy thus expended is given back when the weight is ah
kuved to descend, or the spring is released, just as wrhen the
two plates of a condenser are allowTed to obey their mutual
attractions.G
MAXWELL9S THEORY.
In the following pages we shall make use of ail three analo-
gies.
3. Résistance of Conductors.— Suppose our two réservoirs
to be joined together by a long, horizontal tube of small
cross-section. (Fig. 4.) The water will run slowly through
this tube, and the flow will increase as we increase the dif-
férence of level in the réservoirs and the cross-section of the
tube, or as we diminish its length. In other words, the ré-
sistance of the tube, being due to internai friction, increases
with its length and with a diminution of its area.
Similarly, if we connect two conductors by a long, slender
wire, the flow of electricity will increase with the différence
		“31
		
II	1 III Il 1 Üi	** raicnortRL «?ts»sTa>ice. “	—g;1 		=i	—
Fig. 4.— Ohmic résistance is analogous to friction of water flowing
through a slender tube. The flow dépends upon the différence in level, the
bore of the tube and (inversely) on its length. The energy lost goes into
beat.
of potential and with the cross-section of the wire, and will
vary inversely as its length.
The electrical résistance of a wire is therefore comparable
to the hydraulic résistance of our tube: it is a kind of fric-
tion. The similarity is the more complété, for the résistance
causes the wire to become heated as in the case of mechanical
friction.
This effect is strikingly shown in the well-known experi-
ment of Foucault. When a dise of copper is caused to rotate
in a magnetic field, a considérable force is required to turn
it, and the dise becomes heated, precisely as if the dise were
rubbing against an invisible brake.ELECTRICAL PHENOMENA.
7
4. Induction.— If two wires be placed close together,
and one of them carry a variable current, currents will also
be produced in the second. If the primary current be in-
creasing, the secondary current will be in the opposite direc-
tion to the primary : if the primary be decreasing, the second-
ary will be in the saine direction. The currents in the second-
ary circuit are known as induced currents, and the phenome-
non is called mutual induction.
But this is not ail. A variable current produces electro-
motive forces of induction in the wire traversed by the cur-
rent itself. This force is opposing if the current be increas-
ing, but it tends to augment the current when the latter is
decreasing. This effect is called self-induction.
In our mechanical analogy, self-induction is easily ex-
plained. It seems that, to set electricity in motion, we
must overcome a counter-electromotive reaction; but once
the motion is commenced, it tends to continue of itself. Self-
induction is thus a sort of inertia.
Similarly, a reaction must be overcome in starting a vehi-
cle in motion; but, once it is started, the motion tends to
continue of itself.
To recapitulate, a current may hâve to overcome :
First. The ohmic résistance of the circuit (which always
exists and always opposes the current).
Seco?id. Self-induction, if the current is variable.
Third. Counter-electromotive forces of electrostatic origin,
if there are electrical charges in the neighborhood of the cir-
cuit or upon it.
The last two reactions may become négative and tend to
augment the current.
Compare these reactions with those encountered by a ve-
hicle moving along a road :8
MAXWELL’S THEORY.
First. Ohmic résistance, \ve hâve seen, is analogous to
friction.
Second. Self-induction corresponds to the inertia cf the
vehicle.
Third. The forces of electrostatic origin are like the
weight, which opposes the motion when ascending a grade,
and assists when descending.
For mutual induction the matter is a little more compli-
cated. Imagine a sphere, S, of considérable mass, which car-
ries two arms at diametrically opposite points; and at the
ends of these arms, two small spheres, Si and s2. (Fig- 5.)
Pig. 5.— Mechanical model illustrating the phenomena of mutual in-
duction. The small spheres, sx and s2, represent two mutually inductive
circuits ; the sphere, S, of large mass, the ether which surrounds them.
Any variation in the velocity of sx (primarily current), induces a velocity
in s2 (secondary current), on account of the inertia of S.
S represents the ether, sx the primary current, and s2 the
secondary current.
If we wish to set the little sphere Si. in motion it offers little
opposition; but the sphere S does not start so readily. For
the first instant it remains motionless and the whole System
turns about it as a center, the sphere s2 moving in the oppo-
site direction to Si.
This represents the action of mutual induction. The
spheres Si and s2 correspond to the two conductors; the
sphere S, which we must imagine invisible, is the ether which
surrounds them. When the motion of Si is accelerated, s2
moves in the opposite direction ; similarly, when the primaryELECTRIC AL PUENOMENA.
9
current increases, a secondary current is induced in the oppo-
site direction.
Pursuing the analogy, suppose the motion of Si and s2 to
he retarded by a sort of friction (the ohmic résistance of the
two conductors), while S has no résistance to overcome but
its ewn inertia; and suppose the motive force to act con-
tinuously on Si. When a condition of uniform motion is
finally established, the sphere Si will move at a constant ve-
locity, carrying with it S, which, once in motion, offers no
further résistance. The sphere s2, by virtue of its friction,
will remain motionless," and the whole System will revolve
about it. The primary current has become constant; the
secondary current has ceased.
Finally, if the motive force cease to act on Si, its velocity
will be reduced by its friction. But S, by virtue of its great
inertia, continues to move, carrying with it s2, which acquires
a velocity in the same direction as that of Si. The primary
current decreases; the secondary current flows in the same
direction as the primary.
In this figure, S represents the ether which surrounds the
wires; it is the inertia of the ether which produces the phe-
nomena of mutual induction. The same is true of self-in-
duction : the inertia which must be overcome in starting a
current in a wire is not that of the ether which pénétrâtes
the wTire, but of the ether which surrounds it.
5. Electrodynamic Attraction.— We hâve endeavored above
te give, by analogy, an explanation of electrostatic attrac-
tions, and of the phenomena of induction. Let us now see
what idea Maxwell offers as to the cause of the mutual attrac-
tions of currents.
If electrostatic attractions were due to the tension of a
multitude of tiny springs, or, in other words, to the elasticity
of the ether, the kinetic energy and the inertia of this fluid10
MAXWELL’S THEORY.
would give rise to the phenomena of induction and electrodv-
namic actions.
The complété analysis is much too long to find place liere,
and we must again limit ourselves to an analogy. We shall
find it in a well-known device,— the centrifugal governor.
{Fig. 6.)
Fig. 6.— Model illustrating the
mechanical action of currents. The
halls tend to separate, thus increas-
ing their kinetic energy if the
angular velocity be kept constant.
This energy is supplied from with-
out in overcoming the inertia reac-
tion due to the séparation.
Fig. 7.— Two currents in oppo-
site directions repel each other,
tending to separate and thus in-
crease the kinetic energy of the
System for constant current. This
energy is. supplied from the source
in overcoming the counter E.M.F.
due to the séparation.
The kinetic energy of this apparatus is proportional to the
square of the angular velocity of its rotation, and to the
square of the displacement of the halls from the axis.
According to Maxwell’s hypothesis, the ether is in motion
whenever there are electric currents, and its kinetic energy
is proportional to the square of the intensity of the currents.
This intensity thus corresponds, in the analogy which we are
endeavoring to establish, to the angular velocity of rotation.ELECTRICAL PHEXOMEXA.
11
If we consider two currents in the same direction, the
kinetic energy, for a given intensity of çurrent, will beeome
greater as the currents approach each other. If the currents
üow in opposite directions, it will be greater as they are far-
ther separated. (Fig. 7.)
This being granted, we may pursue the analogy.
To increase the angular velocity of the governor, and hence
its kinetic energy, it is necessary to do work, and hence to
overcome a reaction, called its inertia,
Similarly, increasing the strength of the currents increases
the kinetic energy of the ether; and to do this requires the
doing of work and the overcoming of a reaction, which is sim-
ply the inertia of the ether, and is called induction.
The kinetic energy will be greatest when the currents are
in the same direction and close together ; the work to be done
in producing them and the counter-electromotive force of in-
duction are thus greatest. This is what is meant when we
say, in ordinary language, that the mutual induction of two
currents is added to their self-induction. The reverse is true
if the currents are in opposite directions.
If the balls of the governor be separated, energy must be
supplied if the angular velocity is to be maintained ; because,
for a given angular velocity, the kinetic energy increases as
the balls are separated.
Similarly, if two currents in the same direction be brought
together, work must be done to maintain their intensity, be-
cause the kinetic energy is increased. Hence, there is an
clectromotive force of induction to be overcome, which tends
to diminish the strength of the currents. On the other hand,
it would tend to increase them if they were in the same di-
rection and were separated, or if they were in opposite direc-
tions, and were brought together.12
MAXWELL9S THEORY.
The mutual mechanical actions of currents may be simi-
larly explained.
The centrifugal force tends to separate the balls of the
governor, wliich would hâve the effect of increasing the
kinetic energy if the angular velocity were kept constant.
Similarly, when the two currents are in the same direction,
. they attract each other ; that is, they tend to corne together,
which would liave the effect of increasing the kinetic energy
if the currents were maintained constant. If the currents
are in opposite directions, they repel each other, and tend to
separate; which would again hâve the effect of increasing
the kinetic energy for a constant strength of current.
Thus, the electrostatic phenomena are explained by the elas-
ticity of the ether, and electrodynamic phenomena by its
kinetic energy. But should this elasticity itself be explained,
as Lord Kelvin thinks, by the rotation of minute portions of
the fluid ? Various considérations render this hypothesis
attractive, but it plays no essential part in the theory of Max-
well, which is independent of it.
In ail the preceding we hâve made comparisons with vari-
ous mechanisms; but they are simply analogies,— indeed,
rather crude ones. Moreover, we must not expect to find in
MaxwelFs work a complété mechanical explanation of elec-
trical phenomena, but only a portrayal of the conditions
which ail such explanations must satisfy; indeed, the great
element of permanency in MaxwelFs work is this fact, that it
is independent of ail particular explanations.CHAPTEE IL
MAXWELL’S THEORY.
1. Relations between Light and Electricity.—At the time
when the experiments of Fresnel were forcing the scientifie
world to admit that light is due to the vibrations of a subtle
fiuid, filling interplanetary space, the researches of Ampère
revealed the laws of the mntnal action of electric currents,
and laid the foundation of electrodynamics.
It was but a step farther to suppose that this same fluid,
the ether, which is the seat of luminous phenomena, is also
the medium for electrical action. This step was taken by
Ampère; but the illustrious physicist, in proposing his at-
tractive hypothesis, could hardly hâve foreseen that it would
so soon take a more précisé form and begin to receive its con-
firmation. It was indeed only a dream without foundation
until electrical measurements brought to light an unexpected
fact.
The ratio of the “ absolute electrostatic unit ” to the “ ab-
solute electromagnetic unit” is measured by a velocity. Max-
well devised several methods for deriving the value of this
velocity, and the results which he obtained fell in the vicinity
of 300,000 kilometers per second; that is, the velocity of
light
The observations were soon made so précisé that it was
impossible to attribute the coincidence to chance, and there
was no longer any doubt of the existence of some intimate
relation between optical and electrical phenomena. But the
nature of this relation might still hâve remained a mystery
if the genius of Maxwell had not divined it.
[13]14
MAXWELL’® TÜEORY.
The remarkable coincidence may be explained in the fol*
lowing way:	Along a wire of perfect conductivity an elec-
trical disturbance would be propagated with the velocity of
liglit. The calculations of Kirchhoff, founded on the old
electrodynamic theory, led to this resuit.
But it is not along a wire that light is propagated, but
through transparent bodies, through the air, through space.
Such a propagation as this was not accounted for by the old
electrodynamies. Before the principles oi optics could be
derived from the electrodynamic théories then in vogue, the
latter had to undergo serions modifications without ceasing
to take account of ail known facts. This reconstruction wa&
the work of Maxwell.
2. Displacement Currents.— It is well known that mate*
rial bodies may be divided into two classes : the conductors,
in which we can produ.ee displacements of electricity, that isr
volt aie currents, and the insulators, or dielectrics. To the
earlv electricians, the dielectrics were quite inert, and their
function was simplv to oppose the passage of electricity. If
this were the case, any insulator whatever could be replaced
by a different one without in the least changing the phe-
nomena. The experiments of Faraday showed that this is
not so :	Two condensers of the same form and the same di-
mensions, connected to the same source of electricity, do not
take the same charge, even though the thickness of the insu-
lating layer be the same ; provided the nature of the insulat*
ing material is different. Maxwell had made too profound a
study of Faraday’s researches to overlook the importance of
the dielectrics, and the necessity of giving them their proper
significance.
Moreover, if it be true that light is an electrical phenome-
non, then when light is propagated through an insulating
body, this insulator must be the seat of the phenomenon.MAXWELL’,8 THEORY.
15
Thus there must be localized electrical phenomena in the di-
electrics. But what is their nature ? Maxwell replies boldly
tliey are currents.
Ail the known facts of his time seemed to contradict him ::
never had a current been observed except in a conductor.
. How could Maxwell reconcile his audacious hypothesis with
a fact so well established ? Why do these hypothetical cur-
rents produce manifest effects under certain circumstances,
and yet are absolutely unobservable under ordinary condi-
tions ?
It is because the dielectrics offer to the passage of electric-
ity, not a great-er résistance than the conductors, but a résist-
ance of a different nature. An analogy will make Maxwell’s
idea more intelligible.
If we undertake to compress a spring we encounter an
opposing force which increases as the spring yields to the
pressure. If, now, wTe can exert only a limited pressure, a
moment will arrive when we can no longer overcome the
reacting force ; the movement will cease, and equilibrium will
be established. Finally, wdien the pressure is removed, the
spring will regain its original form, giving back ail the
energy that was expended in compressing it.
Suppose, on the other hand, that we wish to move a body
immersed in water. Here again we encounter a reaction,
which dépends upon the velocity, but which, if the velocity
remain constant, does not go on increasing as the body yields
to the pressure. The motion will thus continue as long as the
motive force acts, and equilibrium will never be established.
Finally, when the force is removed, the body does not tend
to return to the starting point, and the energy expended in
moving it cannot be restored ; it has been completely trans-
formed into heat through the viscositv of the water.
The contrast is manifest, and it is important to distinguisk16
MAXWELL’S THEORY.
between elastic reaction and viscoas reaction. Kow, the di-
electrics behave toward the motion of electricity as elastic
solids do toward the motion of matter, while the conductors
behave likeviscous liquids. Hence there are two kinds of
currents: the displacement cnrrents of Maxwell, which tra-
verse the dielectrics, and the ordinary conduction cnrrents
which flow in conductors.
The former, having to overcome a sort of elastic reaction,
must be of short duration, for this reaction increases as long
Fig. 8.— Model illustrating the flow of a displacement current in a
dielectric C. The pressure in the vessel represents the voltage of the bat-
tery ; the height of the column, the displacement in the dielectric ; the flow
of water, the charging current. The energy expended may be recovered.
as the current continues to flow and equilibrium must soon
be established.
Conduction currents, on the other hand, must overcome a
sort of viscous résistance, and hence may continue as long as
the electromotive force which produces them.
Resuming our hydraulic analogy, suppose that we hâve a
closed vessel containing water under pressure. (Fig- 8.) IfMAXWELL’S TÜEORY.
17
we put tins vessel in communication with a vertical pipe, tlie
water will rise in it, but the flow will cease when the hydro-
static equilibrium is established. If the pipe be large, therc
will be no appréciable friction nor loss of head, and the water
thus raised may be used to do work. We hâve here an il-
lustration of displacement currents.
If, on the other hand, the water be allowed to run out
through a horizontal pipe (Fig. 9), the flow will continue as
long as there is water in the réservoir; but, if the pipe be
small, there will be a considérable loss of energy, and heat
Fig. 9.— Model illustrating the flow of a conduction current in a con-
ductor, R. The flow continues undiminished as long as the pressure is
maintained. The energy expended in friction takes the form of heat and
is lost.
will be produced by the friction. This illustrâtes the action
of conduction currents.
Although it is impossible, and unnecessary, to try to im-
agine ail the details of the mechanism, we may say that ail
takes places as if the displacement currents had the effect of
compressing a multitude of minute springs. When the cur-
rents cease, electrostatic equilibrium is established; and the
tension of the springs dépends upon the intensity of the elec-
trostatic field. The energy accumulated in these springs, that
is, the electrostatic energy of the field, may be restored when-
ever they are allowed to unbend ; and it is thus that mechani-
218
MAXWELL'S THEORY.
cal work is produced when charged conductors are allowed to
obey their electrostatic attractions. These attractions are
thus due to the pressure exerted pn the conductors by the
compressed springs. Finally, to pursue the analogy to the
end, a disruptive discharge may be attributed to the breaking
of sonie springs which are unable to stand the strain.
On the other hand, the energy expended in producing con-
duction currents is lost, and converted into heat, like the
work done in overcoming friction or the viscosity of fluids.
This is why a conductor is heated by the passage of a ouvrent»
Fig. 10.— The old explanation of
the charging of a condenser. The
electricity was supposed to accumu-
late on the surface of the plates,
as indicated by the dotted lines.
The circuit was thus considered un-
closed.
Fig. 11.— Maxwell’s idea of the
phenomenon of charging a con-
denser. The current does not stop
at the surface of the conductor, but
continues to flow, as a displace-
raent current, through the dielectric,
until checked by the elastic reaction.
The circuit is thus completed.
From Maxwell’s point of view, none but closed currents
exist. To the early electricians, this was not the case. They
considered as closed the current which circulâtes in a wire
joining the two terminais of a batterv. But if, instead of
joining these terminais directly, they were connected respec-
tively to the two plates of a condenser, the momentarv cur-
rent which flowed while the condenser was being chargedMAXWELL’g THEORY.
19
was considered as unclosed. (Fig. 10.) It flowed, they said,
from one plate to the other through the wire counected to tlie
battery, and stopped at the surfaces of the plates. Maxwell,
on the contrary, considers that the current continues, in the
form of a displacement current, across the insulating layer
which séparâtes the plates, and is thus completely closed.
(Fig. 11.) The elastic reaction which the current en-
counters in traversing the dielectric explains its short dura-
tion.
Currents may manifest themselves in three ways : by their
heating effects, by their action on magnets and on other cur-
rents, by the induced currents which they generate. We hâve
seen above why conduction currents produce heat and dis-
placement currents do not. Yet, according to Maxwell’s hy-
pothesis, the currents which he imagines should, like ordinary
currents, produce electromagnetic, electrodynamic, and induc-
tive effects.
Why could these effects not be observed? Because a dis-
placement current, however feeble, cannot continue long in
one direction; for the tension of our hypothetical springs,
continually increasing, will soon check it. Thus we cannot
hâve in a dielectric either a continuons current of long dura-
tion or a sensible alternating current of long period ; but the
effects should be observable if the alternations are very rapid.
3. The Nature of Light.—And here we hâve, according to
Maxwell, the origin of light:	A light wave is a sériés
of alternating currents, flowing in a dielectric, in the
air, or in interplanetary space, changing their direction
1,000,000,000,000,000 times in a second. The enormous in-
ductive effect of these rapid alternations produces other cur-
rents in the neighboring portions of the dielectric, and thus
the light waves are propagated from place to place. The ve-20
MAXWELL’S THEORY.
locity of propagation may be shown analytically to be equal
to the ratio of the units, that is, to the velocity of light.
These alternating currents are a kind of electrical vibration ;
but are they longitudinal, like those of Sound (Fig. 12), or
k------------------^------------------------*
Fig. 12.— Mode of propagation of a longitudinal vibration, such as a
Sound wave. The crowding and separating of the parallel Unes repre-
sent condensations and rarifactions of the air, and the arrows indicate
the motion of individual particles.
transverse, like those of Fresnel’s ether? ( Fig. 13.) In the
case of sound, the air undergoes altemate condensations and
rarifactions ; but the ether of Fresnel acts as if it were com-
posed of incompressible layers capable only of sliding upon
each other. If the currents flowed in unclosed circuits, the
1-4

■*>
Fig. 13.— Mode of propagation of a transverse wave, such as light.
There are no changes of density, and the displacements in the ether are
perpendicular to the line of propagation.
electricity would necessarily accumulate at one end or the
other of the circuits, and we should hâve a condition analo-
gous to the condensations and rarifactions of air: the vibra-
tions would be longitudinal. But, as Maxwell admits only
closed currents, these accumulations are impossible, and theMAXWELL’S THEORY.
21
electricity must behave like the incompressible ether of Fres-
nel : its vibrations must be transverse.
Thns we reach ail the conclusions of the wave theory of
light. This, however, was not enough to enable the physi-
cists, wrho were attracted rather than convinced, to accept
absolutely Maxwell’s ideas: ail that could be said in their
favor was, that they did not conflict with any known facts,
and that it were indeed a pity if they were not true. The
experimental confirmation was lacking, and remained so for
twenty-five years.
It was necessary to find, between the old theory and that
of Maxwell, a discrepancy not too minute for our crude
methods of observation. There was only one such f rom which
an experimentum crucis could be derived. To do this was
the work of Hertz, which we shall now discuss.CHAPTER III.
ELECTRICAL oscillations before hertz.
1. Experiments of Feddersen.— Alternating currents were
produced at a very early date by inechanical means, such as
the use of rotating commutators, vibrators, etc. These
were indeed, in a sense, electrical oscillations, but their fre-
quency was necessarily very low.
The discharge of a condenser furnished the means of ob-
taining much more rapid oscillations, and it was Feddersen
who first demonstrated experimentally that, under certain
circumstances, the discharge of a Leyden jar may be oscil-
latory.
Feddersen observed the spark produced in discharging a
Leyden jar, by means of a rotating concave mirror. He also
projected an image of the spark, by such a mirror, upon a
sensitive plate, and thus photographed it under various condi-
tions.
He varied the résistance of the circuit. With a low résist-
ance he obtained an oscillatory discharge, and the arrange-
ment of his apparatus enabled him to see how the frequency
of oscillation varied with the capacity of the condenser and
the self-induction of the circuit.
To vary the capacity he simply changed the number of
Leyden jars, and thus showed approximately that the period
is proportional to the square root of the capacity.
To vary the self-induction, Feddersen changed the length
of the discharge circuit, and showed the period to be approxi-
mately proportional to the square root of the self-induction ;
but approximately only, because in his experiments the
[22]OSCILLATIONS BEFORE HERTZ.
23
length of the circuit reached sometimes several hundred
meters; it was suspended on the wall and formed with this
a condenser whose capacity was not at ail negligible with re-
spect to that of the main condenser.
As for the numerical coefficient, Feddersen could not dé-
termine its value, for lie did not know accurately the capacity
of his condensers ; he could only verify the proportionalities.
Feddersen obtained periods of the order of 10'4 seconds.
By gradually increasing the résistance, which he did by
inserting in the circuit small tubes filled with sulfuric acid,
he obtained, first, continuous discharges, then intermittent
ones; the latter for very large values of the résistance, ob-
tained by means of wet cords.
It is évident that, in a rotating mirror, a continuous dis-
charge should appear as a continuous band of light ; an alter-
nating or intermittent discharge, as a sériés of separate,
bright spots.
The photographs of alternating discharges obtained by
Feddersen presented a peculiar appearance. There was a
sériés of bright and dark points corresponding to the two ends
of the spark; but the luminous points for one end corre-
sponded to the dark points for the other, and vice versa.
This is easily explained:	When a disruptive discharge
takes place in air, the particles torn from the positive élec-
trode become incandescent, but this is not the case with the
négative particles: the positive end of the spark is thus
brighter than the négative.
Feddersen’s photographs thus proved that each end of the
spark is alternately positive and négative. Hence the dis-
charge is not intermittent and always in the same direction,
but is oscillatory.
2. Lord Kelvin’s Theory.— The experiments of Feddersen
may be easily explained :24
MAXWELU8 THEORY.
Suppose two eonductors (in Feddersen’s experiments they
were the two coatings of the condenser) to be connected by a
wire. If they are not at the same potential, the electrical
equilibrium Avili be disturbed, just as the mechanical equili-
from the vertical. Each represents a store of energy, ready to be re-
leased.
brium is disturbed wlien a pendulum is displaced from the
vertical. (Fig. 14.) In either case there will be a tendency
to re-establish the equilibrium. A current will flow through
the wire in the effort to equalize the potentials of the two

Fig. 15.— When the condenser is discharged and the pendulum has
swung to the vertical, the energy is not lost, but has taken the form of
current in the one case, velocity in the other. These are alike in their
tendency to continue.
eonductors, and the pendulum will swing towrard the verti-
cal. (Fig. 15.)
But the pendulum will not stop in the position of equilib-
rium; having acquired a certain velocity, its inertia willOSCILLATIONS BEFORE HERTZ.
25
carry it beyond this position. Similarly, when our conduct-
ors are discharged, the momentary condition of equilibrium
does not last, but is at once destroyed by a cause analogous
to inertia — the self-induction of the circuit. We hâve
learned that, when a current ceases to flow, it induces a cur-
rent in the same direction in a neighboring conductor. A
similar effect is produced in the circuit of the inducing cur-
rent itself, which is thus continued, as it were, by the induced
current.
In other words, a current persists after the cessation of the
Fig. 16.— The current has ceased, and the condenser is again charged,
but with reversed polarity. The pendulum has reached the extreme limit
of its swing, and is ready for the return journey.
cause which produced it, just as a moving body continues in
motion after the force which started it is removed.
Thus, the two potentials having been equalized, the current
continues in the same direction, and charges the conductors
again, but with reversed polarities. (Fig. 16.) In this case,
as in that of the pendulum, the position of equilibrium hav-
ing been passed, the motion must be reversed in order to re-
turn to it: again the momentary equilibrium is established,
and again it is destroyed in the same way ; and thus the oscil-
lations continue.
It is readily shown analytically that the period of oscilla-26
MAXWELL9S THEORY.
tion varies with the capacity; hence, by diminishing this
capacity, which is easily done, we may obtain an “ electrical
pendulum ” capable of producing extremely rapid oscilla-
tions.
3. Other Analogies.— In explaining the theory of Lord
Kelvin we hâve compared the electrical oscillations with
those of a pendulum, but there are many other analogies
which would serve equally well.
Instead of a pendulum, take, for example, a tuning fork:
if its arms be bent from their position of equilibrium, their
elasticity tends to bring them back; but, impelled by their
inertia, they swing past it; their elasticity brings them back
again, and again they swing past, and so on : they perform a
sériés of oscillations.
Here the elasticity of the fork plays the same part as the
weight of the pendulum, and as the electrostatic force in the
oscillatory discharge of the Leyden jar; the inertia of the
fork takes the place of the inertia of the pendulum and the
self-induction of the circuit.
But our hvdraulic analogy is perhaps even better. Sup-
pose two vessels to be joined by a horizontal tube : when the
water is in equilibrium its level is the same in both vessels.
But if in any wav this equalitv of level be destroyed, it will
tend to be re-established ; the water will fall in vessel A,
where it was above the normal level, and will rise in vessel
B, where it was below the normal. The water in the tube
will be set in motion, flowing from A to B. But when the
equalitv of level is established, the motion Avili not cease,
on account of the inertia of the water in the tube ; the water
will rise in vessel B and fall in A. The flow will then
commence in the opposite direction, the phenomenon will
be repeated in the opposite sense, and so on. (Fig. 17.)
Here again we hâve a sériés of oscillations ; but what deter-OSCILLATIONS BEFORE HERTZ.
27
mines their period ? It increases with the horizontal sec-
tion of the vessels (which we shall imagine cylindrical).
Thus, if a litre of water be transferred from one vessel to
the other, the différence of level caused by this transfer will
be less, in proportion as the cross-section of the vessels is
greater. Hence the motive force will be smaller, and the
oscillations slower.
On the other hand, the period will increase with the length
of the tube. To transfer a litre of water from one vessel to
the other, ail the w’ater in the tube must be set in motion;
a.	b-
«_—■	T	—1
Fio. 17.— Hydraulic analogue of the oscillatory discharge of a con-
denser. The period dépends upon the cross-section of the vessels A and B,
and on the length of the tube T, which correspond to the capacity and
self-induction of the circuit. If the friction in the tube is too great the
flow ceases to be oscillatory. (See p. 28.)
hence the inertia to be overcome increases with the length
of the tube, and the oscillations become slower.
We hâve seen in Chapter I that the horizontal section of
the vessels corresponds to the capacity of a circuit, the length
of the tube to its self-induction. The period of an electrical
oscillation thus increases with the capacity and with the
self-induction.
4. Damping.— The oscillations of a pendulum do not con-
tinue indefinitely ; each swing is a little smaller than the pre-
ceding one, and, after a certain number of oscillations of de-
creasing amplitude, the pendulum cornes to rest. This is
due to friction.28
MAXWELL^ THEORY.
Xow, we hâve seen that, in electrodynamic phenomena,
there is an agency which plays the same part as mechanical
friction, i. e., ohmic résistance. Electrical oscillations mnst
then decay like those of a pendulum ; they mnst grow weaker
and weaker, decreasing in amplitude, and finally cease.
This diminution is called damping. (See Fig. 18.)
Friction does not appreciably afFect the period of a pen-
dulum; and similarly, fhe ohmic résistance does not, as a
rule, sensibly change the period of electrical oscillations : they
Fig. 18.—Oscillatory discharge of a condenser, calculated for the
values of capacity C, inductance L, and résistance R indicated above.
grow smaller and smaller, but they do not become much
less rapid.
In certain experiments, however, Feddersen employed very
great résistances, and the period, as we might imagine, be-
came notablv longer. An extreme case is that in which the
discharge ceases to be oscillatory. (Fig. 19.)
Imagine a pendulum immersed in a very thick and viscous
fluid. Instead of descending with an increasing velocity,
it moves slowly, arrives without velocity at its position of
equilibrium, and does not swing beyond. There are no
oscillations.OSCILLATIONS BEFORE HERTZ,
29
It is on this principle that aperiodic or “ dead beat ” gal-
vancnieters are constrncted. The needle is mounted close
to a plate of copper, in winch Foucault currents are induced
by iis motion; hence the needle encounters a considérable
résistance, which retards it as friction would do. Thus,
instead of oscillating from one side to the other of its posi-
tion of equilibrium, wdiich would make the instrument dif-
licult to read, it swings gently up to the point, and stops.
These mechanical illustrations will suffice to show the
nature of the discharge of a Leyden jar where the ohmic
Fig. 19.—- Unidirectional discharge of a condenser. The conditions are
the same as in Fig. 18, except for the increased résistance. Curve A is the
critical case of quickest discharge. For smaller values of R the dis-
charge becomes oscillatory. Curves B and C are for larger values of R.
résistance is very great. The condition of electrical equi-
librium is attained slowly, and is not overpassed. The dis-
charge is no longer oscillatory, but continuous. This is
just wThat was shown by the experiments of Feddersen, which
thus confirai completely the theory of Lord Kelvin.
Friction and analogous reactions are not the only causes
of damping, and the kinetic energy of oscillating bodies is
not ail converted into heat. Consider again a tuning fork,
whose vibrations diminish gradually in amplitude. ITn-
questionably there are frictional effects which slightly heat30
MAXWELL’8 THEORY.
the fork, but, at the same time, we hear a sound : the air
is set in motion, and takes up energy from the fork. Part
oi the energy is thus dissipated by a sort of radiation into
space.
The energy of electrical oscillations also is expended in
two ways: ohinic résistance transforms a part of it into-
heat, but we shall soon see that another part is radiated into
space without losing its electrical character. This is a phe-
nomenon which was predicted by MaxwelPs theory, and
which is contrary to the old electrodynamics.
Thus we see that electrical oscillations undergo two kinds
of damping: by ohmic résistance (analogous to friction)>
and by radiation.CHAPTER IV.
HERTZ’S OSCILLATOR.
1. Hertz’s Discovery.— The displacement currents pre-
dicted by Maxwell’s theory could not, under ordinary circum-
stances, manifest their existence. As we hâve seen, they hâve
to overcome an elastic reaction which increases continually as
long as they continue to flow ; hence they must be either very
feeble, or of very short duration, if they flow always in the
same direction. In order that their effects may be appré-
ciable, they must change frequently in direction; the alter-
nations must be very rapid. Industrial altçrnating currents,
and even the oscillations of Feddersen, are entirely too slow
for this purpose.
This is the reason that Maxwell’s ideas waited twenty years
for experimental confirmation, and to Hertz was reserved the
honor of giving it. This eminent scientist, whose life was so
short and so full, contemplated at first the career of an archi-
tect, but was soon drawn by an irrésistible impulse toward
pure science. Noticed and encouraged by Helmholtz, he was
appointed professer at Carlsruhe: it is there that he made
the researches which hâve immortalized his name, and rose
in a day from obscurity to famé. But he was not destined
to enjoy it long; he had barely time to complété his new
laboratory at Bonn, when illness prevented him from utilizing
its resources ; and soon he died, leaving behind him, besides
his monumental discovery, experiments of great importance
on cathode rays and an original and profound book on the
philosophy of mechanics.
C31]32
MAXWELL9» THEORY.
2. Principle of the Oscillator.— The problem, as has been
explained, was to obtain extremely rapid vibrations. It would
seem, according to what we hâve seen in Chapter III, that it
would only be necessary to repeat the experiments of Fedder-
sen with diminished capacity and self-induction. It is thus
that the vibrations of a pendulum are made more rapid by
diminishing its length.
But it is not enough to construct the pendulum; it must
be set in motion. To do this, the pendulum must be displaced
from its position of equilibrium by some agency ; the cause
must then be removed suddenïy, that is, in a time very short
Fig. 20.— An electrical oscillator, consisting in two conductors Cx, C2,
joined by a wire interrupted by a spark-gap G. The conductors are
charged by an induction coil R, and discharged across the air-gap.
with respect to the duration of a period ; otherwise it will not
oscillate.
If a pendulum be displaced from the vertical by the hand,
for example; then, if, instead of letting go suddenly, the
arm be relaxed slowly without releasing the pendulum, the
latter will reach its position of equilibrium without velocity
and will not swing beyond it.
Thus the time occupied in the release must be very sho*f
with respect to the period of oscillation ; hence, with periods
of a hundred-millionth of a second, no System of mechanical
releaâe could operate, however rapid it may appear with
respect to our ordinary units of ,time.
Hertz solved the problem as follows :
Returning to our electric pendulum (see page 24), let usHERTZ’S OSCILLATOR.
33
eut the wire which joins the two conductors, leaving a gap
of several millimeters. This air-gap' divides our apparatus
into two symmetrical halves which we shall connect to the two
terminais of a ïtuhmkorff coil. (Fig. 20.) The secondary
current will charge our two conductors, and their différence
of potential will increase comparatively slowly.
At first the air-gap will prevent the conductors from dis-
•charging; the air acting as an insulator and keeping our
pendulum displaced from its position of equilibrium. But
when the différence of potential reaches a certain point the
spark of the coil will leap across the gap and open a path for
the electricity accumulated on the conductors. The air-gap
■ceases suddenly to be an insulator, and, by a sort of electrical
trigger, our pendulum is released from the cause which pre-
vented it from returning to equilibrium. If certain rather
eomplex conditions, thoroughly studied by Hertz, are ful-
fflled, the release will be sudden enough to produce oscilla-
tions.
3. Different Forms of Oscillators.— Thus, the essential parts
of an oscillator are :
lst. Two terminal conductors, of relatively large capacity,
which receive from the induction coil initial charges of op-
posite sign, and which exchange their charges at each half
oscillation.
2d. An intermediate conducting wire joining these con-
ductors, through which the electricity flows from one to the
c .ner.
3d. A spark micrometer, placed in the middle of the inter-
mediate conductor. This is the sent of a résistance which
permits the displacing of the electric pendulum from its posi-
tion pf equilibrium: this résistance afterward disappears
suddenly when the discharge takes place, thus releasing the
pendulum.
334
MAXWELL’S THEORY.
4th. An induction coil, whose tenninals are connected to
the two halves of the oscillator, and which furnishes their
initial charges. This is, so to speak, the arm which displaces
the pendulum from its position of equilibrium.
In Hertz's first oscillator (Fig. 21), the two terminal con-
ductors were spheres of fifteen centimeters radius, and the
intermediate conductor a straight wire 150 centimeters long.
Hertz also used square plates instead of the two spheres.
(Fig. 22.)
		
		
Fig. 22.—Hertzs oscillator with flat plates instead of the spherical
conductors.
Bending the intermediate conductor into the form of a
rectangle and bringing the two plates close together so as to
form a condenser, we hâve the oscillator of M. Blondlot
(Fig. 23), which he generally used as a resonator.
By simply replacing the plate condenser by a Leyden jar,HERTZ’8 OSCILLATOR.
35
and lengthening the intermediate wire, we hâve the apparatus
of Feddersen, whose vibrations are so slow that the release
may be made mechanically.
Suppressing the intermediate
conductor, we hâve Lodge’s oscil-
lator reduced to two spheres
between which the discharge takes
place ; but instead of two spheres,
Lodge ordinarily used three or
four. (Figs. 24 and 25.) We
shall see this apparatus again,
much reduced in size, in the
experiments of Eighi and Bose, in Chapter X.
Suppressing the terminal conductors, and reducing the
length of the intermediate wire to thirty centimeters, we hâve
Hertz3s small oscillator. (Fig. 26.) The charge, instead of
being concentrated at the extremities, is distributed over the
en tire length of the wire.
4. Function of the Spark.— We hâve seen how important
it is that the spark be “ good,” that is, that it shall leap sud-
denly, in a time very short with respect to the period of oscil-
S*
Fig. 23.— Blondlot’s oscilla-
tor. The two fiat conductors
(Fig. 22), are brought into
close proximity, so that they
constitute a condenser of com-
paratively large capacity.
Figs. 24 and 25.— Two forms of Lodge’s oscillator. In the former the
oscillations play across between the two middle spheres; in the latter,
they surge, like tidal waves, over the surface of the large sphere.
lation. Many circumstances influence the quality of the
spark. In the first. place, it must pass between two knobs ;
it would be bad if it passed between two points, or a knob and
a point*36
MAXWELL 'S THEOKY.
Again, the surfaces of the knobs must be well polished.
In air they oxidize rapidly and must be frequently cleaned.
Finally, the knobs must be separated by the proper dis-
tance; indeed, it is this which limits the amplitude of the
oscillations. In order to give strong oscillations, our pendu-
lum must be displaced considerably from
its position of equilibrium; that is, the two
halves of the oscillator must receive con-
sidérable charges before the spark occurs.
Now, the discharge will take place when
the différence of potential reaches a cer-
tain value, depending upon the length of
the air-gap; hence, we would naturally be
led to increase this distance ; but if this is
done, the spark ceases to be good.
After a little practice, it is easy to distin-
guish good from bad sparks by their
appearance and sound.
5. Influence of Light.— Hertz observed
another curious effect,— the primary and
secondary sparks seemed to act mysteri-
ously upon each other. When a screen
was placed between the two, the secondary spark ceased to
occur. Hertz thought at first that this was due to some elec-
trical action, but he perceived later 'that the phenomenon
was caused by the light of the spark.
Yet a plate of glass, which allows light to pass, prevented
the action of the sparks upon each other. This was because
the active ravs, in this case, are the ultra-violet, which are
absorbed by the glass; in fact, a plate of fluorite, which is
transparent to ultra-violet rays, does not prevent the action
of the primary spark.
Fig. 26.— Hertz’s
*mall oscillator, con-
sisting in two short
forass rods terminat-
ing in spheres at
the inner ends.HÊRTZ'8 OSCILLATOR.
ST
6.	The Use of Oil.— MM. Sarasin and de la Rive made a
great advance, in causing the discharge to take place in oiL
The knobs of the micrometer no longer become oxidized, the
incessant cleanings are not necessary, and the sparks are much
more regular. Moreover, the disruptive potential being
greater than in air, the electrical pendulnm may be further
displaced before it is released by the discharge. The ampli-
tude of the oscillation is thus increased.
7.	Value of the Wave-length.— Various theoretical consid-
érations enable us to calculate that Hertz’s large oscillatorr
described above, produces oscillations whose frequency is
50,000,000 cycles per second.
We know that the wave-length of an oscillation is the dis-
tance traversed by the disturbance in the time of a complété,
oscillation ; hence, if the velocity of propagation is the same
as that of light, that, is, 300,000 kilometers per second, the
wave-length will be the fifty-millionth part of 300,000 kilo-
meters, or 6 meters.
In the same way we may predict that Hertz’s small oscilla-
tor will give vibrations ten times as rapid, and consequently
of one-tenth the wave-length.
We shall see further on that these theoretical conditions,
hâve been confirmed by the direct measurement of the wave-
lengths.CHAPTER V.
METHODS OF OBSERVATION.
1. Principle of the Resonator.—An oscillator produces in
the space surrounding it displacement currents and phe-
nomena of induction; or again, it produces by induction a
disturbance at one point of a wire, and this disturbance is
propagated along the wire. It remains to be seen how these
facts may be observed.
For this purpose a resonator is generally used. When a
tuning fork vibrâtes, its vibrations are communicated to the
surroundiug air; and, if there be in the vicinity another fork
in tune with the first, this also will commence to vibrate. In
the same manner, an electrical oscillator produces a disturb-
ance in the surrounding medium, and causes a second oscil-
lator in the vicinity to respond, if their periods of oscillation
be the same. The second oscillator thus becomes a resonator.
But there is a great différence between acoustic résonance
and electrical résonance. An acoustic resonator responds
readily to vibrations which are exactly in unison with it ; the
résonance is practically nil if their periods differ, however
slightly. An electrical resonator responds readily to impulses
with which it is in tune, not quite so well to tliose whose
period is a little different from its own, and poorly to those
wdiich are notably discordant.
The reason for the différence is this : Acoustic vibrations
hâve a small décrément — their amplitude is nearly con-
stant — while electrical vibrations are damped rapidly. This
is why the electrical résonance is weaker and less marked.
A resonator is simply an oscillator without the induction
[38]MET H ODS OF OBSERVATION.
39
coil, which is now useless ; for the function of the coil is to
charge the oscillator, while, in this case, it is the external
field which excites the oscillations in the resonator.
Furthermore, any form of
oscillator may be used as a
resonator. Ordinarily the two
terminal conductors are dis-
pensed with, and in most cases
one or other of two forms is
used : the open resonator in which
the conductor is a straight wire
(A D, Fig. 28), and the closed
resonator, which is bent in a cir-
cle with the ends of the con-
ductor almost meeting. (Fig.
27.)
2f*	.•	»	xi	,	uiicrumeier screw ior measur-
. Opération OI tne xlesonator. ing the length of the induced
.	sp&rk
— When a Sound is produced
in an organ pipe it is reflected at one end, returns in
the opposite direction, is again reflected at the other end,
and so on. Ail these reflected waves interfère with each
other, adding their effects if they be in accord, destroying
each other when in opposition. Thus certain tones are re-
inforced and others are extinguished.
The operation of an electrical resonator is quite similar ; the
disturbance travels along the wire, is reflected at each end,
and the cumulative effect of ail these reflected waves, traveling
to and fro, is to reinforce those vibrations whose period is
suitable.
We hâve shown above why it is necessarv to furnish an
oscillator with an interrupter which releases the electrical
pendulum suddenlv. The same considérations do not apply
here, for it is the external field which excites the resonator.
Fig. 27.— A closed resonator
as used by Hertz. M is a
m î nt»Am ûf AT*	Cai»
40
MAXWELL9S THEORY.
But it is not sufficient to hâve oscillations in the resonator, we
inust be able to detect them; and the spark furnishes a con-
venient means of observation. Hence we retain a spark-
gap in the middle of the open resonator. With the closed
resonator it suffices to bring the ends close enough together
to allow the spark to pass. Thus, when the amplitude of the
oscillations reaches a certain point, the différence of po-
tential between the ends of the resonator may be sufficient
to cause a spark to leap across the air-gap, and in this way
only do we become aware of the existence of the oscilla-
e-T

D.
3 e-±.'
A
z
p»*

Fig. 28.— An open resonator, AD, compared to a sonorous tube, MN,
closed at both ends. The current, which is analogous to the velocity of
the air, is a maximum in the middle, but is zéro at the ends, as indicated
by the dotted curves. The wave-length, /, is twice the length of the
resonator.
tions. It is as if we had a vessel containing water in mo-
tion, but were unable to observe the disturbance except
when it became so great as to splash part of the water out
of the vessel.
The second ary sparks produced in the resonator are much
smaller than the primary sparks of the oscillator — they are
only a few hundredths of a millimeter in length.
If a tube closed at both ends contain a vibrating column
of air, the half wave-length of the vibration is equal to the
total length of the tube; and, by analogy, the half wave-
length of the free oscillation of a resonator should be theMETHODS OF OBSERVATION.
41
total length of the wire, if its ends hâve no capacity. The
ends of the conductor are thns comparable to the closed ends
of the sonorous tube ; for the current must be zéro at these
points, beyond which the electricity cannot pass, and where
it cannot accumulate. (Fig. 28.)
This ceases to be true when the capacity of the ends of the
conductor is appréciable, and for this reason the half wave-
length in a closed resonator is a little greater than the length
of the conductor.
We can now understand the operation of an open resonator.
Given a wire, A D, broken in the middle by a spark-gap,


B.lfC. ------------
1-rux.l ' l'.ru».

te-------1-------^1U-------è-------*!

P**
£
V*rW ly>na«.
H.

Fig. 29.— Before the spark occurs in the resonator. the two halves,
though separate, affect each other across the spark-gap. The current is
still maximum at the inner ends, B, C, and each half vibrâtes like a closed
organ pipe, with a wave length, — 4 x AB = 4x CD = 2 x AD.
E C. (Fig. 29.) This gap is very short— only a few hun-
dredths of a millimeter. The end B of A B, and the end C of
C D are thus, as it were, the plates of a condenser, separated
by a very thin layer of dielectric, and hence of considérable
capacity. Consequently, they correspond rather to the ope a
end than to the closed end of a sonorous tube.
If a spark passes, the whole resonator, A D, vibrâtes like
a tube with both ends closed, and the half wave-length is
A D. If the spark does not pass, the two halves, A B and
C D, of the resonator vibrate separately, but after the man-42
MAXWELL’tf THEOKÏ.
ner of a tube with one end closed and the other open. The
half wave-length is thus twice A B, that is, equal to A D, as
before.
3. Other Methods of TJsing the Spark.— The use of a re-
sonator, which distorts the wave by exaggerating certain
harmonies, may be avoided as follows :
Suppose the disturbance to be propagated along a wire, two
points of which are brought close together. (Fig. 30.) The
Fig. 30.— A method of investigating wave forms. A current from the
oscillator travels along the wire ABCD, a loop of which is bridged by a
spark-gap, BD. Owing to the finite velocity of propagation there will be
a différence of potential between B and D, of which the spark-length is a
measure.
wave will reach one of these points before the other, hencc
there will be a différence of potential between them ; and if
this différence be sufficiently great, a spark will leap across.
This method was used by MM. Pérot and Birkeland, who, by
varying the length of conductor comprised between the two
sides of the spark-gap, obtained sufficient data for determin-
ing the form of the wave.
Whether a resonator be used or not, it is évident- that the
spark furnishes a means of measurement. The distance be-
tween the knobs of the spark-gap may be varied by means
of a micrometer screw, and the distance over which the spark
will leap thus determined. (Fig. 31.)METHODS OF OBSERVATION.
43
The phenomenon becomes much more brilliant if a Geissler
tube be used; indeed a tube containing rarified gas is illu-
minated when it is simply placed
in the alternating field produced
by an oscillator.
4. Thermal Methods.— Instead
of observing the sparks we may
study the heating efïect of the
oscillating currents, either in a
resonator or in the wire along
which the wave is propagated.
For measuring the heating of conductors three methods
are available:
First. Measuring the élongation of the conductor;
Second. Measuring the variation of résistance ;
Third. The use of thermo-electric couples.
(1.) The measurement of élongation is not accurate, not-
withstanding the ingenious devices that hâve been used;
hence we shall not consider this method, nor the experiments
based on the motion of heated air in a tube surrounding the
conductor.
(2.) Measuring the change of résistance gives better re-
sults. The bolometric method is used: an ordinary Wheat-
stone bridge has ail of its branches traversed by the current
of a battery, and in addition, the oscillating current is sent
through one of them. (Fig. 32.) Suppose the galvanometer to
stand at zéro, and then pass the oscillations through one arm
of the bridge : this arm is heated, its résistance increases, the
equilibrium is destroyed, and the galvanometer is deflected.
(3.) The oscillating current is caused to traverse a fine
wire, near which (about one-tentli millimeter away) is placed
a thermo-pile. This method is very délicate.
Fig. 31.— A spark micrometer
as used' by Hertz in connection
with the resonator shown in Fig.
27.44
MAXWELL98 THEORY.
5. Mechanical Methods.— Mechanical methods, whether
founded on electrostatic attractions or on the mutual action
of currents, seem at first sight incapable of detecting Hertz-
ian oscillations. These oscillations are so rapid that no
mechanical device can follow
ail the variations of the electri-
cal or magnetic phenomena ;
ail that can be obtained is a
mean vaine of the phenomenon.
But a galvanometer, for in-
stance, receiving a sériés of al-
ternate impulses in opposite di-
rections, would remain at rest;
the mean value of the phenome-
non would be zéro.
Again, if the quadrants of
an electrometer be connected to
the apparatus producing the
oscillations and the needle
charged to a constant potential, the électrification of the quad-
rants will be continually changing sign while that of the
needle is constant; their mutual action will be continually
reversed, and its mean value will be zéro.
In order to obtain a deflection, Herr Bjerknes used an-
other arrangement. He employed a quadrant electrometer
with ail but two opposite quadrants removed. These were
connected respectively to the two terminais of a resonator so
arranged as to give no sparks. The needle of the electrom-
eter was insulated.
At a given moment the needle is charged inductively with
positive electricity at one end, and with négative at the
other, and the quadrants exert a certain action upon it. A
half-period later the charges of the quadrants hâve changed
Fig. 32.— Bolometric method of
measuring oscillations. The fine
wire MN carrying the current to
jbe measured is inserted in the arm
AB, and the bridge balanced. Any
heating due to the oscillations in
MN will cause a deflection.METHODS OF OBSERVATION.
45
sign, but the induced électrification of the needle is also re-
versed, so that the direction of the action is not changed.
6.	Comparison of the Different Methods.— There is an im-
portant différence between the methods founded on the spark
and the thermal or mechanical methods.
The spark simply occurs, or does not occur; and in order
that it may occur, it suffices that the potential be sufficiently
high at any instant ivhatever to break down the air-gap.
Hence it tells us only the maximum amplitude of the oscilla-
tion.
The thermal and mechanical methods, on the other hand,
give us intégral values : they indicate a mean* amplitude de-
pending upon the values of ail the oscillations.
Herr Bjerknes, by employing both methods simultaneously,
succeeded in measuring the damping of the free oscillations
of a resonator. It is clear that, the greater the décrément
of an oscillation, the smaller is the ratio of the mean ampli-
tude to the maximum : thus, by comparing the results of the
two methods of measurement, we may détermine this ratio.
7.	Coherers.— Branly devised a detector which is much
more sensitive than any of the foregoing. It is based on an
entirely different principle, and is known as the “ coherer ”
or “ radio-conductor.”f
Imagine a glass tube of rather small bore filled with
metallic filings. Each of the metallic particles is, in it-
* The word (moyenne) in the original does not convey the strict idea of
“ average ” or “ arithmetical mean,” for it isthe mean square that is gener-
ally indicated in such measurements.—F. K. V.
f The name radio-conductor,” given to this apparatus by Branly,
implies nothing regarding the nature of the phenomenon involved, but
simply that the tube becomes a conductor under the influence of the radia-
tions. The name “ coherer,” introduced by Lodge, though assuming more
definite knowledge on the disputed question, is more generallv used.—
F. K. V.46
MAXWELL’S THEORY.
self, a good conductor, but the electricity encounters a con-
sidérable résistance in passing from one to the other, so
that almost the cntire résistance of the apparatus is seated
in the points of contact between the particles.
Now, experiment shows that the résistance is greatly di-
minished when the apparatus
is exposed to Hertzian radia-
tions — that is, to the induc-
tive forces which proceed
from a Hertzian oscillator,
and which change sign a great
number of times per second.
We shall not attempt to ex-
plain this phenomenon*: suf-
fice it. to say that similar
effects hâve been observed on
exposing a coherer, not to
Hertzian radiations, but to
other influences of an entirely different nature, though
periodic in character and of very short period, such as certain
sound waves.
Whatever the explanation, the Hertzian radiations act as
if they produced a more intimate contact between the me-
tallic particles. A jar or an élévation of température re-
stores the coherer to its original condition of high résistance.
Suppose now a coherer to be connected in circuit with a bat-
tery and exposed to the radiations produced by an oscillator.
(Fig. 33.) When the oscillator is not working, the coherer is
traversed onlv by the continuons current of the battery.
If, now, the oscillator be put in operation, the coherer will
be traversed also by rapidly alternat ing current s produced
Fig. 33.— Early form of filings
coherer, C, with galvanometer G
and battery B.
* For the results of recent experiments, see pages 179-185.METHODS OF OBSERVATION,
47
by induction from the oscillator; but, in this case, as the
alternating currents diminish the résistance, the continuons
current is greatly increased, and a galvanometer in the cir-
cuit will show a marked deflection.
Branly’s detector may be compared to the bolometer de-
scribed above: in each apparatus the oscillations hâve the
effect of changing the résistance of a conductor traversed by a
continuons current, but the variation is due to quite differ-
ent causes: in the one case, to the heating of the wire; in
the other, to a more intimate contact between the particles
of métal.
Moreover, the coherer is vastly more sensitive; we shall
see it again in the experiments of Bose in Chapter X ; in-
deed, it is this device that has made wireless telegraphy
possible.
The coherer has been used in the effort to détermine
whether Hertzian radiations are emitted by the sun, but
the results were négative. Perhaps these radiations are ab-
sorbed by the solar atmosphère.
Experiments show unquestionablv that gases under ordi-
nary pressures are quite transparent to these radiations:
but is this the case with highly rarified gases ? We hâve
seen that a Geissler tube glows in the field of an oscillator.
It does not give light without absorbing energy ; hence rari-
fied gases absorb Hertzian radiations, and it is possible that
those which the sun may émit are absorbed by the upper
strata of the two atmosphères, where the pressure is very low.CHAPTER VI.
PROPAGATION ALONG A W3RE.
1. Production of Waves in a Wire.—A Hertzian oscillator
produces forces of induction in the field which surrounds it.
If we place a long wire in this field, the forces of induction
will generate alternating currents in the part of the wire
rearest to the oscillator, and this electromagnetic disturbance
will travel along the wire.
To force the electromagnetic disturbances to follow the
wire several devices may
be used, among which we
may mention the electro-
static method of Hertz,
and the electromagnetic
method of M. Blondlot.
Hertz's method.— The
spheres of the oscillator
are' replaced by two métal
plates, A and B (Fig. 34),
of large capacity; opposite
these are placed two simi-
lar ones, A' and B', and at the middle of each of the latter
is attached a wire of a certain length. The capacities of
the plates A and B are thus increased by causing each to
form part of a condenser.
Tf the oscillator be put in operation, one of the plates,
say A, will be charged positively, and B negatively. At
the end of a half oscillation the charges will hâve changed
sign ; and so on, the polarity changing with each half-period.
" [48]
Fig. 34.— Hertz’s method of establish-
ing waves in wires. The plates A and B
of the oscillator act electrostatically uçon
two similar plates, A'B', to which
the wires are attached.PROPAGATION ALONG A WIRE.
49
The plates A' and B' are charged inductively witli op-
posite signs to those of A and B, and the wires proceeding
froin them become the seat. of an oscillatory phenomenon
of the same period as that of the oscillator.
M. Blondlofs method.— The oscillator takes the form of
a curved wire ending in a sort of condenser. (Fig. 35.)
Around this first wire is bent another, whose ends are car-
ried out radially to a considérable distance. The two cir-
cular conductors are insulated from each other by a cover-
ing of rubber.
When the oscillations are produced, the oscillator is the
seat of periodic currents
which excite induced cur-
rents of the same period in
the second conductor.
2. Mode of Propagation.
— Is the propagation of a
Hertzian oscillation, that is,
* wire which connects the plates of the
OI an alternatmg current OI oscillator is bent into a circle, and
i.i/.	.	acts inductively upon another loop
Very nign irequency, similar concentric with it, to which the wires
in every respect to the prop- are attached*
agation of a continuons current, such as is furnished by a
battery?
One striking différence was observed long ago by experi-
menters: a continuons current distributes itself uniformly
over the whole section of the conductor; but this is not the
case, even with the low-frequency alternating currents em-
ployed in the arts. In the axis of the conductor the current
is very weak, while its intensity is much greater at the sur-
face. It is as if the surface current shielded the interior
of the conductor from external actions, by the forces of in-
duction which it produces.
With Hertzian oscillations, whose period is very much50
MAXWELL’S THEORY.
shorter, we should expect to see the phenomenon exaggerated.
There should be no current except in a very thin superficial
layer. Bjerknes verified this prédiction in an ingenious
manner.
We hâve seen (page 45) how this scientist measured
the damping of a resonator. This damping dépends upon
the material of which the resonator is made: it is not the
same for a resonator of iron as for one of copper.
Bjerknes plated his iron resonator, by electrolysis, with
a coating of copper, and the copper resonator with a coat-
ing of iron. When the thickness of the coating was greater
than a hundredth of a millimeter, the iron resonator acted
like one of copper, and the copper resonator like one of iron.
This showed that the currents are confined to a shell
whose thickness is of the order of a hundredth of a milli-
meter. This effect is in accord with both the old theory and
that of Maxwell.
But Maxwell’s theory predicts another peculiarity, which,
unfortunately, hardly admits of direct experimental proof.
The altemating currents which flow in a wire produce
forces of induction in the surrounding air. According to
Maxwell, these forces of induction should generate displace-
ment currents in the air itself.
Thus, with continuons currents, we hâve conduction cur-
rents through the whole mass of the conductor and none
at ail in the surrounding air. With high-frequency altemat-
ing currents, on the other hand, there are conduction cur-
rents in the superficial layer of the conductor, none in the
interior, and displacement currents in the air.
3. Velocity of Propagation and Diffusion.— Kirehhoff un-
dertook to compute the velocity of propagation of any elec-
trical disturbance whatever. He assumed, at the start, that
the conductor was perfect, and that the current, encounter-PROPAGATION ALONG A WIRE.
51
ing no ohmic résistance, woulcl only hâve to overcome the
self-induction, which acts like inertia. He showed that,
under these conditions, the velocity of propagation would
be equal to the ratio of the units — that is, to the velocity
of light—300,000 kilometers per second.
Moreover the propagation is uniform : if the disturbance
be confined originally to a certain part of the wire, one
meter long, for example, at the end of a hundred-thousandth
of a second the front of the wave will hâve advanced three
kilometers, and the tailof the wave also three kilometers;
so that the extent of the disturbance will not be changed,
but it will still occupy just one meter of the conductor.
But these theoretical conditions are never realized in
actual conductors, for, besides the self-induction, there is
always an ohmic résistance analogous to friction to impede
the current. What happens then? The front of the wave
advances always with the same velocity — that of light;
but the tail of the wave travc-ls much less rapidly, so that
the space occupied by the disturbance becomes greater and
greater: just as a caravan is spread along the road by the
lagging behind of followers. This is called the diffusion 6f
the current.
The diffusion becomes less noticeable as the period of the
oscillations is shortened. Practically we may say that, with
Hertzian waves, there is no diffusion, and that ail conductors
behave as if they were perfect. Not that their ohmic résist-
ance is less, for it is actually greater, as the current utilizes
only the thin outer shell of the conductor ; but the effect of the
self-induction, which dépends upon the variations of the
current, increases much faster when these variations are ex-
tremely rapid, and thus the ohmic résistance becomes negli-
gible with respect to the self-induction.
These are the effects for which the old theory and that of52
MAXWELL>S THEORY.
Maxwell both provide,— they are agreed on this point. We
shall now see that these prédictions are confirmed by experi-
ment.
4. Experiments of MM. Fizeau and Gounelle.— Fizeau
and Gounelle’s experiments were made in 1850, with an ap-
paratns based on the same principle as the celebrated method
of Fizeau for measuring the velocitv of light.
A dise of wood, having its circumference divided into
thirty-six sectors of alternating wood and platinum, is caused
to rotate with great rapidity. (Fig. 36.) Twto wires, each
differential galvanometer.
terminating in a metallic brush which bears upon the circum-
ference of the dise, may thus be alternately connected and
insulated from each other, as the dise rotâtes. There were
three such pairs of brushes, B and C, E and F, and E' and
F', so arranged that the connections between B C and E F
were opened and closed at the same time, while the connec^
tion E' F' was closed when the other two were open, and
vice versa.
A battery was connected with one terminal to the ground
and the other to a wire A B, attached to the brush B. APROPAGATION ALONG A WIRE.
53
long line wire, ODE E', ran from brush C to the end of
the line D, and retumed to the brushes E and E'. Finally,
two wires F G and F' G' connected the brushes F and F' to
the ground.
Let us now see what would take place if the electricity
were propagated with a perfectly definite velocity, like light
or Sound. We shall dénoté as a “ period ” the time which
elapses from the moment one of the brushes cornes in con-
tact with a sector until the contact ceases — that is, the
thirty-sixth part of the time of one rotation of the dise.
This period will decrease as the speed of rotation increases.
Suppose the time T, required for an impulse to travel the
length of the line C D E, to be equal to an even number of
periods. The electricity coming from the battery will pass
from B to O at the moment the connection B C is closed,
will traverse the line, and arrive at E and E' at the end of
time T. At this moment the connection E F will be closed
and E' F' open, so the current will pass through the wire
F G.
If, on the other hand, T be equal to an odd number of
periods, the current, on reaching E and E' will find E F
open and E' F' closed, and the current will pass through
the wire F' G'.
Thus the speed of rotation could be adjusted so that the
whole current would pass through F G, or through F' G' ; or,
for intermediate velocities, the current would be divided in
different proportions between the two wTires.
The wires F G and F' G' included respectively the two
coils of a differential galvanometer, on whose needle they
acted in opposite senses; so that the deflection of the gal-
vanometer indicated the comparative strength of the inter-
mittent currents in the two wires. The experimenters were
thus enabled to détermine what velocity of rotation was54
MAXWELL’8 THEORY.
necessary to make T equal to a given multiple of the period,
and hence to measure T, winch gave them the velocity of
propagation.
Yarious circumstances, to which we shall refer later, arose
to coinplicate the phenomena, and it was found that the cur-
rent in F G (or F' G'), could never be reduced to zéro, but
showed only a succession of maxima and minima, of which
the former alone could be determined.
The observations of Fizeau and Gounelle gave 100,000
kilometers per second for the velocity in iron, and 180,000
kilpmeters per second for copper.
5. Diffusion of Currents.— It has been noted that the
current could never be reduced to zéro, as would be the case
if the impulse traveled with a perfectly definite velocity.
It appears rather that the wave spreads out as it travels, so
as to occupy more space on the wire when it arrives than it
did at the start. This phenomenon, which the experiments
of Fizeau established beyond a doubt, was called by him the
“ diffusion ” of the current.
We hâve seen above (page 51), the ground on which
this phenomenon might hâve been predicted. The consé-
quences follow readily. The wave must travel as if a part
of the electricity moved wûth the velocity of light, while the
rest followed at a lesser, and variable, velocity. The resuit
would be, as it were, a column with a strong front, advancing
at a velocity of 300,000. kilometers per second, but leaving
behind laggards straggling along the road.
The method of Fizeau measured, not the maximum veloc-
ity — that of the head of the column — but the mean veloc-
ity, which should be much smaller. This explains why his
results are so far below 300,000 kilometers.
The mean velocity in iron is less than in copper, for two
reasons:PROPAGATION ALONG A WIKE.
55
First, because iron is magnetic, and the self-induction is
increased by reason of tbe transverse magnetization of the
wire ; second, because the spécifie résistance of iron is greater
than that of copper, and hence the diffusion is greater.
Fizeau’s experiments are thus not in conflict with the
theory.
6. Experiments of M. Blondlot.—The above discussion
indicates how different is the propagation of a continuons
current, or of an intermittent or alternating current of low
frequency, from the propagation of Hertzian waves.
These last are of very short duration and consist in oscil-
lations of extremely high frequency. We may well surmise
that the effect of diffusion is negligible, the residue lagging
behind very small, and the mean velocity of propagation ex-
tremely close to that of the wave-front, that is, 300,000 kilo-
meters per second.
But the experiments just described did not justify any
conclusions regarding waves of this kind. Further researches
were necessarv, and thus M. Blondlot was led to undertake
the following experiments:
His apparatus comprises two symmetrical Leyden jars, F
and F', of small capacity. (Fig. 37.) The interior coatings,
A and A' are joined by a wire, broken in the middle by a
spark micrometer. The two knobs of the micrometer are con-
nected to a Ruhmkorff coil. The coatings A and A' of the
Leyden jar, the conductor which joins them, and the spark
micrometer constitute an oscillator, which we shall call E.
The outer coating of each of the jars F and F' is divided
into two insulated sections. We shall dénoté as B and C
the two sections of the outer coating of F, and B' and C'
those of the outer coating of F'.
B and B' are connected in two manners:
First. By a moistened cord;56
MAXWELL’S THEORY.
Second. By a short wire, containing a spark micrometer
at its niicldle point. The terminais of the spark-gap are
two met allie points, P and P'.
Similarly O and C' are connected in two ways:
First. By a moistened cord;
Second. By a line wire. This wire runs from C to a point
D at the end of the line, then retums to the point P, men-
tioned above ; after traversing the micrometer, the electricity
Fig. 37.— Blondlot’s method of measuring the velocity of high fre-
quency waves in wires. A and A' represent the inner coatings of two
Leyden jars, connected together through a spark-gap, and constituting an
oscillator, E. BC and B'C' are the outer coatings, each divided into two
sections. CDP and C'D'P' are the line wires, and PP' two points be-
tween which leaps the secondary spark, which is observed in a rotating
mirror. The dotted lines represent wet cords.
must pass from P' to a point D', at the end of the line, re-
turning then to the coating C' of the jar F\ The pôles
of the line thus support four wires, O D, D P, P' iy? D' C' ;
and the electricity, in passing from O to C' by this route,
passing through the micrometer, must traverse the length
of the line four times: twice in going, twice in returning.
Thus there are two routes from B to B' or from C to C' ;
the one through a wet cord of high résistance, the other
through a metallic circuit, interrupted by a spark-gap.PROPAGATION ALONG A WIRE.
57
If the variations of potential are slow, the current will
pass entirely by the moist cord; for the différence of po-
tential between the points P and P' will not be great enough
to cause a spark to leap, and the micrometer will remain an
insulator.
If, on the other hand, the variations of potential are rapid,
a spark will leap, opening a path for the electricity across
^the micrometer: almost the entire current will pass by the
metallic circuit, while the cord will carry only a negligible
amount, owing to its high résistance.
The apparatus opérâtes as follows:
The Ruhmkorff coil charges the inner coatings, A and A',
of the jars; say A positively and A' negatively. The coat-
ings B and C are charged negatively by induction ; the coat-
ings B' and C', positively. Hence a quantity of electricity
must flow from B to B' and from C to C' ; but, as the varia-
tions of potential are comparatively slow, it will flow through
the moist cords.
At a certain moment a spark will pass in the oscillator E.
This discharge is oscillatory, as its appearance plainly
shows. The coatings A and A' are discharged suddenly, and
the charges accumulated on B and C, B' and C', are sud-
denly and simultaneously liberated. Currents will thus flow
baek from B' to B and from C' to C, but this time following
the metallic circuit, for their variations are sudden.
Two sparks will pass in the micrometer P P', which is the
common part of the metallic circuits B B' and C C'. The
first spark will pass at the moment the disturbance starting
from B arrives at P ; the second, when the disturbance start-
ing from C arrives at P. As the path B C is very short, the
interval of time elapsing between the two sparks will be the
time required by the disturbance to traverse the path CDP.
This length, CDP, we shall call the length of the line. It5S
MAXWELL'S THEORY.
is double the length of the wire C D, which runs to the end
of the line, and half the total length of the circuit CDPP'
D' C'.
The time interval between the two sparks was determined
by means of a rotating mirror which threw the light of the
sparks upon a sensitive plate, and the distance between the
two images on the plate was measured.
The first experiments, in which the length of line was a
little over one kilometer, showed, on an average, a velocity of
293,000 kilometers per second ; later, with a length of line
of 1,800 meters, an average velocity of 298,000 kilometers
was obtained.CHAPTEK VIL
MEASTTREMENT OF WAVE-LENGTH AED MULTIPLE
RESONANCE.
1. Stationary Waves.— The experiments above described
show that the velocity of propagation along a wire is the
same as that of light. To détermine the number of vibra-
tions per second it remains to measure the length of wave,
and to divide by this length the distance traversed in a sec-
ond, i. e., 300,000 kilometers.
To this end, Hertz undertook to ntilize the phenomenon of
stationary waves.
Suppose a periodic disturbance to travel along a wire:
when it reaches the end of the wire it will be reflected and
will return in the opposite direction. The resulting dis-
turbance may be obtained by combining the direct and re-
flected waves. Two periodic waves are added together when
they are of the same phase; that is, when the alternating
currents which accompany them are both positive or both
négative at the same time : they annul each other when they
are of opposite phases ; that is, when the currents accompany-
ing one are positive while ihose of the other are négative,
and vice versa.
The two waves, direct and reflected, are of the same phase
and are added if the différence between the distances which
they hâve traveled is an intégral number of wave-lengths* ;
* The author evidently had in mind the case where the wave is reflected
from a capacity (Fig. 38): when the reflection occurs at the free end of a
wire, the positions of nodes and loops are interchanged. (Fig. 39.) See
below.—F. K. V.
[59]60
MAXWELL’S THEORY.
the corresponding points of the wire, where the action is a
maximum, are called “ loops,” or “ antinodes.”
The two waves are opposite in phase, and mutually de-
structive, when the différence between the distances trav-
eled is an odd number of half wave-lengths : the correspond-
ing points of the wire, where the action is nil, are called
“ nodes.”
The distance between two consecutive nodes is equal to
a half wave-length.
Let A and B be two such nodes. (Fig. 38.) At A, the dif-
férence between the distances traveled by the two waves is
(B-
le-
Fig. 38.— Formation of stationary waves in wires terminating in a
large capacity. N, N are nodes, where the current is zéro, and L, L are
loops, where the current reaches a maximum. The ordinates of the dotted
curves indicate relative values of the eurrent. AB = -^ = half wave-
length.	2
equal to an odd number of half wave-lengths, say 2n+l. The
direct wave passes A before reaching B; the reflected wave
passes B before reaching A. When we move from A to B the
distance traversed by the direct wave is increased by the
length A B, while the distance traversed by the reflected wave
is diminished by A B. Thus the différence between the dis-
tances traveled is diminished hy 2 x A B. Now, as the
point B is a node, this différence also must be an odd num-
ber of half wave-lengths, 2n — 1. Thus 2 x A B must be
precisely equal to a wave-length.
Such is the phenomenon of stationary waves as it was atWAVE-LENGTH AND MULTIPLE RESONANCE.
G1
first understood by Hertz, who lioped to find in it a simple
method of measuring wàvedengths.
Unfortunately, as we shall now see, the matter is some-
what more complicated.
Eeflection at the end of a wire may take place in differ-
ent ways. If the wire simply terminâtes abruptly, without
capacity, the electricity cannot accumnlate at the end, and
the current at that point must be zéro. The end of the wdre
is a node. (Fig. 39.)
If, on the other hand, the wire terminâtes in a considér-
able capacity — for example, if the two parallel wires shown
(IIH
te-

Fig. 39.— Formation of stationary waves in wires which terminate
abruptly. The positions of nodes and loops are reversed with reference
to those of Fig. 38.
on pages 48 and 49, be connected to the two plates of a
condenser — then the end is a loop. (Fig. 38.)
Again, the ends of these two parallel wires may be joined.
A wave which has traversed one of the wires in the positive
sense returns by the other in the négative sense, and, inter-
fering with the positive wave following the second wire,
produces stationary waves.
2. Multiple Résonance.—We hâve seen (page 38) that
a resonator responds readily to an oscillator with which
it is perfectly in tune, but that it also responds, though less
readily, to an oscillator of different period.
Consequentlv, it is possible to work with an oscillator62
MAXWELL’S THEORY.
and a reson ator wliose periods differ considerably. This
was done by MM. Sarasin and de la Eive.
They discovered an unexpected law, which they called the
“ law of multiple résonance.” The internode, or distance
between two nodes, which, according to the preceding para-
graphe should be the measure of the half wave-length, changes
when different reson ators are used with the same oscillator,
but remains the same when the oscillator is changed while
using the same resonator. Hence, that which is measured
must be something pertaining to the resonator itself; in
fact, the internode is the half wave-length of the free oscilla-
tions of the resonator — not of those produced by the oscil-
lator.
The following is the explanation offered by MM. Sarasin
and de la Rive:	The wave produced by the oscillator is
complex, and results from the superposition of an infinity
of simple vibrations, which may be called its components.
Such is the radiation of a luminous body which produces,
not monochromatic light, but white light, giving a continuous
spectrum.
Each resonator responds to only one of these components,
and when a resonator is used to measure a wave-length, it
is the wave-length of this component which is obtained: the
other components hâve no effect on the resuit. In other
words, we measure the wave-length of the free oscillation
of the resonator.
In like manner in acoustics, a complex sound made up
of several harmonies may be analyzed by a resonator which
suppresses ail but one of these harmonies.
3. Aniother Explanation—The phenomenon may be ex-
plained in a different manner. The vibrations produced
by an oscillator decay very rapidly; the energy of theWAVE-LENGTH AND MULTIPLE RESONANCE.
63
oscillation is quickly transformée! into heat by the résistance
of the spark-gap, or dissipated by radiation into space.
What is the resuit? We hâve shown above how the re-
flected wave is added to or subtracted from the direct wave,
and that it is this composition of the two impulses which
produces stationary waves. But consider a point, A, at some
distance from the end of the wire. (Fig. 40.) A considérable
time is required for the impulse to travel from A to the end of
the wire and retum after reflection to A; and during this
time, the damping of the direct wave may hâve completely ex-
tinguished it. Thus, on the arrivai of the reflected wave
Fig. 40.— A strongly damped wave-train about to be reflected from the
free end B of a wire. Interférence between the direct and reflected waves
can occur only within the space BC, equal to half the length of the wave-
train.
at A there is no direct watve for it to combine with, and
hence no stationary waves can be formed.
Thus we feee that there are no stationary waves, properly
speaking, except near the end of the wire.
Yet, by using a resonator we may observe a succession of
nodes and loops throughout the whole length of the wire.
How can this be ?
The Paradox is readily explained on the assumption that
the vibrations of the oscillator are damped much more rapidly
than those of the resonator. Wlien the direct wave passes
it sets the resonator in vibration ; when the reflected wrave
returns, the direct wave has been extinguished in the wire,
but the resonator has not ceased to vibrate. It receives a64
MAXWELL’S THEORY.
second impulse from the reflected wave. Will this impulse
increase the amplitude of its oscillations or diminish them ?
Consider an analogy.
A pendulum receives a first impulse which causes it to
move, say from left to right. After a half oscillation it will
be moving from right to left ; after a complété oscillation it
will again be moving from left to right. In general, after
a whole number of oscillations it will move from left to right ;
after an odd number of half oscillations, it will move from
right to left.
Suppose it to receive a second impulse in the same sense as
the first. If this impulse be given after a whole number of
oscillations, when the pendulum is moving from left to right,
it will tend to increase the velocity; if the impulse corne
after an odd number of half oscillations, when the pendulum
is moving from right to left, it will tend to diminish it.
So with the resonator: this apparatus receives a first im-
pulse on the passage of the direct wave; a second, on the
passage of the reflected wave. If, between these two im-
pulses, the resonator perform a whole number of oscillations
— that is, if the différence between the distances traveled by
the two waves be equal to a whole number of wave-lengths
of the resonator — the effects of the two impulses will be cu-
mulative, and a loop will be observed. If, on the other
hand, the différence between the distances traveled be equal
to an odd number of half wave-lengths of the resonator, the
effects of the two impulses will annul each other, and a node
will be observed.
Thus, the distance between two nodes should be equal to
the half wave-length of the resonator; the wave-length of
the oscillator does not enter the resuit.
A few remarks in passing regarding this second ex-
plan a tion :WAVE-LENGTH AND MULTIPLE RESONANCE.
65
We hâve seen what occurs when the two impulses received
by a pendulum are in the sanie sense: the effect is reversed
when they are in opposite senses. Kow, it is évident that
the impulse due to the direct wave and that due to the re-
flected wave may be in the same sense or in opposite senses,
according to the manner in which the reflection is produced
(see page 61) and according to the position of the resonator.
Ilence we hâve an exceedingly simple explanation of the ex-
periments of M. Turpain, which hâve seemed paradoxical
to some persons, but which are sufficiently accounted for by
symmetry.
In the second place, we may ask why an apparatus con-
si sting in two long wires is not équivalent to a large resonator,
but responds indifferently to excitations of ail periods. If
it were not for damping, the reflected waves, interfering as
has been explained on page 39, would produce résonance
effects. But this is not the case : when one of the reflected
waves reaches a given point of the wire, the direct wave has
long been extinguished, and there is no interférence.
4. Experiments of Garbasso and Zehnder.— Between the
two explanations proposed above, experiment alone can décidé.
Zehnder attempted to observe directly the continuons spec-
trum postulated in the theory of MM. Sarasin and de la Rive.
He employed a sort of grating which should separate the
different components of a complex wave emitted by the oscil-
lator, just as the ordinary grating used in optics séparâtes
the different colors which constitute white light.
Garbasso endeavored, by means of a complicated appa-
ratus which we cannot describe here, to imitate the dispersion
produced by a prism when acting on white light.
These experimenters obtained the results which they ex-
pected, thus apparently confirming the explanation of Sarasin
and de la Rive.
560
MAXWELL'S THEORY.
The experiments seem conclusive, but they are not. In-
deed, it may be sbown by a simple calculation that a damped
vibration bas the characteristics of a complex vibration giving
a continuons spectrum in which the intensities are distributed
according to a particular law.
Hence it is not sufficient to prove that the vibration emitted
by an oscillator behaves as if it had a continuons spectrum ;
Fig. 41.— A strongly damped oscillation, such as is produced by a
dumb-bell oscillator. The logarithmic décrément = .26.
it must also be shown that, in this spectrum, the intensities of
the various components do not vary according to that par-
ticular law.
5. Measurement of the Décrément.— Quite on the contrary,
a sériés of experiments which we shall now consider hâve
shown, not only that the intensities do vary according to this.
law, but that the second explanation is the true one.WAVE-LENGTH AND MULTIPLE RESONANCE.
67
It was first necessary to prove the fondamental hypothesis
on whieh this second explanation rests — to be certain that
the damping of the oseillator is much more rapid than that
of the resonator.
We hâve seen (page 45) how Herr Bjerknes measured the
décrément of a resonator.
Fig. 42.— A feebly damped oscillation, such as that of a resonator with
a logarithmic décrément of .034. The waves which excited the resonator
are here supposed to hâve subsided, and the oscillations are undergoing
tbeir normal decay.
For an oseillator he fonnd a “ logarithmic décrément of
0.26, while he obtained, for two resonators, 0.002 and 0.034.
* The term “ logarithmic décrément,” as used by Bjerknes, dénotés
the natural logarithm of the ratio of two adjacent maxima — separated
by a complété period. This differs from the ordinary définition of
Kohlrausch and others, which involves the ratio of two consecutive tum-
ing points — sbparated by a half period — and which gives values of
the logarithmic décrément half as great as the above.— F. K. V.68
MAXWELL'8 THEORY.
Tliat is to say, to reduce the amplitude of the oscillations
to one-tenth of its initial value, nine oscillations were suffi-
cient in the case of the oscillator, while the two resonators re-
quired, respectively, more than 60 and more than 1,000.
(See Figs. 41 and 42.)
ïhus the vibration of an oscillator is damped much more
rapidly than that of a resonator.
6. Experiments of Strindberg.— To complété the proof it
was necessary to show that if, by any artifice, the damping of
the resonator could be made greater than that of the oscillator,
the phenomena would be reversed ; that is, the internode
would dépend no longer upon the resonator, but on the
oscillator.
This was done independently by M. Décombe in France,
and M. Nils Strindberg in Sweden.
I cannot write this name without reminding the reader that
M. Strindberg, not content with serving science by his intelli-
gence, would also contribute his courage. Ile accompanied
M. Andrée on his perilous æronautic journey in the polar
régions.
To accomplish the desired resuit it was necessary to di-
minish the damping of the oscillator and increase that of the
resonator.
To diminish the décrément of the oscillator it was first
necessary to stop the loss of energy in the spark. This
seems at first impracticable, for, without an interrupter, the
release of the “ electric pendulum ” is impossible and its
oscillations cannot be started. But M. Strindberg met the
difficulté by a simple artifice. A primary oscillator was pro-
vided with a spark-gap. It acted by induction upon a second-
arv oscillator which was entirely similar to the first, but,
being set in vibration by the action of the primary, did not
require an interrupter. This secondary oscillator had theWAVE-LEXGTH AXD MULTIPLE RESONANCE.
m
same period as the primary, but a smaller décrément, and it
was used to produce the disturbance in the wires by means of
the arrangement of M. Blondlot. (See page 49, Fig. 35.)
Again, it was easy to increase the résistance of the reso-
nator; and, as the résistance is a sort of friction, this had
the cffect of increasing the damping of the oscillations.
7.	Experiments of Pérot and Jones — There are other more
direct methods of proof. We liave seen that, notwithstanding
the damping, true stationary waves are formed ; but only near
the end of the wire. The study of these stationary wave9
enables us to détermine the form of the disturbance produced
by the oscillator. But this study, to be successful, must be
carried on without the aid of a resonator; for we hâve seen
that resonators produce secondary effects which persist far
from the end of the wire, and are then interpreted as the
phenomenon of “ multiple résonance.” These disturbing
effects must be suppressed.
The various methods described on pages 42-44, which are
independent of the resonator, hâve been used to this end.
M. Pérot used the spark without a resonator.
Mr. Jones employed a thermal method, based on the use
of a thermo-pile.
Herr Bjerknes used a mechanical method.
Ail these experiments confirmed the second explanation.
8.	Experiments of Décombe.— Even these methods did
not seem suflSciently direct to M. Décombe. He wished to
study the disturbance at the moment it was produced by the
oscillator; indeed, we may well inquire if the oscillation is
not changed in passing from the oscillator to the wires, or in
traveling along the wires.
To this end M. Décombe undertook to photograph the spark
of the oscillator by means of a rotating mirror. This had
been done by Feddersen (cf. Chapter III), but with oscilla-70
MAXWELL’S THEORY.
tions of much lower frequency. With Hertzian oscillations
the difficulties were much greater; indeed, they would hâve
been insurmountable with the apparatus used by Hertz him-
self (50,000,000 vibrations per second). M. Décombe had to
be content with an oscillator giving 5,000,000 vibrations,
whereas the apparatus of Feddersen gave only 20,000 to
400,000.
The different sparks which correspond to the successive
oscillations produce, on the sensitive plate, an image con-
sisting in separate points, because of the motion of the mirror.
The motion must be sufficiently rapid to keep these points
distinct and separate from one another. M. Décombe’s
mirror made 500 révolutions per second.
In order that the plate might receive an impression, not-
withstanding the extreme brevity of the exposure, M.
Décombe found it necessary to carry to the extreme every
means at his disposai, and to put ail the chances in his favor.
He had to use an oscillator with a small décrément, to pro-
duce the spark under oil, where it is smaller and more bril-
liant, and to use a particularly energetic developing solution.
The optical apparatus was so arranged that the luminous
spots would be very small and very intense.
Ail the details of tins experiment reflect the greatest crédit
upon their author. Success rewarded his efforts, and he ob-
tained images whose study reveals the existence of a simple
damped oscillation, in conformity with the second explana-
tion.
The oscillator, it is true, is not that of Hertz, and its oscilla-
tions hâve only one-tenth the frequency of his, but the différ-
ence is sufficiently small to justify us in reasoning from one
to the other.CHAPTER VIII.
PROPAGATION IN AIR.
1. The Experimentum Crucis.—Ail the experiments which
we hâve thus far considered are incapable of deciding between
the old theory and that of Maxwell.
Both théories agréé that electrical disturbances should be
propagated along a conducting wire with a velocity equal to
that of light. Both take account of the oscillatory character
of the discharge of a Leyden jar, and consequently of the os-
cillations produced by a Hertzian oscillator. Both assert that
these oscillations should produce electro-motive forces of in-
duction in the surrounding medium, and hence should excite
a resonator placed in the vicinity.
But according to the old theory, the propagation of in-
ductive effects should be instantaneous. If, indeed, there be
no displacement currents, and consequently nothing, electri-
cally speaking, in the dielectric which séparâtes the inducing
circuit from that in which the effects are induced, it must
be admitted that the induced effect in the secondary circuit
takes place at the same instant as the inducing cause in the
primary; otherwise in the interval, if there were one, the
cause wrould hâve ceased in the primary circuit, while the
effect is not yet produced in the secondary; and, as there is
nothing in the dielectric which séparâtes the two circuits,
there is nothing anywhere. Thus the instantaneous propaga-
tion of induction is a conclusion which the old theory cannot
escape.
According to Maxwell's theory, induction should be propa-
gated in air with the same velocity as in a wire; that is, with
the velocity of light.
[71]MAXWELL’,8f THEORY.
Here, then, is the experimentum crucis : we must détermine
with what velocity electromagnetic disturbances are propa-
gated by induction through the air. If this velocity be in-
finité, we must adhéré to the old theory ; if it be equal to the
velocity of light, we must accept the theory of Maxwell.
IIow, then, can this velocity be measured? We cannot
measure it directly ; but we hâve seen that the wave-length is,
by définition, the distance traveled in the time of one vibra-
Fig. 43.— Hertz’s apparatus for observing stationary waves in air.
O is the oscillator, M a plane mirror of zinc fastened against a stone wall,
and R a closed resonator. When the resonator is moved from the mirror
towards the oscillator the spark-length varies in the manner indicated by
the curve. Only two nodes, A and C, are observed, owing to the damping
of the waves. As the mirror is not a perfect conductor, the node A is
apparently behind the surface.
The figure is drawn approximately to scale.
tion; and we hâve also seen how the wave-length in a wire
may be measured.
If the wave-length in air is the same as that in a wire, then
the velocity of propagation in air is the same as the velocity
a long a wire, and the theory of Maxwell is true.
The problem is thus reduced to the measurement of the
wave-length in air.
In making this measurement the same method may be used
as in the case of propagation along a wire.
We hâve seen that the direct wave transmitted along the
wire was caused to interfère with the wave reflected at the
end of the wire. In like manner, the direct wave transmitted
through the air may be made to interfère with the wave re-PROPAGATION IN AIR.
73
flected from a plane metallic mirror. This mirror should be
so placed that the direct wave will strike it normally, and
consequently, the reflected wave will travel in the opposite
direction to that of the direct wave. (See Fig. 43.)
Under these conditions we should obtain true stationary
waves if the vibrations of the oscillator were not damped;
but, because of this damping, and for the same reasons as
were developed in Chapter VII, the phenomenon of multiple
résonance will occur. It is needless to repeat the discussion
given on pages 61-64: the phenomena in this case are
exactly similar.
If a resonator be moved between the oscillator and the
mirror, a sériés of nodes and loops ^vill be observed ; the
nodes are the points where the resonator fails to respond to
the oscillator, the loops are those wThere the intensity of the
phenomenon is a maximum.
The intemode, or distance between two nodes, is equal to
the half wave-length of the vibration of the resonator in air;
just as in the case of propagation along a wire, the internode
was equal to the half wave-length of the vibration of the re-
sonator when traveling along a wire. If, then, the internode
in air is the same as along a wire, the wave-length in air is
equal to that in a wire, and Maxwell’s theory is true.
2. Experiments at Karlsruhe.— This is the experimentum
crucis which Hertz first performed at Karlsruhe. He did not
at once obtain the expected resuit.
Along a wire his resonator gave an internode of 3 meters ;
in air it seemed to show an intemode of 4.50 meters, or 9
meters wTave-length. This experiment seemed undeniably to
condemn the old electrodynamic theory, which demanded an
infinité w7ave-length ; but it seemed none the less to condemn
the theory of Maxwell, which involved a wave-length of 6
meters.74
MAXWELL’® THEORY.
This failure is still unsatisfactorily explained. It is prob-
able that the mirror was too small with respect to the wave-
length, and that diffraction entered to disturb the phenomena.
Perhaps also, the reflection of the waves from the walls of the
room or from the cast-iron columns which divided the roorn
into three sections, may hâve exercised a disturbing influence.
However this may be, the smallest oscillators gave a dif-
ferent resuit, and showed the same internode in air as in a
wire; doubtless the smaller wave-length was not too great
with respect to the dimensions of the mirror.
3.	Expérimenta at Geneva.— Still, the question was not
settled, and illness prevented Hertz from continuing his ex-
periments. MM. Sarasin and de la Rive took them up with
sufficient précautions to eliminate ail sources of error.
Their mirror was 8x16 meters, and they worked in a very
large and unencumbered room. The results were as clear with
the 75 centimeter resonator (having the same wave-length as
Hertz’s large oscillator) as with the smaller ones. These ex-
periments must thus be regarded as conclusive.
In conformity with Maxwell’s theory, the internode was
the same in air as in a wire.
4.	Use of the Small Oscillator.— The experiment may be
more easily repeated with the small oscillator of Hertz, which,
we hâve seen (page 35), consists in a short rod of métal
divided in the middle.
Parabolic mirrors are in common use for gathering the
light emanating from a small source into a beam of parallel
ray s. Such an apparatus is called a parabolic projector or rè-
flector.
The radiations produced by an oscillator may be treated in
almost the same way; only the dimensions of the oscillator
are comparable to those of the mirror, so that the former is
more like a luminous line than a luminous point.PROPAGATION IN AIR.
75
Hence, instead of giving the mirror the form of a para-
boloid of révolution witb the source of the radiations at its
focus, it is made in the form of a parabolic cylinder and the
oscillator is placed in its focal line. (Fig. 44.) Thus a
parallel beam of electrical radiations is obtained.
T°
-50
-H 00 CM.
Fig. 44.— Hertz’s apparatns for concentrating the radiations of an
oscillator by means of a parabolic mirror of zinc. (From “ Electric
Waves,” Eng. Trans.)
In like manner the resonator, which is just like the os-
eillator, may be placed in the focal line of a second parabolic
mirror. This mirror concentrâtes the parallel rays upon the
Tesonator.76
MAXWELL’S THEORY.
ïlowever, in the interférence experiments just described,
the second mirror should not be used, for it wonld act as a
screen to shield the resonator from the reflected wave.
5. Nature of the Radiations.— The field which surrounds
an oscillator is traversed by electromagnetic radiations: the
theory enables us to formulate the laws of their distribution,
and these hâve been further confirmed by experiment, at least
in their general characteristics, which are ail that our présent
means of investigation enable us to détermine.
These laws are rather complex, and, in order to simplify
their exposition, we shall consider only those points of the
field which are a long distance from the oscillator.
Imagine a sphere of very large radius, having its center at
the middle of the oscillator. At each point of this sphere
there is a variable electromotive force, which passes through
zéro and changes sign twice during each oscillation but does
not change its direction. There is also a magnetic force
which varies in a similar manner.
What is the direction of these two vibrations — the one
electric, the other magnetic ?
Trace on the sphere a System of meridians and parallels,
as on a terrestrial globe ; the pôles being at the points where
the sphere is eut by the axis of the oscillator produced. The
electric force at any point will be tangent to the meridian;
the magnetic force, to the parallel. (Fig. 45.)
The two vibrations are thus at right angles to each other,
and both are perpendicular to the radius of the sphere — that
is, to the direction of propagation, corresponding to what is,
in optics, the direction of the ray of light. These two vibra-
tions are thus transverse, like those of light.
The amplitude of these vibrations varies inverselv as the
distance from the oscillator, hence the intensity varies in-
versely as the square of the distance.PROPAGATION IX AIR.
77
The vibration maintains, as we hâve seen, a constant di-
rection ; hence it is comparable to the vibrations of polarized
light, rather than those of ordinary light, which constantly
change their direction while remaining perpendicular to the
path of the ray.
Another question présents itself:	What is it tliat corre-
sponds to the plane of polarization in optics ? Is it the plane
Fig. 45.— A portion of the spherical wave-front proceeding from an
oscillator. The full lines indicate the magnetic force, the broken Unes,
the electric force. The direction of propagation is perpendicular to both
of these, and is therefore radial.
perpendicular to the electrical vibration ? Or is it the plane
perpendicular to the magnetic vibration? We shall see in
Chapter XI how it may be shown that the former of these
hypothèses is correct.
Another différence from the radiations emitted by a source
of ordinary light: the intensity is not the same in ail direc-
tions. It is a maximum at the equator and zéro at the pôles78
MAXWELL^ THEORT.
(retuming to the network of meridians and parallels which
we snpposed to be traced on our sphere).
Aside from these différences, the mode of propagation of
an electromagnetic disturbance through air is the same as
that of light. In the case of propagation along a wire, also,
we had displacement currents; but these were sensible only
j
Fig. 46.— A portion of the wave-front surrounding a perfectly con-
ducting wire transmitting oscillations. The magnetic force forms closed
circles concentric with the wire, which slide along without expanding :
hence the intensity of the wave remains constant. The electric force, and
hence the displacement currents, are radial, and so are strongest close to
the wire.
in the air in the immédiate neighborhood of the wire. In-
stead of spreading out in ail directions, the disturbance was
propagated along a single line ; consequently its intensity was
maintained, instead of diminishing according to the law of
inverse squares. (Fig. 46.)CHAPTER IX.
PROPAGATION IN DIELECTRICS.
1. Maxwell’s Relation.— When, in a condenser, we replace
the layer of insulating air by a layer of some other insulat-
ing substance, we find that the capacity of the condenser is
multiplied by a coefficient which is called the “ spécifie in-
ductive capacity ” of this substance. The theory demands
that the velocity of propagation of electric waves in a dielec-
tric be inversely proportional to the square root of the spé-
cifie inductive capacity of the dielectric.
Again, the velocity of light in a transparent medium is
inversely proportional to the index of refraction. Hence
the spécifie inductive capacity should be equal to the square
of this index. This is the theoretical relation of Maxwell.
It is poorly verified, except for sulphur. This may be
explained in two ways:—either the index of refraction for
very long waves, such as electrical oscillations, is not the
same as the optical index of refraction — which would not be
at ail surprising, since wre know that different radiations are
unequally refrangible, and that the index for red is differ-
ent from the index for violet — ; or the square of the electrical
index of refraction may itself be different from the spécifie
inductive capacity as measured by static methods in an in-
variable field — which could be explained by varions second-
ary effects, such as residual charges.
Hence the necessity of measuring the spécifie inductive
capacity by two sorts of methods : the dvnamic methods, based
on the use of electrical oscillations, which will give the elec-
trical index of refraction ; and the static methods, in a con-
stant field.
[79]80
MAXWELL’S THEORY.
2.	Dynamic Methods.— The veloeity of propagation is the
same in air or along a metallic wire stretched in the air.
So also, the veloeity of propagation through a dielectric
shonld be the same asHhe veloeity along a wire immersed in
the dielectric. Hence it is sufficient to measure the latter.
We hâve seen how the wave-length of an electrical oscilla-
tion may be determined bv measuring the distance between
the nodes on a wire, by means of a resonator (see page
59). If the wire be immersed in a dielectric, the veloeity
of propagation is diminished : as the period remains the same,
the wave-length and the distance between the nodes are di-
minished in the same ratio. Thus we may simply measure
this ratio, which is the reciprocal of the electrical index of
refraction.
Suppose again that the resonator used for exploration be
made of a condenser whose plates are joined by a wire (Blond-
lot’s resonator). If a laver of insulating material be placed
between the plates, the capaeity of the condenser is mul-
tiplied by the spécifie inductive capaeity; the period of vi-
bration to which the resonator responds is thus increased,
and, consequentlv, also the distance between the nodes.
If the wire along which the electrical oscillations are propa-
gated, and the resonator with its condenser, be immersed in
the same dielectric, the two effects should exactly balance
eacli other, and the distance between the nodes should be
unchanged. This is found to be the case.
These methods of measuring the electrical refractive index
are analogous to the interférence refractometer in optics.
We may also make use of the refraction of the electric rays
by a prism of the dielectric ; or, better still, of total reflection.
3.	Static Methods.— To measure an inductive capacitv in
a constant field we must compare two capacities. This
may be done :PROPAGATION IN DIELECTRICS.
81
First. By discharging a condenser through a ballistic gab
vanometer, which measures the quantity of electricity which
flows ;
Second. By charging and discharging a condenser a great
number of times per second, and comparing the intermittent
current thus produced with a continuons current through a
given résistance (MaxwelFs method) ;
Third. By connecting two condensers in sériés, and prov-
ing the equality of their capacities by showing that the po-
tential of the middle plates is the arithmetical mean of the
potentials of the terminal plates ( Gordon s method) ;
Fourth. By measuring the attraction between two electri-
fied spheres immersed in the dielectric ;
Fiftli. By connecting in opposition two electrometers whose
needles and whose corresponding pairs of quadrants are re-
spectively in met allie connection, and which are immersed,
one in a dielectric, the other in air (Differential electrometer) ;
Sixih. By studying the déviation of the lines of force in
an electrostatic field occasioned by the introduction of a prism
of dielectric material (FéroFs method of equipotential sur-
faces).
4. Eesults.— These different methods give very discordant
results. For resin, the following values hâve been obtained
for the spécifie inductive capacity, which we shall call s :
Square of the optical index.
By equipotential surfaces ..
With Hertzian oscillations
By ballistic galvanometer ..
By another static method ..
By the method of attraction
2.0
2.1
2.12
2.03
2.88
5.44
For alcohol, water, and ice we find still greater discrep-
ancies.
6.82
MAXWELL’*8f THEORY.
Alcohol.— a. The static methods gave for values in
the neighborhood of 4.9, that is, quite different from the
optical index;
b.	Yet, Stchegtiœf, using Gordon’s method with oscilla-
tions produced b y a Ruhmkorff coil, found for	value not
far from the optical index.
e. Methods founded on the use of Hertzian oscillations
gave a value in the neighborhood of 4.9.
Water.— a. M. Gouy, by a method of attraction, found:
e =80.
The value of £ varies, of course, with the impurities in the
water, which render it more or less conducting; 80 is the
value which £ approaches as the conductivity of the wTater
approaches zéro..
6. Herr Colin measured e by determining the wave-length
in a wire immersed in water. He found that e dépends upon
the conductivity of the water and on the température. His
values are near to that of M. Gouy.
c.	Only one expérimenter lias found for e a value approach-
ing the square of the optical index, £ =1.75.
Ice.— A static method gave
s = 78,
a value close to that obtained by M. Gouy for water.
M. Blondlot, on the other hand, found, by the use of
Hertzian oscillations
e = 2.5
and M. Pérot, by the same method, obtained a similar value.
Thus we find an enormous différence between the values
of MM. Blondlot and Pérot, on the one hand, and the num-
ber 78, on the other.
5. Conducting Bodies.— Substances transparent to light
are, in general, bad conductors ; the metals, on the contrary,
are very good conductors and very opaque. There is noth-PROPAGATION IN DIELECTRICS.
83
ing paradoxical about this. The dielectrics offer to elec-
trie waves an elastic reaction (see Chapter II) which re-
turns the energy imparted to them ; hence they permit the
oscillations to pass. Conductors, on the other hand, offer a
viscous résistance which destroys the kinetic energy to con-
vert it into heat ; hence they absorb electric waves and light.
Indeed, it is found that the metals stop electrical waves
like a screen ; they» make an imperfect screen for oscillations
of very long period, but their opacity is almost absolute for
Ilertzian waves. The experiments of Herr Bjerknes, cited
above (page 50), show that these radiations cannot pene-
trate a métal to a depth greater than a hundredth of a mil-
limeter.
Nevertheless, Prof essor Bose, whose very sensitive appa-
ratus will be described later, apparently observed the péné-
tration of metals by his radiations; but M. Branly has re-
cently shown that a metallic envelope is impénétrable, even
to the very rapid oscillations obtained by Professor Bose,
provided that the envelope is absolutely closed. Even the
smallest opening invites diffraction sufficient to affect the
very sensitive detector of Professor Bose.
6. Electrolytes— Thus ail conducting bodies are opaque ;
ail insulators are transparent. This rule admits of apparent
exceptions.
Certain substances, like ebonite, are insulators ivithout
being transparent. But it is found that, although opaque
to visible light, they transmit Hertzian radiations.
There is no more reason for surprise at this than at the
passage of red light through a red glass which will not trans-
mit green light. Besides, these substances, wThich are trans-
parent to electrical waves of long period, would naturally
act as dielectrics in a static field, where the period may be
regarded as infinité.84
MAXWELL’S THEORY.
On the other hand, certain liquids, like sait or acidulated
water, are conductors of electricity but transparent to light.
This is becanse such liquids, which are decomposed by a cur-
rent and are called electrolytes, hâve a conductivity of a very
different nature from that of metals.
The molécules of the electrolyte are decomposed into
“ ions/’ and the electricity is transported from one electrode
to the other by these ions, which travel through the liquid.
Hence the electrical energy is not transformed into heat, as
in the case of metals, but into Chemical energy. Doubtless
this process, which dépends upon the comparatively slow
movement of the ions, has not time to take place if the vibra-
tions are as rapid as those of light. In fact, the electrolytes
are somewhat transparent even to Hertzian waves.CHAPTER X.
PRODUCTION OF VERY RAPID OSCILLATIONS.
1. Very Short Waves.— Blondlot’s oscillator gives a wave-
length of 30 meters, the large oscillator of Hertz a wave-
length of 6 meters, and the small oscillator of Hertz, 60
centimeters. In other words, we hâve :
Vibrations
per second.
Witli the oscillator of Blondlot........................... 10,000,000
With the large oscillator of Hertz......................... 50,000,000
With the small oscillator of Hertz......................... 500,0 0,000
But this is not the limit.
The learned Italian physi-
cist, Sig. Righi, and after
him, the young Hindoo
professor, Sagadis Chunder
Bose, constructed appara-
tus which enabled them to
go much farther.
Theoretically it was only
necessary to decrease the
size of the apparatus; but
this also diminished the in-
tensity of the oscillations,
and extremely sensitive detectors had to be devised to ob-
serve them.
2. Righi’s Oscillator.— This oscillator consists in two
spheres of brass, A and B (Fig. 47), fixed in the centers of
two dises of wood, glass, or ebonite. These dises form the
[85]
O o-
Fig. 47.— Righi’s oscillator. This
is similar to Lodge's (Fig. 24), but the
balls are smaller and the spark occurs
in a vessel of oil. The waves emitted
are thus very short, but not correspond-
ingly feeble.86
MAXWELL’S THEORY.
bottom and top of a cylindrieal vessel witli flexible sides,
whose diameter is much greater than its height. In one of the
dises is a small bole for filling the vessel with vaseline oil.
The flexibility of the side walls of the vessel permits the use
of varions arrangements for regulating the distance between
the spheres.
The spark leaps between the two spheres, as in Lodge’s oscil-
lator ; but, owing to the small dimensions of the spheres, the
wave-length is very small.
We hâve seen above the advantages of having the spark
occur in oil. It is through this artifice that the oscillations
retain sufficient intensity, notwithstanding the smallness of
the apparatus ; for we hâve seen that the use of oil strengthens
the oscillations, while improving the regularity of the
sparks.
For charging the oscillator, Righi used, not an induction
coil, but a Holtz statical machine. This has also been used
with Hertz’s oscillators.
It is important to note that the spheres A and B are not
connected directly to the two pôles of the Holtz machine,—
these pôles are connected metallically to two other spheres,
C and D ; the sphere C being placed at a short distance from
A, the sphere D close to B. Thus three sparks are produced ;
the first. between C and A, the second between A and B, and
the third between B and D. The first and last occur in air,
and the second in oil.
It is the second spai'k that has the oscillatory character.
The two others, which take place in air, serve only to charge
the two spheres A and B. When these hâve received sufficient
charges, the spark A B breaks through the oil and the oscilla-
tions commence.
It is important to properlv adjust the lengths of the three
sparks: Righi gave the middle spark a length of about oneVERY RAPID OSCILLATIONS.
87
millimeter, and the others two centimeters. The diameter of
the spheres A and B was about four centimeters. The wave-
length was about ten centimeters, hence the frequency was
3,000,000,000 vibrations per second.
With spheres of eight millimeters diameter Sig. Righi ob-
tained oscillations four times as rapid.
3. Besonators— Notwithstanding the improvements intro-
duced by Righi in the construction of his oscillator, its effects
are still very feeble, and especially sensitive resonators are
required to detect them.
Two principles guided the learned Italian in designing his
resonator: first, the sparks are much longer, for a given
différence of potential, when they play across the surface of
an insulating body than when they leap through free air;
and second, as the electromagnetic effects are propagated only
on the surface of a métal, the thickness of the metallic parts
of a resonator may be reduced without détriment.
Righi deposited electrolytically, on a plate of glass, a thin
film of silver in the form of a rectangle much longer than
wide. Across the middle of the rectangle the silver film is
eut through by a diamond, leaving a gap a few thousandths of
a millimeter wide. Tt is across this gap that the sparks play.
They respond to very small différences of potential, since
the space to be bridged is so narrow and the sparks pass
across the surface of the glass.
The sparks are observed by means of a small microscope.
This resonator opérâtes in the same manner as the recth
linear resonators of Hertz. The rays of electrical force
emanating from the oscillator are rendered parallel by a
mirror in the form of a parabolic cylinder, and another simi-
lar mirror concentrâtes them on the resonator.
This very sensitive apparatus is well adapted to measure-
ment. If the resonator be rotated, the action becomes a maxi-88
MAXWELL'S THEORY.
mum when the resonator is parallel to the oscillator, that is,
to the line joining the centers of the two spheres, A and B;
it is zéro when the resonator is perpendicular to the oscil-
lator, and in other positions it
takes intermediate values. Thus,
the position of the resonator
when sparks began to appear af-
fords an indication of the in-
tensity of the radiations.
4. Bose’s Oscillator.—Prof. Ja-
gadis Chunder Bose has obtained
still more rapid oscillations. His
oscillator consists in three metal-
lic spheres, A, B and C (Fig. 48);
the two spheres A and C con-
nected to the pôles of a Ruhmkorfï coil, the middle sphere,
B, insulated. Sparks leap between A and B, and between
B and C. This is another form of Lodge’s oscillator.
The sparks occur in air; still, the électrodes must not be
allowed to deteriorate if the discharge is to retain its oscilla-
tory character. To this end, Prof essor Bose uses spheres of
platinum instead of brass, and, instead of operating his coil
with a vibrating interrupter, he uses a hand break. Each
motion of the hand gives him a single sériés of decreasing os-
cillations, in place of an uninterrupted stream of sparks
which would rapidly destroy the électrodes.
With these précautions, the discharges continue to be os-
cillatory without the necessity of frequent cleaning and polish-
ing of the balls.
The radiations are feeble, but Professor Bose dépends for
his results on the sensitiveness of his detector. He finds the
intensitv of the action less important than its regularity and
constancy, without which measurements would be impossible ;
(nearly full size). The plati-
num sphere B, which is insu-
lated, is the seat of the oscilla-
tions. The spheres A and C are
connected to the induction-coil.
L is a cylindrical lens for con-
centrating the radiation.YERY RAPID OSCILLATIONS.
89
indeed, in his estimation, very strong oscillations would be
detrimental, for reflection and diffraction might produce
secondary radiations capable of affecting the detector and dis-
turbiug the observations.
The coil and battery are inclosed in a double metallic case,
alinost entirely closed, so that they can exert no disturbing
influence on the exterior. The tube containing the oscillator
is mounted on the box, and the radiations are rendered par-
allel by a cylindrical lens of sulphur or ebonite.
This apparatus gives a wave-length of six millimeters,
which corresponds to 50,000,000,000 vibrations per second.
Vibrations 10,000 times as rapid would suflice to impress the
retina (they would correspond to the orange rays of the
spectrum) ; thus, says Prof essor Bose, we are within thirteen
octaves of visible light. It has been found possible to pro-
duce a pencil of parallel electric rays with a cross-section of
one or two square centimeters.
5. Bose’s Detector.— The detector is based on the princi-
ple of the Branly coherer, or radio-conductor. The coherer
is an instrument of marvelous sensitiveness, but it is some-
what eapricious in its action. At times it becomes so ex-
traordinarily sensitive that the galvanometer is deflected with-
out any apparent cause ; and again, when it seems to be work-
ing admirably, its sensitiveness suddenly disappears. Per-
haps some of the particles corne into too intimate contact ; or
again, the contact surfaces lose their sensitiveness through
fatigue due to prolonged activity.
Professor Bose modified the original coherer to overcome
these defects. Pièces of fine Steel wire were wound into
spirals, which were placed side by side in a narrow groove in a
block of ebonite, each tiny spiral touching its neighbor in a
well-defined contact. At each end of the groove were pièces of
brass, one fixed and the other capable of sliding, and both90
MAXWELL'*8f THEORY.
connected to the terminais of a battery, A screw regulated
the pressure of the movable block upon the first spiral, and
this pressure was transmitted from spiral to spiral, so that
it was uniform over ail the contacts.
The current from the battery entered at the upper spiral,
and, passing from one to the other through the contacts be-
tween them, left through the lower spiral.
When electromagnetic radiations impinge upon the ap-
paratus, the résistance offered by the sériés of contacts is
diminished, the current traversing them from the battery is
increased, and the variation is shown by a galvanometer.
As ail the points of contact lie in the same straight line,
the radiations may be concentrated upon them by means of a
cylindrical lens.
The sensitiveness of this apparatus is exquisite, and it re-
sponds to ail radiations over the range of an octave. It may
be made sensitive to radiations of different kinds by varying
the electromotive force of the battery which opérâtes it.
The apparatus is inclosed in a metallic case with no open-
ing but a narrow slit. It is thus protected from ail radiations
except those which are concentrated on this slit.CHAPTER XI.
IMITATION OF OPTICAL PHENOMENA.
1. Conditions of Imitation—According to Maxwell’s
îdeas, light is nothing more nor less than an electromagnetic
disturbance propagated through air, space, or other trans-
parent medium. The electrical radiations emanating from
an oscillator differ from light only in their period, and it is
simply because their wave-length is too great that they do not
impress the retina.
We hâve seen that these disturbances travel at exactly the
same velocity as light, but this is not sufficient; it must be
shown that they possess ail the properties of light, and that
we can reproduce with them ail optical phenomena.
The great length of the waves is, however, a serious ob-
stacle ; to reproduce the conditions under which optical phe-
nomena are observed, ail dimensions must be increased in
proportion to the wave-length, according to the law of sim-
ilarity.
If we are using, for example, the large oscillator of Hertz
(wave-length six meters), a mirror, to bear the same relation
to its radiations as a mirror one millimeter square bears to
light, would hâve to be ten kilometers square. Even with
Bose’s little oscillator, we should need a mirror ten meters
square.
It is évident that this condition can be only imperfectly
fulfilled, but the approximation will be doser as we use
shorter waves. Hertz obtained fairly good results with his
small oscillator ; but, as might be expected, Righi and Bose,
who used waves only one-tenth and one-hundredth as long,
achieved a much more perfect imitation.
[91192
MAXWELL’8 THEORY.
2. Interférence.— We considered in Chapter VIII the in-
terférence produced between the electric waves proceeding
directly from an oscillator and those reflected by a metallic
mirror. In these experiments the two interfering rays — the
direct ray and the reflected ray — were traveling in opposite
directions. (Fig. 49.)
Here the conditions are very different from those which
obtain in optical apparatus designed for the study of inter-
férence, where the two rays travel in the same sense and inter-
sect at a very acute angle. The more acute this angle, the
/. 6. 3. ^
^y /			, y	f
				
E—(r	14	*			
I IV»				
				
		< N	✓ >	
7		: z 5.		
Fig. 49.— Interférence by reflection from a single mirror, M. 1, 2, 3,
etc., represent wave fronts differing in phase by a half period, 1 to 4
being direct and 4' to 7 reflected and reversed in sign. The planes in
which two opposite wave fronts, 4-4', 5-3, 2-6, etc., meet are interférence
striæ, separated by a half wave-length, A
2
wider are the interférence fringes, and hence the easier to
observe. In optics we do not ordinarily produce interférence
between rays traveling in opposite senses, for this would give
fringes of a few ten-thousandths of a millimeter only.
Not until quite recently did Wiener succeed in observing
the optical bands produced under such conditions, though
it is this phenomenon that is utilized in M. Lippmann’s proc-
ess of color photography. M. Lippmann places the sensitive
plate against a surface of mercury, which acts as a mirror.
The direct ray interfères with the ray reflected from the mer-IMITATION OF OPTICAL PHENOMENA.
93
<mry, which travels in the opposite direction, and a sériés of
équidistant striæ are pxodueed in the sensitive film. These
striæ are entirely analogous to the electrical striæ considered
in Chapter VIL
But Righi achieved a better imitation of the ordinary ex-
periments in interférence. He caused the electrical waves to
be reflected by two mirrors inclined at a small angle. (Fig.
posing wave-fronts, 1-2, 2-3, etc., intersect at A, B, C, D, etc., in planes
nearly parallel to the rays, and separated by a distance, A B, depending
upon the acuteness of the angle of intersection. A resonator moved in
the direction A B would show successive maxima and interférence points.
50.) When proper précautions are taken to shield the re-
sonator from the direct ray by means of a metallic screen, the
interférence of the two reflected rays may be studied. This
is Fresnel’s experiment of two mirrors.
Again, the two mirrors may be placed in parallel planes a
short distance apart (Fig. 51), and we hâve an imitation of94
MAXWELL’S THEORY.
the interférence apparatus used by Michelson for the optical
construction of standard centimeters or decimeters.
Finally, in place of two rays reflected by mirrors, we may
cause interférence between two rays refracted through prisma
of sulphur. This is Fresnel’s experiment of the biprism.
3. Thin Filins.— One of the most brilliant of optical inter*
ference phenomena is that of Xewton’s colored rings, to which
soap bubbles owe their rich coloring. It is due to the inter-
Fig. 51.— Interférence between two parallel rays in the same direction.
The two reflected rays hâve the same phase différence at ali points, and
the degree of interférence dépends upon the distance between the mirrors
Mx and M2.
ference of the rays reflected from the two surfaces of a thin
film.
The phenomenon of thin films may be reproduced electri-
cally; but ail is relative. In optics, a film, to be thin, must
be measured in thousandths of a millimeter; with wave-
lengths 10,000 to 500,000 times as great, Righi used “ thin
films ” of paraflSn, having a thickness of one or two centi-
meters.
4. Secondary Waves.— A phenomenon which Prof essor
Righi studied in detail is that of secondary waves. The op-IMITATION OF OPTICAL PHENOMENA.
95
tical analogue is more difficult to observe, and will be dis-
cussed in the following chapter.
If a resonator be exposed to the radiations emitted by an
oscillator, it enters into vibration, and becomes, in its turn, a
source of radiations. These may be detected by a second re-
sonator, shielded from the direct radiations by a metallic
screen.
The secondary radiations produced in this way by a re-
sonator may interfère with the direct radiations, and again,
secondary radiations produced by two resonators may inter-
fère with each other.
Again, by virtue of the phenomenon of multiple résonance,
of which we hâve already spoken in detail, an oscillator may
set in action two resonators of different periods, and these
may react upon each other.
Righi has shown that a mass of dielectric becomes, like
a metallic resonator, a center from which secondary radiations
are emitted.
This is not surprising. How does a resonator respond to
its excitation ? We hâve said before, in the same manner as
does an organ pipe (cf. page 39). In this pipe, a sonorous
wave excited by any cause whatever is reflected at the two
ends of the pipe, and so travels to and fro. If there be har-
mony between the pitch of the tone and the length of the
pipe, ail these reflected waves will be concordant and cumula-
tive and the sound will be reinforced.
In a metallic resonator, an electrical disturbance is reflected
at the two ends of the wire, and the waves thus driven to and
fro may combine and reinforce each other in the same way.
If we consider a mass of dielectric, wre find a similar state
of affairs; the electrical disturbances are reflected from the
opposite limiting surfaces of the mass, as, in a resonator, they
are reflected by the ends of the wire.96
MAXWELL’S THEORY.
Thus a mass of dielectric is actually a resonator.
Ail these secondary waves * produce, by their mutual inter-
férence, a complicated set of phenomena which Professor
Kighi deserves much crédit in unraveling.
5. Diffraction.— The phenomena of diffraction becomes
more noticeable as the wave-length is increased, hence it is
easy to imitate them with electric waves. Ail the diffraction
Fig. 52.—A Hertzian
wave is plane-polar-
ized, i. e., the vibra-
tion takes place al-
ways in one plane.
Fig. 53.—In ordinary
light the plane of the
vibration is constantly
changing.
Fig. 54.—Rotation of the
plane of polarization of a
Hertzian wave, A, by a grid
of parallel wires, G, which
absorbs the component B,
parallel to the wires and
transmits the component C,
at right angles.
phenomena due to a slit, or to the edge of an indefinite screen,
hâve been reproduced.
But if, for a metallic screen, we substitute a dielectric, the
phenomena become more complicated, for we must take into
considération the secondary waves emitted by the dielectric.
The application of Huvghens’ principle and the purely geo-
metrical theory deduced from it is not always sufficient. It
is generally satisfactory in optics, because, on account of the
small wave-length, diffraction produces only slight dévia-
tions. But even in the experiments of M. Gouy on the am-IMITATION OF OPTICAL PHENOMENA.
97
plified diffraction produced by the edge of a very keen razor,
the geometrical theory is found wanting.
Professor Bose made the imitation of diffraction phe-
nomena complété by constructing actual gratings, and nsing
them for measuring the wave-lengths of his electrical oscilla-
tions.
6. Polarization— Electrical vibrations are always polar-
ized, becanse they are always parallel to the axis of the oseil-
lator. They are thus analogous to the vibrations of plane
polarized light, whose direction is constant (Fig. 52), and
not to those of ordinary light, whose direction varies con-
stantly while remaining always in a plane perpendicular to
the ray. (Fig. 53.)
We can nevertheless imitate the action of a polarizer which,
wrhen traversed by a ray of light already polarized, changes
the orientation of the plane of polarization.
For this purpose Hertz used a “ grating ” or grid composed
of a number of parallel wires stretched on a frame. We hâve
seen that a métal interrupts electrical waves simply becanse
it is a conductor. Such a grid is a conductor in onlv one di-
rection — that of the wires. Hence it will absorb onlv the
vibrations parallel to this direction, and will transmit those
perpendicular to it. (Fig. 54.)
It is important not to eonfound this polarizing grid with
the diffraction grating of Professor Bose, which acts like those
used in optics. Their functions are entirely different, and
the différence is found in the fact that, in the polarizing grid,
the distance between the wires is less than the wave-length,
while in the diffraction grating it is greater.
The polarizing grid has no optical analogue, though it may
be compared to tourmaline, which absorbs vibrations oriented
in a certain plane.
798
MAXWELL*& THEORY.
7. Polarization by Reflection—Metals and dielectrics re-
flect electric waves ; their effects should be the same as in the
case of metallic and vitreous reflection of polarized light.
This has been verified by Trouton and Klemencic. Prof essor
Righi thought at one time he had found the contrary resuit,
but when he recognized the existence of secondary waves, and
succeeded in disentangling their laws, he fully confirmed the
conclusions of his predecessors.
An important point was thus settled beyond a doubt : elec-
trical vibrations are perpendicular to the plane of polariza-
tion, like luminous vibrations in the theory of Fresnel.
Fig. 55.— A circularly polar-
ized ray may resuit from re-
flection of a plane-polarized one.
Fig. 56.—In an elliptically
polarized ray, the amplitude
of the vibration has maxima
and minima in two perpen-
dicular planes.
Reflection from a metallic surface produces, as with light,
an elliptic or circular polarization.
Righi’s apparatus serves readily to demonstrate this polari-
zation. Give the resonator different orientations: if in one
position there is complété extinction, the polarization is plane
(Fig. 52) ; if the sparks hâve the same intensity in ail
azimuths, it is circular. (Fig. 55.) In an intermediate
case, where the spark passes through a minimum without
vanishing, the polarization is elliptical. (Fig. 56.)
8. Refraction.— At an early date prisms and lenses were
constructed of sulphur or paraffin, which acted on electric
waves as prisms and lenses of glass act on light.IMITATION OF OPTICAL PHENOMENA.
99
Réfraction affects the plane of polarization according to
the saine laws as in optics. The effect may be made more
évident by multiple reflections and refractions, in imitation
of the optical phenomena with piles of glass plates.
The following curious experiment is due to Professor Pose.
We know that powdered glass is opaque to light on account
of multiple reflections, but that its transparency is restored
by pouring on Canada balsam, which has the sanie index of
refraction as glass. In like manner, if we fill a box with
irregular bits of resin, electric waves cannot pass through it ;
but its transparency is restored when we fill the chinks with
kerosene.
We may note in passing that certain substances, like sul-
phur, are opaque to light because they are composed of min-
ute crystals whose faces produce reflections. They behave
like powdered glass. But they are quite transparent to elec-
tric waves, because the crystals are very small compared to
the wave-length, and, with reference to these radiations, the
substances must be considered homogeneous.
9. Total Réfection.— The phenomena of a total reflection
and the resulting circular polarization are easily imitated,
but there is a curious circumstance which seems well worthy
of note.
According to theory, when a ray of light undergoes total
reflection, a portion of the disturbance finds its way into the
second medium, in conformity with certain particular laws.
The effect is not visible, however, for the disturbance péné-
trâtes only a superficial layer whose depth is a wave-length.
This theoretical déduction cannot be verified directlv in
optics; we must rest content with indirect experiments com-
prising an intermediate phenomenon similar to that of îïew-
ton’s rings.
With very long waves, however, the vérification becomes100
MAXWELL’S THEORY.
possible, and is quite satislactory ; so that liere it is electric
waves which reveal to us one of the secrets of light.
10. Double Réfraction.— Crystals are doubly refracting to
electric waves, but as they can be used only in very thin layers
we liave the same phenômenon as occurs in the polarizing
microscope when a thin crystalline film is interposed between
the polarizer and the analyzer.
Prof essor Bose used for polarizer and analyzer the polariz-
ing grids of Hertz.
Gare must be taken not to confuse two phenomena, whose
effects, in this apparatus, are similar. They are generallv
superimposed, and require much care in their séparation.
Crystalline bodies hâve different indices of refraction for
vibrations in different planes : this is double refraction, prop-
erly speaking. But, in addition, these different vibrations
are unequally absorbed: this is called, in optics, dichroism.
Both phenomena hâve been demonstrated. Dichroism is
observed especially in bodies of lamellar or fibrous structure,
such as wood, a book, or a lock of hair. Their mode of ac-
tion is comparable to that of the polarizing grid of Hertz.
Professor Bose has shown that dichroism to electric waves
is always accompanied by unequal electrical conductivity in
the two directions.CHAPTEE XII.
SYNTHESES OE LIGHT.
1. Synthesis of Light.—Ail these experiments put in* évi-
dence the complété analogy between light and rays of elec-
tric force.
If the period of these rays, short though it is already,
were a million times shorter yet, they would not differ from
ravs of light.
\\Te know that the sun sends us radiations of various kinds ;
some are luminous, because they impress the retina; others
are dark — ultra-violet or infra-red — and are manifested by
Chemical or calorific effects. The former do not owe the
properties which enable us to perceive them to any differ-
ent nature, but to a sort of physiological accident. To the
physicist, the infra-red differs no more from red than red
from green — simply the wave-length is greater. The wave-
length of Hertzian radiations is very much greater yet, but
the différence is only one of degree; and \ve may say, if
Maxwell’s ideas are correct, that the illustrious professor
of Bonn accomplished an actual synthesis of light.
The synthesis, however, is not yet complété, and a first
difficulty arises from the great wave-length.
We hâve seen that light does not follow exactly the laws
of geometrieal optics, and the déviation, caused by diffrac-
tion, becomes more considérable as the wave-length increases.
With the great wave-length of Hertzian oscillations these
phenomena must become of great importance and influence
everything else. Doubtless it is fortunate, at least for the
[loi]102
MAXWELL’S THEORY.
moment, tliat our methods of observation are so crude ; other-
wise, the simplicity which at first attracted us would give
place to a maze in which we should be hopelessly lost. We
hâve here a probable explanation of certain anomalies which
hâve thus far been inexplicable.
From this point of view, the smallness of our bodies and
of everything that we use is the only obstacle to a perfect
synthesis. To giants, measuring length in thousands of
kilometers, that is, in millions of wrave-lengths of Hertzian
oscillations, and reckoning time by millions of vibrations,
Hertzian ray s would be the same as light is to us.
2. Other Différences.— Unfortunately there are still other
différences, notably the fact that Hertzian oscillations are
very rapidlv damped, while the duration of a luminous wave
is counted in trillions of vibrations. This explains, as we
hâve seen, the phenomena of multiple résonance, which
hâve no optical analogue.
But this is not ail ; let us realize what ordinary light is.
In the tenth of a second (that is, during the persistence of an
impression on the retina) the direction of the vibration, its
intensitv, its phase, and its period change millions of times;
yet they remain practically constant throughout millions of
vibrations,— in short, the number of vibrations per second
is reekoned in millions of millions.
With Hertzian oscillations the case is far from being the
same :
First. They do not take ail possible orientations, as do
the vibrations of ordinary light; but maintain a constant di-
rection, like those of polarized light.
Second. Their intensity, far from remaining constant dur-
ing millions of vibrations, decreases so rapidly as to disap-
pear after a few oscillations. After being extinguished, the
vibrations do not recommence at once with a new intensity8YNTHESIS OF LIGHT.
103
and a different phase and orientation, but there is a long
pause — much longer than the period of activity — whicli
is not broken until the vibrator of the Ruhmkorff coil again
opérâtes.
We hâve seen (page 77) that the amount of energy
radiated hy an oscillator is not the sanie in ail directions ; it
is maximum at what we hâve called the equator and zéro at
the pôles. Why does not light follow the same law ?
A source of light, at any given instant, does not émit the
same amount of energy in ail directions ; here also, there is a
maximum at the equator. But, in a tenth of a second the
equator lias changed so many times that it lias taken ail pos-
sible orientations; consequently, our eye, which perceives
onlv averages, déclarés the illumination uniform.
What would be required to accomplish a complété syn-
thesis of light? We should hâve to concentrate into a small
space an immense number of oscillators oriented in ail pos-
sible directions ; these oscillators should operate either simul-
taneously or in succession, but without interruption—that
is, each must be set in action before the oscillations of its
predecessor hâve entirely ceased. Finallv, to observe the
radiations, we should need an instrument which would in-
dicate mean values of the energy received, and whose impres-
sions, like those on the retina, would persist during trillions
of the oscillations observed.
The resuit would be an analogue of white light, even if
ail the oscillators had the same period, because of the damp-
ing.
To obtain something analogous to monochromatic light we
should require oscillators, not onlv of practicalty the same
period, but with a very small décrément.
3. Explanation of Seeondary Waves.— We hâve considered
(page 94) the seeondary waves discovered by Righi,104
MAXWELL’S THEORY.
which are emitted by resonators or dielectric masses placed
in the vicinity of an oscillator. These phenomena seem, at
first, to hâve no optical analogue.
We cannot compare them to what takes place when a body,
absorbing the light which passes through it, is heated suffî-
ciently to become luminous itself. In this case the trans-
formation is not direct, but must pass through the interme-
diate State of heat; moreover, there is no essential relation
between the phase of the vibrations emitted by the incandes-
cent body and that of the radiations which excite them.
ITence, these vibrations cannot interféré ivith each other.
Again, we cannot make a comparison with the phenomena
of phosphorescence, because neither can the vibrations
emitted by a phosphorescent body interfère with the vibra-
tions which excite them.
The analogy must be sought elsewhere.
If secondary waves are formed, it is because part of the
primary radiation has been diffracted by the resonator or the
dielectric mass. But this kind of “ diffraction ” differs
greatlv from that to which we are accustomed.
First, the déviation is enormous, because the dimensions
of the diffracting body are comparable to the wave-length.
In the second place, the phenomenon dépends upon the
nature of the diffracting body, and not only on its form as
the geometrical theory of Fresnel demands ; but this theory is
only approximate, and applies only to small déviations, as
is shown by the experiments of Gouy on the light diffracted by
the edge of a razor.
Finally, the secondary waves produced by resonators are
not generally of the same nature as the incident waves ; but
so in optics, the nature of the diffracted light is not the same
as that of the incident light,— thus, if the incident light is
white, the diffracted light is generally colored.SYNTHESIS OF LIGHT.
105
But, in the experiments of Hertz and of Righi, this altera-
tion of the radiations by diffraction appears in a quite un-
usual garb, and we hesitate to recognize it.
The damping of the oscillator is more rapid than that of
the resonators, hence it happens that the secondary waves
which correspond to diffracted light still persist after the
incident waves hâve disappeared. This paradoxical case of
diffraction seems quite natural when we reflect a little.
A damped vibration may, from a certain point of view, be
compared to a complex vibration whose components hâve no
décrément What happens at the end of a certain number of
oscillations? We find that each of the components has re-
tained its original intensity, while the résultant vibration is
extinct How can this be ? The résultant is extinguished, be-
cause the components are mutually destroyed through inter-
férence.
Diffraction will analyze this complex vibration, as it ana-
lyzes white light, separating the different colors. If it al-
lows only one of the components to remain, the mutual inter-
férence of the components can no longer resuit in destruction.
The incident light, in which ail the components are found
together, may thus become extinct while the diffracted light,
which contains only one, shines still.
With ordinary light such a phenomenon never occurs;
neither would it be produced with Hertzian light if, instead
of a single oscillator, we had a great number of them, ar-
ranged as explained on page 103. They would corne into
action at irregular intervals, but independently of each other,
and they would be so numerous as to preclude the possibility
of the concert being ever interrupted. Thus the synthesis of
light would be more perfect, and under these conditions the
incident waves would no longer sink to zéro.106
MAXWELLTHEORY.
A recent experiment lias shown more clearly tlie optical
analogies of Righi’s secondary waves.
Garbasso caused Hertzian radiations to fall upon a sort of
discontinuons screen made up of a number of precisely simi-
lar resonators. This screen reflected secondary radiations
whose period and décrément were the same as those of the
resonators.
ïhis phenomenon, whose analogy to that of secondary waves
is évident, could be reproduced optically. A plate of silvered
glass was ruled with two rectangular sériés of very fine
équidistant cuts, extremely close together, which removed the
film of silver, leaving a multitude of minute silver rectangles,
like so many tiny resonators.
This apparatus acted on infra-red light as did Garbasso’s,
of which it was a miniature reproduction, on Hertzian rays.
4. Miscellaneous Remarks.— Two luminous rays arising
from different sources cannot interféré, for the following rea-
son:	We may imagine each of these rays to hâve been pro-
duced, as described above, by a great number of oscillators
vibrating independently of each other and coming into action
at irregular intervals.
During a tenth of a second, ail these oscillators operate suc-
cessively, and the différence of phase of the two interfering
rays changes a great number of times ; sometimes they are cu-
mulative, sometimes mutually destructive ; and the eye, which
perceives only a mean effect, sees neither reinforcement nor
diminution — it sees no interférence. A single pair of oscil-
lators would produce interférence fringes, but the different
Systems of fringes produced by the different pairs of oscil-
lators are not regularly superimposed ; they mingle promis-
cuouslv, and we see only a uniform illumination.
These considérations do not apply to the case of two
Hertzian rays, each produced by a single oscillator, with aSYNTHESIS OF LIGHT.
107
single interruption of the primary of the coil. The inter-
férence fringes do not mix, for there is only one System of
fringes. The two rays, though of different origin, may inter-
fère.
The interférences cannot ahvays be easily observed, be-
cause, generally, the second oscillator will not commence to
vibrate until too long after the first has corne to rest ; but the
resuit could be accomplished by feeding the two oscillators
from the saine coil, if the damping is not too great. Here
again is a différence from optics.
We may ask again, what is the optical analogue of propaga-
tion along a wire? The corresponding optical phenomenon
cannot be observed, because, owing to the smallness of the
wave-length, it is confined, whether in air or in a métal, to
a layer one or two thousandths of a millimeter in thickness.
However, we can find an approximation in the luminous
fountain phenomenon, where the light follows a stream of
liquid. This comparison, less crude than it seems, is never-
theless very imperfect, because metallic wires are conductors,
while the liquid stream acts, with respect to light, as a trans-
parent dielectric.
Yet, it is doubtless possible to reproduce with Hertzian ray s
the luminous fountain phenomenon. We could then arrange
a sériés of models with dielectrics of increasing spécifie in-
ductive capacities, £ (cf. Chapter IX), and the case of a me-
tallic wire would appear at the end of the sériés as a limiting
case.PART TWO.
THE PRIHCIPLES OF WIRELESS TELEG-
RAPHY.PART TWO.
THE PRINCIPLES OF WIRELESS TELEG-
RAPHY.
CHAPTER I.
GENERAI* PRINCIPLES.
1. Methods of Signaling Through Space.— In the fore-
going chapters M. Poincaré has outlined the fundamental
principles which underlie ail electrical phenomena, accord-
ing to MaxwelFs theory, and has shown how they applv to
the production of electromagnetic waves in space.
Let us now carry the application a step farther, and see
how these principles are involved in the practical problem
of signaling through space without line wires : a problem
which is daily growing in importance, and which promises
to bear fruit of great industrial value.
The problem is not a new one, nor are the methods of
solving it altogether novel, for we hâve seen them fore-
shadowed on every page of the preceding chapters; but
the development of the art has brought out many interest-
ing features which we cannot afford to overlook.
We hâve seen that there are three modes bv which an
electrical disturbance may be transmitted through space;
[HH112
WIRELESS TELEGRAPHY.
a.	By electromagnetic induction,
b.	By electrostatic induction,
c.	By electromagnetic waves ;
the last being a complex phenomenon including the effects
of both the others.
Ail three of these modes of transmission hâve been used
by different experimenters for the sending of signais,
though the last is the only one that has attained great prac-
tical value ; but as they ail illustrate important principles,
we shall consider each briefly before passing on to the more
practical phases of the subject.*
In order to crystallize our ideas, let us return to a me-
chanical analogy.
Suppose we hâve a body of water with operators sta-
tioned at two points of the surface, A and B. The operator
at A may send a signal to the operator at B by any one of
three methods:—
a.	By setting the water in motion and causing it to flow
as a whole from A to B — a Dynamic method;
b.	By raising or lowering the level over the Avhole sur-
face — a Static method;
c.	By raising and lowering the level at frequent periodic
intervals and starting a sériés of progressive impulses over
the surface — a Wave method.
Let us see how these three illustrations apply to the three
methods of signaling through space.
* A fourth method which might be included under the general
head of “ Wireless Telegraphy ” is the method of electric conduc-
tion, in which the earth or sea is made part of a circuit carrying a
current, which spreads out over a wide area. Part of this current
is shunted out at the desired point through two immersed or buried
plates, and is made to opéra te a receiving apparatus. As the trans-
mission here is wholly through a material medium, and not through
space, we need not consider it further.GENERAL PRINCIPLES.
113
2. Method of Electromagnetic Induction.—Imagine a
screw propeller, rotating beneath the surface at A, and
causing currents of water to flow toward B. (See Fig. 57.)
At B is a second screw, pivoted so that it can rotate under
the influence of the water currents, but restrained by a
spring. The transmission from A to B is then of a
dynamic nature ; the energy used to rotate the sending pro-
peller is stored up in the moving water, which, when it
strikes the screw at B, causes it to rotate until arrested by
the compression of the spring, and so gives up part of its
Fig. 57.— Dynamic method of signaling through water. The propeller
at A sets up currents in the water, which tend to rotate the screw at B,
compressing the spring S and moving the index I. The arrows on the
stream lines indicate the direction of flow.
energy to move an index and produce a signal. But the
receiving screw will not move unless the speed of the pro-
peller, and hence the velocitv of the water, changes. When
the propeller is started, the index moves in, sav, a positive
direction ; when it is stopped, the index moves back in the
négative direction; if the motion be reversed, the index
moves again in the négative direction, and so on.
This is analogous to the process of signaling by electro-
magnetic induction. Imagine a loop of wire (C, Fig. 58),
carrying a current. We hâve seen that such a current
represents a supply of energy, which is stored in the
8114
WIRELESS TELEGRAPHY.
medium surrounding the loop as a miode of motion.* If
we hâve a similar loop of wire at D, containing a volt-
meter, any change in the current at O will cause a de-
flection of the voltmeter at D and give a signal. The
analogy is qui te close: an increase in the current at 0
will produce a deflection in one direction, while a diminu-
tion or reversai of the current will cause a deflection in
the opposite sense.
The kinetic energy stored in the medium is manifested
in the magnetic field surrounding the loop: if we place a
Fig. 58.— Method of signaling by electromagnetic induction. A cur-
rent in the loop of wire at C sets up a magnetic field in the surrounding
medium. Any change in the intensity of this field will induce an EMF.
in the loop at D, and give a signal. The Unes of magnetic induction may
be compared to the stream lines in Fig. 57.
compass needle at any point of this field, it will take a
position parallel to the direction of the magnetic force at
that point, just as a pivoted vane immersed in the water
will place itself parallel to the direction of flow. Thus we
* It must be borne in mind that Maxwell does not attempt to
define the actual nature of this disturbance in the ether, but simply
shows that it follows ail the laws of the analogous phenomenon in
matter.GENERAL PRINCIPLES.
315
may plot a sériés of lines of magnetic induction, or “ lines
of force,” throughout the field, corresponding to the stream
lines in the water. The direction of the lines indicates
the direction of the magnetic force, and their proximity to
each other, its intensity; just as the stream lines indicate
the direction and velocity of the flow. The significance
of these lines will appear later.
3. Preece’s Apparatus.—Sir William Preece has ap-
plied this mdthod with considérable success over short
Fig. 59.— Preece’s apparatus for signaling by electromagnetic induc-
tion. An intermittent current produced by the battery B and commutator
C, and controlled by the key K, is sent through the line L, and returns
through the earth. The receiver comprises a similar line, L2, whose cir-
cuit includes a téléphoné receiver, T.
distances.* His sending apparatus comprises a long wire,
supported on pôles as high as possible above the earth, and
grounded at both ends ; the circuit including a battery and
a key for making and breaking the current. (See Fig. 59.)
* See Preece Ætheric Telegraphy, Jour. Instn. Elec. Engrs., v. 27,
p. 869, 1898.116
W IRE LE S 8 TELEGRAPEY.
The receiving apparatus is a similar conducting circuit
parallel to the first, and including a téléphoné. Each
time the current is made or broken in the sending circuit, a
current is induced in the receiving circuit and a click is pro-
duced in the téléphoné. In his improved apparatus Preece
used a rotating commutator, in sériés with the sending
key, whereby the circuit was made and broken some 200
times per second. The resulting succession of impulses
produced a musical note in the téléphoné, which, when
Fig. 60.— Static method of signaling through water. The pump at A
raises or lowers the level of the water, and a gauge at B indicates the
variation of pressure.
interrupted by the sending key, gave easily readable dots
and dashes.
With this apparatus Preece worked successfully acro3S
the English channel, but the size and cost of the equipment
increases very rapidly with the distance, and a practical
limit is soon reached. The reason for this will appear
later.
A similar method was used by Phelps for telegraphing
to and from moving trains. In this case, a coil of wire
encircling the car created a magnetic field whose variations
induced currents in an ordinary telegraph wire stretched
along the track.GENERAL P RI N CI P LE S.
117
4. Method of Electrostatic Induction.— The second
method of sending a signal across our body of water is the
static method — by raising or lowering the level of the
water, and hence the pressure at a given point beneath the
surface, without causing any motion of the wàter, as a
whole, from A to B. For example, a pump at A forces
water into the réservoir, thus raising its level, and a pres-
sure gauge fixed below the surface at B indicates the change
of level to the receiving operator. (Fig. 60.) There is a
Fig. 61.— Electrostatic method of signaling through space. An in-
sulated conductor C is charged to a high potential by a generator, G, and
produces an electrostatic stress in the surrounding medium. The charge
induced on a similar conductor, D, is indicated by an electrometer, E.
The curves radiating from the conductor C are Unes of electric displace-
ment in the ether.
transitory flow of water from A outward in ail directions
while the pump is in operation, but no general motion from
A to B, as there was in the case of the dynamic method.
The flow continues only while a change of level . is taking
place, and represents but little energy. Practically ail the
work done by the pump is expended in raising the level of
the whole body of water : it is thus stored up as potential
energy, due to the increased élévation, and manifested by
an increase of pressure at every point beneath the surface.
So with the electrostatic method of signaling: an insti-118
WIRELESS TELEGRAPHY.
lated conductor, supported above the earth at C (Fig. 61),
is charged to a liigh potential by a high-voltage generator,
and sets up a state of electrostatic stress throughout the
surrounding medium. This is ma de manifest at D by an
electrometer connected to a second elevated conductor.
Aceording to Maxwell’s theory, when a current flows from
the generator to charge the insulated conductor, a sériés
of displacement currents are set up in the ether, flowing
in ail directions from the conductor; but these currents
are necessarily of a transient nature, and exist only while
the displacement is taking place, which results in the
strained condition of the ether which is called an electro-
static field. This field, like the magnetic field described
in section 2, represents an accumulation of energy; but
the energy is of a potential nature, and results from the
displacement of the medium against its elastic reaction.
It, also, mav be represented by a sériés of lines — the lines
of electric force, or electric displacement — each line fol-
lowing the direction of the displacement in the medium,
and the proximity of the lines at a given point being a
measure of the intensity of the field, or the degree of dis-
placement, at that point.
Thus far the hydraulic analogy has held, but here it fails
in an important point. The displacement in the ether is
of an elastic nature, and the intensity of the reaction, or
electric force, dépends upon the degree of the displacement.
Hence, near the charged conductor, where the displacement
is great and the lines of displacement are close together, the
electric force is comparatively large, and it diminishes
rapidly as we proceed from the conductor. In the water,
however, where the force of gravity is constant and inde-
pendent of the displacement of the particles of water, theGENERAL PRINCIPLES.
119
level will be the same ail over the surface, and an increase
in pressure at A will be transmitted undiminished to B.
This discrepancy will disappear if we assume, in place
of water, a semi-solid or jelly-like substance, sufficiently
mobile to flow under the influence of gravity, yet with an
elasticity which resists any displacement with a con-
tinuously increasing force. If now we raise the level at A,
the whole surface will slope downward from this point, as
shown in Fig. 62, and the pressure due to this raising of
Fig. 62.— A more complété mechanical analogue of the electrostatic
method. The medium here is an elastic jelly, which is strained by the
pressure of the pump, and the displacement diminishes rapidly with the
distance.
the level will diminish rapidly as we proceed from A
toward B.
5. Dolbear’s and Edison’s Apparatus.— Prof. Dolbear
constructed an apparatus on this principle, as illustrated
in Fig. 63.* An induction coil having an automatic break
is provided also with a key for opening and closing the
circuit at will, and its secondary terminais are connected,
one to a wire leading to a gilded kite, the other to ground.
At each interruption of the primary circuit, the aerial con-
ductor i3 charged to a high potential, and electrifies by
induction a similar conductor at the receiving station.
* A. E. Dolbear, U. S. pat. ISTo. 350, 299, Oct. 5, 1886.
m
120
WIRELESS TELEGRAPHY.
Here the induction coil is replaced by a téléphoné receiver,
and the currents set up in the téléphoné by the altemate
charging and discharging of the aerial wire produce a
buzzing sound similar to that emitted by the vibrator on
the coil.
Messrs. Edison and Gilliland devised a System embody-
ing the same idea in a somewhat different form, which they
used for telegraphing to and from moving trains.* (Fig.
64.) On the roof of the car is fixed a sheet of métal at-
tached to the secondary of an induction coil, the other
terminal of the secondary being grounded. The primary
of the coil is worked, as in Dolbear’s apparatus, through a
key and an automatic break. The métal roof acts by elee-
trostatic induction on a wire stretched along the side of
Fig. 63.— Dolbear’s apparatus for electrostatic signaling. A kite K,
at the sending station is charged by an induction coil I, and another
kite K » at the receiving end is grounded through a téléphoné, T.
the track, and the currents induced therein are made to
operate a téléphoné at the receiving end of the line. When
it is desired to receive a message on the train, the induction
coil is eut out and a téléphoné receiver substituted, while
at the sending station a large plate suspended near the
See Eng. pat. No. 7,583, June 22, 1885.GENERAL PRINCIPLES.
121
line wire is charged and discharged by means of an induc-
tion coil. Charges are thus induced in the line wire, which
carries them on until they are picked up, again by
electrostatic induction, by the train.
This System was put in practical operation on several
railroads, but was finally abandoned on account of the
limited use that was made of it.
6. Method of Electromagnetic Waves______• We now corne to
the third and most important method of signaling through
space — the method of electromagnetic waves.
Fig. 64.— Electrostatic method of train telegraphy. At the land
station (which is shown in sending position), the line wire W is excited
by an elevated metallic plate P, charged by an induction coil I, operated
by a key K and vibrator V. On the train a similar apparatus is con-
nected to the metallic roof R of the car. Here the key is shown elevated,
thus putting the téléphoné T in connection for receiving.
We hâve seen that either an electrostatic or an electro-
magnetic field may exist by itself, without the other, but
the condition on which this is possible is that it shall be
invariable. If an electrostatic field varies, it at once pro-
duces an electromagnetic field, and vice versa.
Let us retum to our hydraulic analogy :
In the arrangement shown in Fig. 60, as long as the
pump is not working and the level of the water is constant,122
WIRELESS TELEGRAPHY.
there is no motion ; but let us endeavor to change the level
by starting the pump, and at once a flow is produced.
Similarly in Fig. 57 : as long as the propeller is working
uniformly and the water is moving with constant velocity,
no variation in pressure occurs; but let the speed of the
propeller vary or the velocity of the water change in any
way, and at once a presure is produced and a change of
level occurs. The water in motion possesses kinetic
energy: if we cause it to give up any of this energy by
checking its velocity, it exerts a pressure which causes its
Fig. 65.— Wave method of signaling over water. The pump here has
no valves, and alternately sucks in and expels water at periodic intervals.
The waves thus started move a swinging vane, V, at the receiving sta-
tion B. Energy is radiated, but the water only moves to and fro.
level to rise, and the energy takes the potential form due
to the increased élévation.
Suppose now, instead of the continuously acting pump
in Fig. 60, we hâve a simple cylinder and piston, whereby
water may be forced into and out of the réservoir alter-
nately. (Fig. 65.) When the piston is pushed down, a
quantitv of water is forced out and the level is raised ; but
this élévation in one spot can exist only for a moment:
the very act of raising the crest gives the water a velocity,
which it imparts with its neighboring particles, shovingGENERAL PRINCIPLES.
123
them before it ; they in turn communicate their motion to
others, and so the élévation travels outward in an expand-
ing ring. But while this is taking place the piston has
been raised, drawing the excess of water out of the réservoir
and creating a dépréssion. The particles which hâve been
moving outward in the crest now start back to fill this de-
pression, and in so doing create a trough to be filled, in
turn, by other particles left behind by the advancing crest.
We thus hâve a sériés of élévations and dépréssions, mov-
ing outward in ever-expanding circles, trough following
crest and crest following trough, carrying with them the
energy supplied by the pump : in other words, a train of
waves.
These waves, when they reach the receiving station at B,
may be detected, either by a pressure gauge wdiich follows
the variations of level, as in Fig. 60, or by a pivoted vane
which moves with the motion of the water, as shown in
Fig. 65.
Let us now see how this compares with what takes place
wThen an oscillator is set in operation:
Take again our elevated conductor, but instead of the
continuous current generator put an alternator giving a
current of very high frequency. At the moment when the
generator is giving its maximum voltage and the conduc-
tor is fully charged, we shall hâve a State of affairs similar
to that shown in Fig. 61 ; the whole surrounding medium
being in a state of electrostatic stress, or electrical dis-
placement. But this state of displacement was not pro-
duced instantaneously throughout the whole field — the
disturbance began at the conductor and spread outward
writh the velocity of light, in somewhat the same way as
the hydraulic crest expanded from the center where it was
formed.124
WIRELE88 TELEGRAPHY.
Before the electrostatic field has become fully established
the voltage of the generator begins to diminish, and the
conductor discharges itself through the wire to ground.
The State of stress about the conductor gives way to a con-
dition of change, and displacement currents flow back along
the lines of force toward the conductor. While this is
taking place, the current flowing from the conductor to
ground is setting up an electromagnetic field, the lines of
magnetic force forming closed circles about the wire*
Fig. 66.— Signaling by electromagnetic waves. The conductor C (cf.
Fig. 61) is charged by a source G of high-frequency alternating current,
and excites waves which expand until they reach the receiving conductor
D, and induce alternating currents which are detected by the receiving
apparatus R. The circles indicate the lines of magnetic force at the
moment when the conductor C is discharged, the potential V = 0, and the
current I is a maximum. Fig. 61 shows the electrostatic lines when 1 = 0
and V = Max.
Finally, when the conductor is completely discharged and
the current in the wire is a maximum, the electrostatic
field has entirely disappeared from the région close about
the oscillator, and we find in its place the electromagnetic
field shown in Fig. 66.
But this is not ail ; the very act of changing the inten-
sity of the electrostatic field produces a magnetic force.
If a conduction current in the vertical wire produces a
magnetic field about it, why should not a displacement cur-GENERAL PRINCIPLES.
125
rent in the ether do the same ? This is, in f act, the case,
according to MaxwelFs theory. Kot only close about the
wire do these circles of magnetic force exist, but wherever
the electrostatic field is varying the magnetic field goes with
it. As the advancing crest of a water wave is accompanied
by an outward motion of the particles which compose it, so
the electrostatic crest of our electric wave carries with it a
dynamic field of magnetic force, whose ever-enlarging cir-
cles swell outward with the expanding wave.
It is this union of electrostatic and magnetic fields in a
progressive disturbance which constitutes a true electro-
magnetic wave. Neither alone can produce a wave, but
both must be combined inseparably in the proper mutual
relation. We shall see more of this in Chapter IV.
Now returning to the sending apparatus (Fig. 66), we
shall see that the frequency of the generator has a great
effect on the energy of the waves given off. The strength of
the electrostatic field about the elevated conductor dépends
simply upon its charge, and hence upon the voltage of the
generator; but the strength of the magnetic field which
goes to make the wave dépends upon the intensity of the
displacement currents, and hence on the rapidity with
which the conductor is charged and discharged. If the
frequency of the generator is low, the magnetic effect qf
the oscillation is small, and the energy of the electrostatic
field is mainly returned to the generator when the con-
ductor discharges. As the frequency increases, the in-
creasing magnetic energy of the field unités with a portion
of the electrostatic energy and is radiated into space, never
to retum.
Take another mechanical illustration : — A fan is moved
slowly to and fro; it sets the air in motion and stirs a126
W1RELEH& TELEGRAPHY.
floating feather near by, but the motion is so slow that no
appréciable pressure is produced, and no waves are given
off. Now let the fan move faster and faster, until it ap-
proaches the condition of a vibrating tuning fork. Here
the air is set in motion so violently that great variations
in pressure are produced, along with the variations in ve-
locity — true waves are given off, and the Sound of the fork
is heard at a long distance.
So with our oscillator: if the frequency is low, as in
Dolbear’s experiments, we get only a local electrostatic
disturbance ; but if the frequency be sufficiently high, there
is true radiation of electromagnetic waves, which travel
to a long distance.
In practice, the highest frequencies attainable by me-
chanical means — a few thousand per second — are in-
adéquate for the purpose ; the amount of energy radiated
is too small to be of practical value. We are forced to
make use of the free oscillations which occur when a charge
surges back and forth on a conductor, giving off a train
of waves of short duration but large amplitude.
7. Comparison of the Three Methods.— One feature that
is eommon to both the inductive methods, and makes them
impracticable for long-distance signaling, is the fact that
the intensity of the field — magnetic in one case,
electrostatic in the other — diminishes very rapidly as
the distance from the source increases. This may be seen
by referring to Figs. 58 and 61. If the lines of force
were straight, diverging in ail directions from the source,
like rays of ligh-t, the intensity of the field would follow
the simple law of inverse squares, and doubling the
distance from the source would give us one-quarter the
strength of field.GENERAL PRINCIPLE8.
127
But this is far from being the case. In Fig. 58 we see
that the lines of magnetic induction are nearly straight
where they start from the source, but they soon bend back
to form closed loops interlinking with the sending circuit,
and only a small proportion of them reach any considérable
distance. Calculation shows that where the distance be-
tween the sending and receiving circuits is great compared
with their diameters the strength of field falls off practi-
cally as the cube of the distance, and if perchance there
were a mass of magnetic material near the receiving
station, this would still farther deflect the lines from the
receiving loop, and so diminish the effect.
When we consider that the energy of the field, which i?
available for working a receiver, varies as the square of its
intensity, the limitations of this method become only too
apparent. Lodge has calculated that to send a message
from London to New York, under the conditions of
Preece’s experiments, would require a sending circuit en-
circling the whole of England and a receiving circuit as
big as the state of New York.*
A similar line of reasoning applied to the electrostatic
method would disclose a similar resuit: that we must
greatly increase the size and power of the sending ap-
paratus for a comparatively small increase in the distance
to be covered.
Again, the rate at which the energy of the field may be
taken up by the receiver dépends upon the rate at which
its intensity is varied. A rapidly varying field — mag-
netic or electrostatic — produces powerful inductive ef-
* See O. J. Lodge, Jour. Instn. of Elec. Engrs., y. 27, p. 800 et seq.,
1898.128
WIRELE88 TELEGRAPHY.
fects, while a slowly varying field parts with its energy
less readily. In the case of the electromagnetic method,
the rate of variation is limited by the self-induction of the
sending circuit — that species of inertia which opposes any
change in the current. If we increase the size of the cir-
cuit with a view to strengthening the signais, we increase
also the self-induction, and so lose in speed much of what
we gain in strength of the field.
In the case of the electrostatic method this objection is
not so évident, for the self-induction of the aerial wire is
small, and its capacity, though it acts in the same direc-
tion, is still small enough to permit of very rapid varia-
tions. But we must not overlook the effect of the induction
coil or generator which charges the conductor. This has
a large self-induction, and so the apparent advantage of
this method is lost.
But one may ask, why not use the induction coil simplv
to charge the conductor, and then suddenly short-circuit
it, and allow the charge to flow rapidly back to earth ?
This is precisely what is done in using the method of
electromagnetic waves, and therein lies its great advantage.
When the potential of the conductor reaches a certain point,
it breaks down an air-gap separating it from the ground,
and the spark thus formed acts as a short circuit of low ré-
sistance and allows the charge to flow rapidly to earth.
As we hâve already seen, such a discharge is not a simple
undirectional one, but oscillâtes back and forth with a
frequency depending upon the self-induction and capacity
of the apparatus, and starts a train of waves radiating in
ail directions from the source.
Here we hâve the advantage, not only of extreme
rapidity of change in the field, but also of the fact that theGENERAL PRINCIPLES.
129
energy itself is radiated in straight lines, and so fails off
with the distance much less rapidly than does the energy
of the electrostatic or magnetic field, winch is not radiated
at ail, but makes a short excursion into space, only to re-
tum again to its source.
Returning to the hydraulic analogy, we see from Fig. 62
that, in the statical method, the energy spent in raising the
level of the fluid is not radiated — it. is simply stored in
the medium, to be retumed to the source as soon as the
pressure is relieved — and the élévation at a given point
can never be greater than that due to the statical stress in
the medium. When a wave is sent out, on the other hand,
the crest started from the source travels outward, carrying
energy with it. If the wave were shut in between parallel
walls, it would continue undiminished in height, except
as the energy were wasted in friction ; if it be allowed to
spread out in ail directions, the amplitude will f ail off as
the circumference of the wave front increases, but still the
height of the crest is much greater than the élévation caused
by simple statical pressure from the source.
The same thing takes place when electromagnetic waves
are sent out by an oscillator. As the wave expands, dis-
tributing its energy over a constantly widening area, the
intensity of the field which accompanies it — the height of
the crest, as it were — f ails off simply in proportion as the
distance from the source increases, and hence the energy
of the field as the square of the distance.
The reason for this we shall see later. For the présent
let us be content with this simple comparison of the three
methods, and look now more closely at the wave method of
signaling.
9CHAPTER IL
TELEGRAPHY BY HERTZIAN WAVES.
1. Early Experiments.—The early experiments with this
method of wireless telegraphy, which the Germans appro-
priately call “ spark telegraphy,” were made with much the
same apparatus as that used by Hertz in his researches
on free waves in space (see p. 74); indeed Hertz pos-
sessed ail the essential features of a working telegraph
System :— an oscillator emitting electric waves, which in
this case were concentrated by a parabolic reflector, and a
detector of these waves, which was an open resonator placed
in the focus of a similar reflector, with a spark-gap to make
its oscillations visible. (Fig. 44.) This apparatus was
effective from end to end of the room at his disposai —■ a
distance of sixteen métrés.* Other experimenters, as we
hâve already seen, improved the apparatus, and Lodge
succeeded in working up to a distance of forty to sixty
yards with such success that he considered a transmission
of half a mile quite feasible; but none of these investiga-
tors seemed to see any practical utility in their experiments
until Sig. Guglielmo Marconi appeared on the scene.
This distinguished young Anglo-Italian, a pupil and com-
patriot of Prof. Righi, was quick to perceive the possibili-
tés of the radiations which Righi had been investigating,
and proceeded at once to turn them to practical account.
The first important problem to be solved was the per-
* Hertz, Electric Waves, Eng. trans., p. 176.
[130]TELEGRAPHY BY HERTZIAN WAVES.
131
fection of a sensitive and reliable detector. The coherer
of Branly offered great possibilités, but, in its crude
form of a tube several inches long filled with filings and
having its ends plugged with corks, it was very unreliable.
Sometimes it was so sensitive as to be affected by trifling
mechanical disturbances, such as the Sound of the opera-
tor’s voice, and again, for no apparent reason, it would
refuse to work altogether. By dint of careful experiment-
ing Marconi overcame these difficulties and succeeded in
producing a coherer which could be depended upon to
work fairly well under ail conditions. It consisted in a
small glass tube (see Fig. 67), 4 cm. long and .25 cm.
Fig. 67.—Marconi’s coherer — longitudinal section. T is the glass tube,
with leading-in wires terminating in the silver électrodes, E, E'. In the
space between these are the filings.
bore, in which were fitted two silver plugs attached to
terminal wires. The plugs were about one mm. apart, and
the space between them contained a mixture of specially-
prepared silver and nickel filings. The whole tube was
then exhausted and hermetically sealed.
In its normal condition this apparatus has an extremely
high résistance, but under the influence of electrical oscil-
lations its résistance drops immédiately to between 100
and 500 ohms. When this r>ccurs a local circuit is com-
pleted through a battery and a sensitive relay, which in
tura opérâtes the recording apparatus and sets in motion132
WIRELESS TELEGRAPHY.
an automatic tapper, which strikes the tube of the coherer
and restores it to its original condition of high résistance.
Fig. 68.— Marconi’s early transmitting apparatus, consisting in a Righi
oscillator, deed, mounted in the focus of a parabolic reflector, f. b is a
key for operating the induction coil c.
2. Marconi’s Early Apparatus.*— An early form of Mar-
coni’s apparatus is shown in Figs. 68 and 69. The trans-
See Marconi, Eng. pat. No. 12,039, June 2, 1896.TELEGRAPHY BY HE RT ZI AN WAVES.
133
mitting device is a Kighi oscillator (see p. 85) mounted in
the focal line of a parabolic reflector of copper or zinc.
The receiver comprises a similar mirror with a coherer
placed in its focus. To the terminais of the coherer are
attached two strips of copper, whose length is carefully
adjusted so as to màke die period of oscillation of the
coherer System the same as that of the oscillator. Wires
Fig. 69.— Receiver of Marconi’s early apparatus. The coherer j is
mounted on the focus of a second parabolic mirror l, and is provided with
wings, k, whose length is adjusted to résonance, n is a relay and h the
recording apparatus.
from the ends of the coherer pass through holes in the
mirror to the relay and recording apparatus, which is
shown diagrammatically in Fig. 69.
This arrangement, crüde as it is, was used success-
fully over distances of two or three miles ; but it has seri-134
WIRELÈSS TELEGRAPHY.
©us limitations. In the first place, the wave-length of the
©scillator is very short — only a foot or so, depending
upon the size of the halls. Such short waves are peculiarly
susceptible to absorption by obstacles — a few buildings or
a small hill in the path of the waves being sufficient to eut
them' off quite effectually — while a longer wave, com-
parable in size to the obstacles themselves, would be hardly
affected at ail. In like manner, a rock protruding from
the water stops the small waves which pass over the sur-
face, while the larger ones seem scarcely to notice it.
Another and even more serious drawback is the feeble
intensity of the waves. How to increase their energy, and
hence the distance over which signaling would be possi-
ble, was the next important problem to be solved.
The charged balls of the oscillator, at the moment the
spark occurs and the oscillations commence, represents a
certain amount of energy. This energy oscillâtes back and
forth until it is ail radiated into space or frittered away
in the spark-gap, but no new supply can be drawn from
the source, however powerful, until this is used up and a
new spark occurs. Ail the energy of a wave-train must
be stored in the charged conductors before the oscillations
commence.
There are two ways of increasing this energy: first, by
increasing the potential to which the conductors are
charged, and second, by increasing the capacity of the
conductors themselves. The former may seem at first
glance the more promising, as the energy of the charge
increases as the square of the potential; but a practical
limit is soon reached. The différence of potential dépends
upon the length of the spark-gap ; if we increase this be-
yond a certain point, the spark is so attenuated that theTELEGRAPHY BY HERTZ!AN WAVES.
135
discharge is not strong enough to keep it hot, and most
of the energy is wasted in overcoming its ohmic résistance,
This difficulty may be part-ly overcome by causing the
spark to break through oil, and so making a high potential
possible with a short, thick spark which does not waste
much energy. But even here a limit is soon reached, and
it is necessary to fall back on the second means of increas-
ing the energy,— by increasing the size and capacity of the
oscillator. This has the further advantage of increasing
fe=£
t2
U
t
CiO
Fig. 70.— Marconi’s improved apparatus with two capacity areas,
the wave-length, and so reducing the interférence of
obstacles.
3. Improved Apparatus— Another form of Marconi’s
apparatus is shown in Fig. 70.* The two halls of the spark-
gap are connected respectively to two plates of métal, hung
on insulators, which constitute the capacity areas of the
oscillator ; — in other words, Hertz’s “ large oscillator ”
*Loc. cit.136
WIRELESS TELEGRAPHY.
(p. 34), magnified. The receiving apparatus is similar
but the spark balls are replaced by a coherer.
A similar arrangement was used by Sir Oliver Lodge,
who employed large conical surfaces, arranged like an
hour-glass, in place of the fiat plates.* (Fig. 71.)
The results of these experiments fully confirm the
theory : with the larger apparatus, the distance over which
signais could be transmitted was greatly increased, and it
was found possible to dispense entirely with the large and
Fig. 71.— Lodge’s conical capacity-areas.
cumbersome reflectors which had been used to concentrate
the radiations. Indeed, Marconi showed that, other things
being equal, the distance of transmission depended upon
the size of the plates, their height above the earth, and, to
some extent, their distance aparté The second point is a
matter of importance, for it reminds us that the radia-
tions we are dealing with are polarized, and that the dis-
placements in the ether take place in a direction parallel
See Lodge, Eng. Pat. No. 11,575, Feb. 5, 1898.TELEGRAPHY BY HERTZIAN WAVES.	137
to the axis of thé oscillator.* If the axis be horizontal,
the waves striking the earth will indnoe currents parallel
to the oscillator, and much of the energy will be absorbed
and wasted in ohmic losses. Elevating the oscillator only
diminishes the trouble, but does not remove it.
Lodge placed his oscillator in a vertical position, and so
avoided this difficulty, only to fail into another — the very
Fig. 72.— Popoff’s receiver. AB, the coherer ; CDE, the relay ; GH, a
combined bell and tapper.
practical one of supporting his large cônes in the air and
of providing meians for getting at the spark-gap and other
apparatus which must be placed between them.
But happily another solution was forthcoming.
*Approximately — See Chap. IV.138
WIRELE 8 8 TELEGRAPHY.
4. The Antenna.— About this time, Prof. Popoff, who
had been using a modified form of the coherer in investi-
gating the effects of distant lightning discharges,* under-
took to apply the same apparatus to wireless telegraphy.
His arrangement is shown in Fig. 72. The coherer, relay,
and tapper were arranged in essentially the same way as
Marconi’s (p. 138), but instead of attaching capacitv areas
to the two terminais of the coherer, he connected one end
to the ground and the other to a vertical wire, or antenna,
supported by a mast.
This antenna played an important part in picking up
the waves produced by lightning discharges, and the in-
tensity of the effect was found to dépend upon the height
of the antenna — the longer the wire, the stronger the
impulse. At first ’glance it would seem that the same
should be true in wireless telegraphy; a high antenna,
cutting a large slice out of the advancing wave-front, should
take up more energy from the ether and give a stronger
signal than would a shorter one. But in practice this was
not the cafce; a limit was reached beyond which an in-
crease in height had very little effect.
We must seek the explanation in the fact that the radia-
tions dealt with in the two cases differ widely in character.
We know that a lightning-flash is oscillatory, and that it
sets up a train of waves which, as Popoff found, may be
detected at long distances ; in fact, when a discharge occurs
between two clouds, or between a cloud and the earth, we
hâve practically a huge Hertzian oscillator, giving off elec-
tromagnetic waves of tremendous energy. As the size of
* Popoff, Jour. of Russian Physico-Chemical Soc.y v. 28-29, p. 896,
1895.TELEGRAPHY BY HERTZIAN WAVES.
139
the oscillator détermines the wave-length of its radiations,
and as ordinary Hertzian waves may be measured in
métrés, we might expect a great cloud-oscillator to émit
waves several miles in length — and this is inaeed the case.
Now, when a wave strikes an an tonna, it indnces an
electromotive force in the wire. If the wire is vertical
and the distance from the source is great, we may assume
that the wire is parallel to the wave-front, and that ail
parts of it are affected alike. The disturbance started at
the top travels downward with the velocity of light, gatlier-
ing force as it goes ; but by the time it has reached a point
a half wave-length distant from the top, the incoming
wave is reversed in phase, and the direct impulse re-
ceived at that point destroys the efïect of that transmitted
from the top. At shorter distances, where the différence
in phase is smaller, the interférence is less complété ; but
there is evidently no advantage in lengthening the antenna
beyond the quarter wave-length of the oscillations to be
received, unless the object is to reach above the région
affected by obstacles. With lightning discharges, the
limit is never reached; but with the radiations of short
wave-length used for telegraphing at the time of Popoff’s
experiments, it is little wonder that the results were dis-
appointing.
Marccni went a step farther, and used an antenna at
the sending station also — and this marks an epoch in the
history of wireless telegraphy.* At once the limits in the
size of the apparatus were removed indefinitely ; the wave-
length and power of the radiations were greatly increased ;
the interférence of obstacles began to disappear: as the
* Eng. pat. No. 12,039, June 2, 1896, claim 16.140
WIRELESS TELEGRAPHY.
apparatus was perfected, the available range of signaling
was increased at a surprising rate; ere long, the news
came to the world that signais had been received across
the Atlantic océan, and commercial wireless telegraphy
became an acknowledged fact. But there were many
things to be leamed before this point was reached, and we
must now trace the development of the idea along this new
line.CHAPTER III.
/
THE GBOUHEtED OSCTLLA1TOB.
1. Development of the Antenna.— The advent of the
grounded oscillating System found many investigators in
the field, and each contributed something to the develop-
ment of the new idea.
Marconi at first considered it important to hâve a large
capacity area on the end of the antenna, after the fashion
Fig. 73.— Marconi’s grounded antenna, with large capacity area. uE is
tlie transmitting antenna, with spark gaps de, ee, ed; wE, the receiving
antenna with coherer, j.
of the Hertzian oscillator, and we find him nsing the ap-
paratus shown in Fig. 73, with an elevated metallic plate
connected to ground through his original Righi spark ap-
[141]142	WIRELESS TELEGRAPHY.
paratus.* Guarini employed a similar arrangement in the
form of a cage or cône of wires,f hung from a mast
(Fig. 74), and many other forms of capacity areas wero
devised ; but it was found that such devices were of com-
parativelv limited value,
and they gradually gave
place to a simple vertical
wire, supported by a mast
and connected at its lower
end to the spark apparatus.
Here we must note the
distinction between this-
form of apparatus and
that used by Hertz for the
study of waves in wires.
(See p. 48.) In the lat-
ter case, the connection
between the oscillator
proper and the wire was
through a condenser — a
sort of elastic coupling —
and the size and capacity
of the wire were so small
that it exerted compara-
tivelv little influence upon
the oscillator, which was thus free to perforai its own
vibrations and communicate them to the wire. The effect
is analogous to that of a large and heavy pendulum
connected to a smaller and lighter one through a spring.
The natural period of the small pendulum has, of course,,
a slight effect on the vibrations of the larger one, but
* Marconi, En g. pat. No. 12,039, June 2, 1896.
t Guarini, in Elec. Rev., v. 51, p. 921, 1902.THE GROUNDED OSCILLATOR.
143
the latter prédominâtes and forces the other to swing
with it.
So, in Hertz?s experiment, the disturbance in the wire
was a forced oscillation of the period of the main oscillator,
and it was at first suggested that the same might be true
in the case of an antenna grounded through a pair of spark
balls — that the real seat of the oscillation is in the mass-
ive conductors near the spark, and that the antenna play s
only a secondary part, as a radiator. But this is not the
case. The coupling between the different parts of the
apparatus is here a rigid metallic connection, and the Sys-
tem vibrâtes as a whole with a period determined mainly
Fig. 75.— Various forms of multiple-wired antennæ :— a, Braun’s grid ;
b, Slaby’s cylinder ; c, Marconi’s fan.
by the height and dimensions of the antenna. For a single
wire, the wave-length is equal to four times the height, but
w7here the antenna is loaded with a capacity area, the wave-
length is correspondingly increased. It is true that ail
irrégularités in the apparatus hâve their effect in pro-
ducing harmonies or inharmonic overtones, and herein lies
one disadvantage of the large capacity area. It is much as144
WIRELES8 TELEGRAPHY.
if we tried to. strengthen and deepen the tone of a piano
cord by hanging a weight in the middle. We might suc-
ceed to some extent, but the note would be harsh and
jarring: the better means is to increase the mass of the
whole string uniformly by winding it with wire. This is,
in effect, what is done with the antenna : when the energy
Fir». 76.— Large multiple antenna used by Marconi in trans-atlantic
experiments.
of the oscillation of a single wire is liot sufficient, the
capacity is increased by adding other wires in parallel con-
nection, as shown in Fig. 75. Braun* arranged them as a
harp or grid (a) ; Slaby,f as éléments of a cylinder (b) ;
Marconi,% as a fan (c) ; but the form which seems most.
effective for long-distance installations is that of an in-
verted cône or pyramid made up of a large number of wires
or cables, radiating from a common vertex, from which a
connection is made to the spark apparatus. Fig. 7 6 illus-
* Braun, Eng. pat. No. 12,420. App. June 14, 1899.
t Slaby, Die Funkentelegraphie, p. 66.
t See Righi-Dessau, Die Télégraphié ohne Draht, p. 473.THE GROUNDED OSCILLATOR.
145
trates such an antenna as used by Marconi in his trans-
atlantic work.
Not the least advantage of the multiple^wired antenna
lies in the fact of its comparatively low résistance. The
currents which surge back and forth through the wire,
though of short duration, are surprisingly heavy — often
many times what would be sufficient to melt the wire if
they were continuons. Moreover, as we hâve already seen,
such rapid oscillations are confined to a thin skin on the
outside of the wire, and do not penetrate into the interior;
hence, the résistance of the wire dépends, not upon its
area, but on its circumference, and a number of thin wires
are much more effective than a single large one of the
same cross-section. As a large amount of energy may be
wasted in ohmic lusses in the wire, this considération is
of no small moment, especially where prolonged oscilla-
tions are desired.*
* As a nuraerical example, take the case of a certain multiple antenna
wliose constants the writer has carefully measured. It consista in five
copper wires, fifty feet long, connected at their lower ends to a single
ground wire .10 inch m diameter, and twenty feet long.
The capacity of the antenna to ground is .00022 microfarad, hence its
initial charge at a potential of, say 20,000 volts, would be
Q=CV=.00022 x 10"g x 20,000=44 x 10-7 coulombs.
The frequency, N, of the oscillation is almost exactly 3,000,000 cycles
per second; hence the current in the wire during the first swing, before
the energy becomes dissipated, would be
I=Q x 4N x
2 y2
= 58 amps.
The effective thickness of the “skin” at this frequency is about .001
inch, hence the cross-section of wire in the ground connection available
for carrying the current is n x —x 77^ = .0003 square inches, and the
current density,
58
.0003=
10 ~ 1U00
= 19,000 amps. per square inch.
The résistance of such a shell of copper is about .0008 ohms per cen-
1014G
WI RELE88 TELEGRAPH Y,
Still anotlier reason Avhy a large capacity area is of
limited utility is found in the considérations outlined
on page 125. If \ve increase the capacity of the antenna
without a corresponding increase in its height, the energy
of the System is indeed augmented, but the proportion of
this energy radiated during each oscillation is diminished,
owing to the lower freqnency. The rate of radiation is not
improved by the increased capacity of the oscillator, though
the total energy given off is greater.* When the coherer is
used for receiving, it is the former that counts, and the
“ whip crack ” discharge of a strongly damped radiator is
more effective than a prolonged oscillation of the same
energy. Of la te, however, improved receivers and tuned
apparatus hâve made prolonged oscillations of great value,
as will appear in the chapter on syntonie methods.
2. Other Means of Increasing the Radiation.— Thus far,
we hâve been content with increasing the energy of the
oscillations by increasing the capacity of the oscillator.
Let us now take up the second means of doing this — by
increasing the potential to Avhich it is charged.
timetre, or sav .5 ohm for the 20 foot ground wire. Hence the rate of
dissipation of energy due to the résistance of the ground connection alone
is I2R = 582 x .5 = 1700 watts, or over two horse-power.
During the first oscillation this amounts approximately to 1700 watts
1
1 3x i06
sec. = .00057 joules.
The total initial energy of the System
was W =-îcV2==i- x .00022 x 10 6 x (20,000)2=.044 joules, htnee
Z Z
00057	1	*
1-=^th of the entire energy is lost during one oscillation, in
the ground connection alone.
If the ground wire were replaced by four smaller wires of the same
aggregate cross-section, this loss would be halved.
* See Hertz, Electric Waves, trans. by D. E. Jones, p. 150; also, H.
M. Macdonald, Electric Waves, p. 76.THE G ROUX DE D OSCILLATOR.
147
We hâve seen (p. 134) that the potential is dépendent
on the length of the spark, and that this is limited by the
ability of the current flowing across the gap to keep the
spark hot and afford a path of low résistance for the oscil-
lations. With the increasing capacity of the oscillator this
trouble disappears; indeed, with verv powerful sparks,
the other extreme is reached, and the difficulty is to keep
the spark cool. The air in the gap, and also tlie terminais
themselves, get so hot that the gap has not time to recover
- ~\7j 9> 9 A MA MM A 9^

Htgggtütimi
VÔW'iV'ib'iSVé

Fig. 77.— A high-frequeney transformer, as constructed by Prof. Elihu
Thomson. P is the primary winding, S the secondary, and T a glass tube
separating the two. V is a vessel of oil in which the coils are immersed,
and G G are glass or rubber bushings for further insulating the terminais.
its insulating character between the successive discharges
of the induction coil. Hence, it is impossible to charge
the oscillator to a high potential, and the spark dégénér-
âtes into a continuons flaming arc, quite useless for pro-
ducing oscillations. This trouble may be avoided by
“ blowing out ” the arc with an air blast or a magnet ; but148
WIRELESS TELEGRAPHY.
as it is encountered only with high-power apparatus, we
shall pass it by for the présent.
In ordinary cases the potential is limited mainly by
mechanical considérations, in building and insulating high-
voltage apparatus, hence it
is often désirable to raise the
potential of the antenna above
that occurring at the spark-
gap.
The most usual and verv
effective wav of doing this, is
by means of an air-core trans-
former or “ Tesla coil.” (See
Fig. 77.) This is an induction
coil or transformer made with-
out iron; both primary and
secondary coils are wound
with only a fevT turns of thick
wire, and the whole is im-
mersed in oil, to insulate it
against the enormous différ-
ences of potential which occur between adjacent parts of
the windings.
The primary winding is connected, through a spark-
gap, across the terminais of a strongly-insulated con-
denser, such as a battery of Leyden jars, and the latter
is charged by an induction coil or other source of high
voltage, and allowed to discharge across the gap. (Fig.
78.) As soon as the spark occurs, oscillations are set up
in the primary winding, as in the case of Feddersen’s
experiments, (p. 22). These, by virtue of their high fre-
quency, hâve a very powerful inductive effect on the
Fig. 78. — Transmitting appa-
ratus with high-frequency trans-
former. P is the primary of the
transformer, whose circuit is closed
through the condenser C and spark-
gan G; S, the secondary, connected
to the antenna A and to ground,
and I the induction coil.THE GROÜNDED OüCILLATOR.
149
secondary, and thus it is possible to step up the voltage
to a degree far greater than that indicated by the simple
ratio of turns of the primary and secondary windings.
One terminal of the secondary winding is connected to
ground and the other to the antenna, which is thus charged
to a much higher potential than that which occurs at the
spark-gap.
To obtain the best resuit s, the two circuits should be
“ tuned,” so that they both perforai free oscillations of the
same period; but as this subject wdll be treated more fully
in the chapter on Sélective Signaling, we shall postpone
considération of it until then.
3. The Induction Coil.— The development of the an-
tenna, and its increasing size and capacity, brought with
them the demand for more powerful apparatus for charg-
ing the oscillator. With the small oscillators of Hertz and
his followers, an ordinary induction coil was quite suffi-
eient* but the new devices imposed new requirements which
the old-fashioned coils are inadéquate to meet.
The standard induction coil of the laboratory grew out
of the demand for long sparks and high voltages, and a
coil was rated aecording to the length of spark it could
generate, with little regard to other considérations. The
resuit is a coil with a slender iron core and primary, sur-
rounded by a prodigious amount of secondary wire of the
smallest size that can be wound and handled. This is pre-
cisely what is not wanted for wireless telegraph work.
We hâve seen that very long sparks are not désirable; a
one-inch spark is ample for sending messages a hundred
miles or more with a plain antenna, and even for long-
distance work the tendency is to use short sparks and
increase the radiation by other means. The attempt to150
WIRELESS TELEGRAPHY.
use the old standard coils for such service is much like
trying to drive a steam engine by liquid air, let out of a
pressure cylinder through a tiny orifice. The pressure is
there, and, it may be, the energy ; but by the time the air
reaches the piston, most of its power is lost, So a coil
with a long and fine secondary, of enormous self-induction
and résistance, is sluggish in its action — it takes a con-
sidérable fraction of a second to give up its energy. After
the antenna is once charged and the spark occurs, ail the
energy remaining in the coil is wasted, and goes onlv to
heating the air-gap and its spark-terminals, and spoiling its
insulation for the next spark.
What is wanted is a coil which gives a large amount of
energy at a moderate voltage, and yields it up quickly;
giving the pendulum, as it were, a quick, powerful shove
and letting go. Now the energy available for each dis-
charge is ail stored in the magnetized iron core before the
primary circuit is broken, and is thence transferred,
through the secondary, to the antenna, with greater or less
rapidity, according to the résistance and inductance of the
circuit and the capacity of the antenna.
The outcome of these various considérations is a coil
having a core of ample cross-section and moderate length,
with a secondary of comparatively few turns of coarse
wire, so wound as to secure the maximum effect from the
magnetism of the core. Such a coil will give a clean,
snappy spark at a high rate of interruption, where another
having many times the weight of wire would refuse to
work above a dozen breaks per second. An y attempt to
increase the speed of the break would resuit in a hot,
flaming arc, wasting energy, and useless for producing
radiation.THE GROÜXDED OSCILLATOR.
151
The influence of the rate of interruption on the possible
speed of signaling is too manifest to require comment.
4. High-power Generators.— Where a still greater sup-
ply of power is required, the induction coil is often re~
placed by a high-voltage transformer, operated by an
altemating current generator. This arrangement lias
many advantages, not least of which is the doing away
with the primary interrupter or vibrator — an apparatus
Fig. 79.— Transmitting apparatus fed by an A-C generator D. whose
voltage is stepped up by a transformer T. A condenser, C, of large ca-
pacity compared to the antenna, is connected in a local circuit, C G L,
coupled to the antenna through the inductance coil L.
whieh gives much trouble when c-alled upon to handle any
considérable amount of power, notwithstanding the many
ingenious shapes that it has taken in the process of adap-
tation to various classes of work.
A typical form of this class of transmitters is that used
bv the DeForest Wireless Telegraph Co. (See Fig.
Y9.) The voltage of an ordinarv commercial alternator is
stepped up to, say, 25,000 volts by an oil-insulated trana-152
WIRELESS TELEGRAPHY.
former, whose secondary is connected to the terminais of
the spark-gap. The spark-gap is not included directly in
the antenna circuit, for this has not sufficient capacity to
absorb ail the energy of the alternator at the voltage used ;
but the current is fed into a condenser of larger capacity,
included in a branch circuit containing an inductance,
which is also made part of the antenna circuit. The two
circuits thus interconnected vibrate together, the energy
of the condenser being fed to the antenna as the energy of
the latter is radiated. Usually the antenna circuit must
conta in an additional inductance to make its period the
sanie as that of the branch circuit, its capacity being less.
It is convenient to combine the two inductances in one
coil having several terminais to facilitate tuning, as shown
in the figure, in which case the apparatus approaches very
closely the arrangement described on page 148, wliere a
transformer was used to couple the closed oscillating cir-
cuit to the antenna. Indeed this doubly-connected coil is
practically a transformer, in which the same wire plays
the part of both primary and secondary windings.
With such a high-power generator, the energy that mav
be applied to the oscillator is practically unlimited, and
the ability of the latter to receive it may be increased al-
most indefinitely by the use of condensers of sufficient
size. The trouble cornes in handling the energy at the
spark-gap, and causing the discharge to maintain its oscil-
latory character, despite the tendency to heat and form an
arc. Air-blasts, magnetic blow-outs, and multiple spark-
gaps hâve ail been used with varying degrees of success,
in the effort to keep the spark cool.THE GROUNDED OSCILLATOR.
153
A particularly effective device due to Prof. Fleming* is
shown in Fig. 80. An alternating-current generator of,
say, 25 k\v. capacity feeds a step-up transformer, T0, by
which its voltage is raised to 20,000 v. The shaft of the
altemator carries, or is geared to, a radial arm whose end
passes close to two metallic sectors, set at diametricaUy
opposite points. The arm is set in such a position that it
cornes opposite to one of the sectors, B0, when the voltage is
Fig. 80.— High-power generator of Prof. Fleming. D is an altemator,
feeding a step-up transformer T0. R is a rotating arm driven by the alter-
na tor shaft, and passing close to the sectors BoB,. Tj and T2 are air-
core transformers, Cj and C2 the primary and secondary condensers, G
and air-gap, and A the antenna. The low-frequency oscillations of the
nrimary oscillating circuit, CjRBjPj, excite the secondary oscillating circuit,
C2GP2, whose high-frequency oscillations charge the antenna A.
a maximum, and a spark leaping across charges a condenser
of large capacity, Ct, to practically the full voltage of the
transformer. When the arm has made a half révolution
and reached a position opposite the second sector, P1? an-
* See Eng. pat. No. 18,865. Application filed Oct. 22, 1900.154
WIRELESS TELEGRAPHY.
other spark occurs, whereby the condenser is discharged
throngh the primary of an air-core transformer, T1? and
oscillations are produced which are again stepped-up to a
still higher voltage.
As the capacity of the condenser Ci must be very large
to absorb the whole output of the generator, these oscilla-
tions are of comparatively low frequency — say 10,000 per
second — too slow to be used directly for radiation by an
antenna ; so they are employed as a secondary source of
power, to excite a secondary oscillating System, consisting
in a spark-gap, G, a condenser, C2, and a second air-core
transformer, T2, whose secondary is connected to the
antenna and to ground. When the condenser C> is
charged to a potential high enough to break down the air-
gap, a powerful spark occurs, and a new set of oscillations
is started with a high frequency, depending on the capacity
and inductance of this second circuit and independent
of the frequency of the primary oscillation. This capacity
and inductance are made small, so that the secondary cir-
cuit may be tuned to the natural period of the antenna, and
thus the maximum efficiency of radiation be secured.
The energy of this secondary oscillation is much greater
than it could be made by any of the ordinary means. We
shall see (p. 215) that the secondary voltage of an oscilla-
tion transformer such as Ti may be much higher than that
which would be indicated by the simple ratio of tums, for
the ratio of the capacities, Ci: C2, is also an important
factor. Thus, notwithstanding the small capacity of the
condenser C2 (.02 microfarad as against 0.5 mf. for C1? in
a case cited bv Prof. Fleming), the increased voltage en-
ables it to absorb most of the energy of the latter, and make
it effective in radiation.THE GROÜXDED OSCILLATOR.
155
This apparatus is used by Marconi for transatlantic
work. Its radiation has the advantage, not only of great
energy, but also of small damping and correspondingly
small intensity. Thus its signais may be detected by an
apparatus two thousand miles away, properly tuned to
receive them, while passing unnoticed by the untuned in-
struments on vessels in between. We shall see more of this
in Chapter VI.CHAPTEE IV.
PROPAGATION OF GROTJNDEOD WAVES.
1. Three Hypothèses Suggested.— We hâve seen that
the waves emitted by a Hertzian oscillator are polar-
ized transverse vibrations, of essentially the same nature
as light, but enormously greater wave-length ; that they
travel ordinarily in straight lines, but are subject to re-
flection, refraction, diffraction, etc., precisely like ordinary
light. Are the radiations of a grounded antenna of the
same nature, and subject to the saine laws, or hâve they
some peculiar properties which set thern apart from the
free Hertzian waves which we hâve been considering?
The answer to this question is of great practical im-
portance to the wireless telegrapher, as it will go a long
way toward determining his ability to surmount ob-
stacles in the path of his waves. To dodge a little hill or a
group of houses is a simple matter of diffraction when the
wave-length is great, but to cross a mountain range or the
hill of water due to the curvature of the earth is quite
another matter. The curvature of the earth is a con-
sidérable factor, even in comparatively short transmis-
sions — a distance of forty miles between stations being
sufficient to put the highest masts below the horizon —
yet it is now an every-day matter to send signais for hun-
dreds or even thousands of miles. How can we explain
this ?
[156]PROPAGATION OF GROUXDFD WAVES.
157
Three hypothèses hâve been offered to explain the propa-
gation of signais from an antenna : —
lst. That they are free waves in the ether, exactly like
those studied by Hertz, except for their wave-length.
2d. That ether waves hâve little to do with the case,
except to waste energy bv shooting into space ; but that the
signais are transmitted by alternating currents in the earth
or sea.
3d. That there are electric waves of a peculiar char-
acter, which glide over the conducting surface, following
its curves as a rain drop follows a window-pane.
Xone of these ideas are absurd, and we must call upon
experimental facts to décidé between them.
2. The Free-Wave Hypothesis.— Take first the hy-
pothesis of free waves. There are four ways in which
such a wave may pass an obstacle : by passing through it,
by diffraction around it, bv reflection or refraction by sur-
rounding objects. The first involves the conductivity of the
obstacle — we know that a conductor is opaque to the
waves, while a dielectric is transparent. Xow, sea-water,
though a good conductor for slowly-varying currents, is
quite transparent to light; and even with waves a million
times longer, such as are produced by the smaller Hertzian
oscillators, it acts as a more or less perfect dielectric. ( See
p. 83.) But the waves used in wireless telegraphv, at a
frequency of a million cycles per second, are several hun-
dred times longer still, and Prof. J. J. Thomson has shown
conclusively that, at these frequencies, sea water is a good
conductor, and hence quite opaque to such waves.* So, if
we are to accept the “ free-wave ” hypothesis, we must find
* See J. J. Thomson, Proc. Roy. Soc. 45, p. 269, 1889.358
WIRELESS TELEGRAPHY.
a way to get around tlie surface of the océan, not
through it.
Diffraction lias been suggested, but we must remember
tliat the amount of déviation dépends upon the relation of
the wave-length to the sharpness of the diffracting edge.
M. Gouy, in his experiments on the diffraction of light bv
the edge of a very keen razor, obtained remarkable dévia-
tions (p. 96) ; but here the thickness of the edge was com-
parable to the wave-length of light. The waves that we are
dealing with are to the earth as light waves are to a sphere
one inch in diameter, and it is difficnlt to conceive of such
an extreme case of diffraction in the light of our présent
knowledge.
A gain, diffuse reflection may be considered, as we see it
in the afterglow when the sun lias sunk below the horizon.
But here, the sun?s rays are reflected from material parti-
cles, large in proportion to the wave-length, suspended in
the air. As the diameter of the particles approaches the
wave-length of the light, the reflection becomes less and
less perfect. The longer waves are naturally the first to be
affected ; thus, on a perfectly clear day when the suspended
particles are very minute, the longer red and yellow rays
are scarcely reflected at ail, and the sky looks blue, on
account of the prépondérance of the shorter waves.
Where are the material bodies in the upper air large
enough to reflect waves 1,000 feet long ? Clouds will not
do, for a fog-bank is found to be quite transparent. The
water which composes the fog and clouds, even if itself a
conductor, is concentrated in minute globules separated bv
insulating air, so the cloud as a whole acts as a good
dielectric. Sonie think that the rarified upper strata of
the air are suffîciently good conductors to reflect the waves,PROPAGATION OF GROÜNDED WAVES.
130
as the sound of a distant thunder-clap is drawn out into a
long roar, but this remains to be proved, and the trend of
experimental evidence is against it.
Then there is refraction. But at-
mospheric refraction is not sufficient
to carry light any considérable distance
around the globe, and the spécifie in-
ductive capacity of air is not large
enough to indicate any great différence
in the index of refraction for long elec-
tric waves. (See p. 79.)
Finally we hâve the experimental
fact that the transmission of waves
from a free oscillator has never been
successful over long distances, notwith-
standing the many attempts at concen-
trating them by mirrors, etc., whereas
the radiations from a grounded an-
tenna travel indefinitely over the sea
with no appréciable loss in intensity
beyond tliat which is accounted for by
the atténuation of the waves through
spreading out over constantly-widening
areas. The free-wave hypothesis does
not account for this.	atin^nne^show dfsatH-
3. The Altemating-Current Hypothesis. SîEi °tfhe ^nsity
J i	ofeurrent. N N, nodes,
— Consider now the second explana- l, îoop.
tion : that alternating currents are set
up in the earth, flowing in and out of the antenna, and that
these currents travel through the ground, themselves giving
rise to the signal at the receiving station without the aid of
ether waves.
Fig. 81.—Vibration of160
WIRELESS TELEGRAPHY.
That such currents exist is undeniable. We may look
upon a grounded antenna as half of a Tlertzian oscillator,
such as is shown in Fig. 81. We hâve learned that the
ends of an oscillator are nodal points, where the potential
varies but there is no current, while at the spark-gap in the
middle there is a loop, where currents surge back and forth
but there is no change of potential. If we take away the
lower half of the oscillator and in its place put an infinité
Fig. 82.— Vibration of grounded oscillator. Distribution of potential
and current as in Fig. 81, but currents spread out in conducting plane.
conducting plane, the oscillations will go on as before ; the
point where the oscillator meets the plane will still be a
loop, and the currents wfill surge in and out, losing them-
selves in the conducting surface. (Fig. 82.)
The same is true of a sonorous tube containing a vibrat-
ïng column of air. (Fig. 83.) If the tube be closed at
both ends, the vibrations will surge back and forth with a
node at each end and a loop in the middle. If we eut the
tube in two, leaving the eut ends open, each half willPROPAGATION OF GROUNDED WAVES.
161
vibrato precisely as before, with a node at the closed end
and a loop at the open one, and currents of air will rush
in and out of the open end. (Fig. 84.)
But are the currents which radiate in ail directions from
the oscillator sufficient to account for the signais received
at a distance? Unquestionably there is also a large
Fig. 83.— Vibration of air column in tube closed at both ends. Nodes
at ends, loop in middle.
amount of energy given out in ether waves, for a receiver
placed near the oscillator is affected, whether it be
grounded or not. If these ether waves travel outward in
straight lines, we should expect the intensity of the signal
to fall off rapidly when the distance becomes so great that
the earth cuts off the direct radiation ; but this is not the
Fig. 84.— The same tube eut in two, with ends left open. Each half
vibrâtes as before, and air currents rush in and out of the open ends.
case : the atténuation follows the same law, however great
the distance.
Again, if a coherer be placed in a hole in the ground, it
will operate when uncovered ; but if the hole be filled with
earth, the oscillations produce no effect.*
* See Poincaré, Notice sur la Télégraphie sans Fil, Ann. du Bur.
des Long. 1002, Notice A, p. 12.
11
L
N162
WIRELESS TELEGRAPHY.
We must look for something more than earth currents
to explain the phenomena.
Yet there is evidence that the conductivity of the sur-
face over which the radiations pass has a marked effect on
their atténuation. The transmission is very much better
over sea than over land, and some kinds of land are better
than others: dry, sandy ground gives very poor trans-
Fig. 85.— The electrostatic field about a charged Hertzian oscillator
just before the air-gap breaks down and the oscillations commence. The
“ lines of force ” indicate the direction of the displacement in the ether,
and hence the stress or “ electric force ” which accompanies it. Each line
may be considered the axis of a Faraday tube of induction.
mission, while a rich, fertile soil is much better. Again,
the same line may work better at one time than at another :
when the soil is moist the transmission is good, but when
it is dry or frozen the signais are much weaker. In short,
a good conducting surface is favorable to the transmission
of signais, while a poor conducting surface is not. Thus
we are forced to the conclusion that, although the dis-PROPAGATION OF GROUNDED WAVES.
163
turbance really has its seat in the ether, it is very closely
connected with the surface over which it glides.
4. Propagation of Free Waves.— Let us now look
again at the principles which underlie etheric disturbances,
and see if there is any way in which they may be caused
to follow a conducting surface.
First we must review the action of an ordinary Hertzian
oscillator.
Take, for simplicity, a straight wire, interrupted in the
middle by a spark-gap. (Fig. 85.) Suppose the two
knobs which constitute the spark-gap to be suddenly con-
nected to a source of constant high potential, not quite
sufficient to break down the air-gap. A current flows from
the knobs toward the ends of the apparatus, and con-
tinues until both conductors are completely charged. But
the current does not stop at the surface of the conductors ;
it takes the form of a System of displacement currents,
flowing ont through the ether, and forming loops from end
to end of the oscillator, thus completing the circuit.
While these currents are flowing the ether is being
strained, and when finally the condition of equilibrium is
reached and the currents cease to flow, the whole medium
is in a state of electrostatic stress. If we explore the field
with a small electrified body and plot, from point to point,
the direction in which it tends to move, we get a sériés of
curves such as are shown in the figure. These “ lines of
electric force/’ we should remember, represent the dis-
placement in the ether, whose behavior is analogous to that
of an elastic solid ; it tends to move back to the position of
equilibrium with a force proportional to the displace-
ment
To give the idea a little more concrète form, let us1G4
WIRELESS TELEGRAPHY.
imagine, as Faraday did, that each of these lines is the
axis of a “ tube of force/’ or, more accurately, a “ tube of
induction,” wliose cross-section
varies from place to place, so
that the whole electrostatic field
is filled with these tubes,
starting at one side of the os-
cillator and ending at the other.
Each tube may thus be consid-
ered, apart from the others, as
a closed volume in which the
displacement has occurred, form-
ing part of the circuit through
the conductor. The total in-
duction or “ electric flux ” (as
we may call it on account of
its analogy to a magnetic flux),
across any section of this
tube, is constant, and equal
to the total intégral current
which has flowed in that part
of the conductor which com-
plétés its circuit — that is, to
the charge on the surface of
the conductor in which the
tube terminâtes.* (See Fig. 86.)
Here we must guard against a too literal interprétation
of the figure, and remember that the assumption of the
* Owing to the unfortunate choice of units in our C. G. S. System,
it is necessary to introduce here the constant factor 4^, but this
does not affect the principle involved. With a suitable choice of
units the factor would disappear.
Fig. 86.— A portion of
the oscillator, Fig. 85, en-
larged, showing a single
Faraday tube. The tube
terminâtes in opposite
charges, +Q and —Q. on the
surface, equal to the inté-
gral value of the current
which supplied these charges,
and also to the induction,
cr “ electric flux ” across
any section of the tube.*PROPAGATION OF GROUNDËD WAVES.
1G5
Faraday tubes is simply an arbitrary convention, which
puts into concrète form the broad principles which we are
studying. The mathematical analysis from which these
principles are derived involves no such arbitrary assump-
tions — they are introduced simply to give tangible form
to the unknown something represented by Maxwell’s équa-
tions, so that we may trace its actions and détermine its
properties.
With this understanding we may go a step farther, and
assume that the tubes hâve an individual existence, apart
from the space which they embrace ; that they may travel
from place to place ; and that any change in the intensity
of an electrostatic field is due to the motion or distortion
of the tubes, which are themselves indestructible; and
each unit tube is of constant strength, measured by a unit
positive electrical charge at one end and a unit négative
charge at the other. Thus an open-ended tube always be-
gins and ends on matter; though, under certain circum-
stances, as we shall see presently, a portion of a tube may
form a loop, which becomes detached and goes ofï into
space, closed on itself.
This concrète conception of the tubes as individuals is
similar to the considération of vortex motion in fluids. A
vortex ring, for instance, is simply a State of disturbance
in the air; but if we look upon it as a concrète entity, we
find that it possesses inertia, elasticity, etc., and we see
that two rings may collide and rebound, aud may exhibit
many other properties of material bodies. Furthermore,
a vortex filament must either be closed on itself or must
end at the bounding surface of the fluid, just as our Fara-
day tubes do.IGG
WIRELESS TELEGRAPHY.
So also our Faraday tubes possess certain peculiar
properties, and a complété mathematical analysis of the
subject shows that nearly ail, if not ail, electrical phe-
nomena may be explained
by a considération of their
actions.
In the first place, thev
possess inertia. Not oniy
does a tube represent a sup-
ply of potential energy, due
to the strained condition of
the ether which it em-
braces, but it may also pos-
sess kinetic energy due to
its velocity. This kinetic
energy is closely allied to
the energy of the magnetic
fïeld, for a Faraday tube, when in motion, is always ac-
companied by a magnetic force perpendicular to the tube
and to its direction of motion, and this magnetic force, as
we already know, represents a stock of kinetic energy.
Furthermore, the tubes act and react upon each other;
similar tubes, i. e., tubes of the same sign, repel each other,
while opposite tubes attract each other ; and each tube has
a tendency to contract in the direction of its length. Tlius,
two similarly charged spheres are mutually repellant, and
the similar tubes emitted by each radiate into space (Fig.
87), while oppositely charged conductors are drawn to-
gether, by virtue of the tension of the tubes which spring
across from one to the other.* (Fig. 88.)
Fig. 87.— Illustrating the mutual
repulsion of the similar. Faraday
tubes emanating from similarly
charged bodies. i. e.. the tendency
of the tubes to expand laterally.
*For a more complété discussion of the properties of Faraday
tubes, see J. J. Thomson, Recent Researches in Electricity and Mag-
ne tiam, Chap. I.PROPAGATION OF GROÜXDED WAVES.
167
Now let us apply these principles to our oscillator. We
hâve the conductors charged, and the Faraday tubes form-
ing bridges through space from one to the other. Now let
the air-gap break down and
a sudden current rush across	f	^^	\
from conductor to conductor.	\	\	/	\	/	j
This means that the opposite	y
charges on the two conduct-
ors move toward each other, / /\
carrving with them the oppo-	'	/	Vx	^J	\	'
site ends of their Faraday	l	----^	I
tubes. This gives the tubes	'
draw up to the conductors,
leaving a vacant space, as it were, into which the longer
tubes are forced vertically by the pressure of others out-
side; or, what amounts to the same thing, the opposite
sides of the loops are drawn together by their mutual at-
traction, so that the tubes take a pear-shape, and the sides
finally meet and coalesoe. The outer loops of the tubes then
separate and go off by themselves, repelled by the inner
portions, which now draw up to the oscillator until their
ends meet. But the tubes do not disappear and go out of
existence when their ends corne together — their inertia
carries them on, their terminal charges separate, and tliey
again expand until the conductors are charged once more
with reversed polarities. Then the same thing is repeated,
the tubes again contract, and again a sériés of closed loops
are sent off, carrying with them a part of the energy of the
oscillation.
nearest to the oscillator the
opportunity to contract and
Fig. 88.— Oppositely charged
bodies attract each other : i. e.,
the Faraday tubes tend to con-
tract in the axial direction.168
WIRELESS TELEGRAPHY.
In Figs. 89 to 92, we may trace the progress of the
oscillation through a complété cycle, at intervals of an
Fig. 89.— The field of electric force about a rectilinear oscillation
(after Hertz) at the moment t = 0 (conductors discharged). The oscii-
lator is shown in the center, drawn to scale, and each line of the field
represents the axis of a unit Faraday tube. The closed loops indicate a
detached wave just starting outward.
Fig. 90.— The field about the oscillator after an eighth period (t =
VsT). The conductors are becoming charged and Faradav tubes are spread-
ing out from them, filling the région within the circle Kj.
eighth period, as worked out mathematically by Hertz;*
See Hertz, Electric Waves, Eng. trans. p. 141 et seq.PROPAGATION OF GROUNDED WAVES.	169
the lines of force in the diagrams representing the axes of
our unit Faraday tubes. Fig. 89 represents the state of
Fig. 91.— For time t^r^T. The conductors are now fully charged,
the circle has enlarged to R2, and the tubes hâve expanded to fill it.
Fig. 92.— For t =: % T. The conductors are discharging, the loops are
expanding and flattening, and the wave is beginning to be detached.
When t increases to % T, the wave will take the form shown in Fig. 89.
(In Hertz’s notation, X dénotés the half wave-length.)
affairs when the conductors are completely discharged, and
ail the energy of the oscillation is in the form of current —
no tubes of force proceed from the oscillator. Fig. 90170
1VIRELESS TELEGRAPHY.
shows the conditions an eighth-period la ter, and the Unes
within the circle represent the growing electrostatic field.
In Fig. 91 the circle has expanded, and the lines hâve
spread out to fill it : the current is now zéro, and the whole
energy of the oscillation is represented by the electrostatic
field within the circle. In Fig. 92 the current has reversed,
and the tubes begin to contract into a pear-shape — alreadv
one of thein has formed a detached loop, which is commenc-
ing its outward joumey, while the inner portion is shrink-
ing back to the oscillator.
We now reach the end of a half-period, and the condi-
tions are the same as they were at the beginning, though
reversed, and if we reverse the arrows which show the di-
rection of the displacement in the ether, Fig. 89 will cor-
rectly represent the new State of affairs : the Faraday tubes
hâve ail withdrawn into the oscillator, leaving behind a
nuinber of detached loops, which are expanding through
their mutual repulsion and beginning to travel outward.
In Fig. 90 the reversed set of tubes are beginning to ex-
pand, forcing the detached wave outward, and flattening it
on its inner side. So it travels on with the velocity of
light, followed by another and another, each expanding
laterally into a crescent-shape as it goes, until finally, at a
great distance from the oscillator, the tubes of induction
form almost perfect semicircles with their common center
at the oscillator ; in other words, a spherical wave-front.
Thus far we hâve considered only the electrostatic field
which goes with the waves. How about the magnetic field ?
This follows simply from the law that a moving Faraday
tube always produces a magnetic force perpendicular to it-
self and to its direction of motion. Thus the lines of mag-
netic force will form closed circles with their centers inPROPAGATION OF GROUNDEU WAVES.
171
the axis of the oscillator, expanding as the waves travel
outward, always remaining perpendicular to the Faraday
tubes and forming parallels of a sériés of conoentric spheres
of which the Faraday tubes, or electrostatic lines, are the
meridians. (See also p. 76.) The energy of.the wave is
thus half electrostatic and half magnetic, just as in a water
wave the energy is part potential, due to the élévation of
the crest and the dépréssion of the trough, and part kinetic,
due to the outward motion of these crests and troughs.
(See p. 122.)
This distribution of magnetic force does not apply to
the région immediately surrounding the oscillator, for here
the magnetic field due to the currents in the oscillator itself
is superimposed on that due to the expanding wave: for
example, when the conductors are discharged and the tubes
hâve ail withdrawn into the oscillator (Fig. 89), the mag-
netic force is not zéro, but is very large -— the current is a
maximum, and the whole energy of the oscillation is in the
magnetic field close about the oscillator. But these limit-
ing conditions do not concern us here, as we onlv hâve to
deal with the waves at a considérable distance from tlieir
source, where the effects of direct induction hâve given
place to pure radiation. Here there is a real transfer of
energy in the direction of propogation, as in the case of
progressive water waves, where élévation and velocity oc-
cur together; but near the oscillator the conditions ap-
proaeh more nearly those of stationary waves in a closed
vessel, where élévation alternâtes with velocity, and the
energy simply oscillâtes between the potential and kinetic
forms without being transmitted through space.*
* This transition from stationary to progressive waves may be illus-
trated by the hvdraulic model, Fig. 65, p. 122. Directly over the dis-172
WIRELESS TELE G RAP H Y.
These are free waves in space — the peculiar discovery
of Hertz — traveling outward with the velocity of light in
ever-expanding spheres. The direction of propagation
must always be perpendicular to the wave-front, i. e., to
the plane of the Faraday tubes and the lines of magnetic
force, hence the disturbance travels radially in straight
lines, like light, Indeed we know that it is practicallv
polarized light of immensely magnified wave-length.
5. Propagation of Guided Waves—Now suppose the
whole System to be divided into two symmetrical halves by
an infinité sheet of conducting material, passing through
the center of the oscillator. (See Fig. 85.) This plane,
being cverywhere équidistant from the oppositelv charged
ends of the oscillator, is a plane of zéro potential ; as ail
the Faraday tubes eut it at right angles, no electromotive
forces are induced in it ; the opposite electrical charges on
its opposite faces due to the impinging of the Faraday
tubes, being infinitely close together, neutralize each other,
and the résultant is zéro — indeed, everything goes on as
if the plane wrere not there.
Again, suppose the lower half of the System to be re-
moved. The oscillator will vibrate as before (see p. 160) ;
currents will surge into and out of the plane, i. ethe Fara-
charge pipe the crest simply rises and falls, velocity alternating with
élévation. To use the familiar alternating current terminology, the
velocity and pressure are in quadrature, and the vibration is “ wattless,”
so to speak. The wave is stationary. At a distance from the source
the crests move outward and the troughs move inward, the velocity and
pressure are in phase, and power is transmitted hy the progressive wave.
So, near the oscillator the electric force and the magnetic force are in
quadrature, and represent energy at rest — the inhérent energy of the
oscillation —; at a distance they are in phase, and represent a transmis-
sion of power.PROPAGATION OF GROUNDED WAVES.
173
day tubes which now terminate in the plane carry their
charges to and froni the oscillator as they move in and out ;
the tubes which are detached travel outward as before,
carrying their charges with them. These moving charges
constitute a sériés of alternating conduction currents in
the plane,* completing the circuits of the displacement cur-
rents in the dielectric which, we know, accompany the mov-
ing Faraday tubes. Thus the half-wraves travel on, pre-
cisely as if the other halves were présent.
Xow suppose the conducting sheet to be curved. A per-
fect conductor is, by définition, one in which only vanish-
ingly small electric forces can exist, Ilence there can be
no tangential component of the electric force, and the tubes
of induction must ahvays be perpendicular to the surface.
If, then, the surface be curved, the Faraday tubes must
accommodate themselves to the curvature, and the wave,
whose direction of propagation is always perpendicular to
the tubes, will follow the surface, as illustrated in Fig. 93.
If this is not évident, imagine for the moment that the
tubes do impinge obliquely on the curved sheet. This in-
volves the production of an electromotive force in the sur-
face, hence a current will flow. This means that the
charges at the ends of the tubes are moving with a velocity
different from that due to the normal progress of the wave,
and thus the grounded ends of the tubes move more or
* The experimental proof of the équivalence of a moving charge to
an electric current was one of the masterpieces of the late Prof. Henry
Rowland. He showed that a rapidly rotating dise carrying cliarged con-
ductors on its periphery caused a deflectionof a sensitive magnetic needle
suspended near by, and that the magnitude of the effect agreed substan-
tially with that demanded by the theory. The effect is very minute,
and has escaped other less skillful experimenters.174
W IRE LE 8 S TELEGRAPHY.
less rapidlv than their upper parts, according to the direc-
tion of the curvature, nntil they again become normal to
the surface and the currents cease. This explains the ob-
served fact that the currents in the earth are greater when
the wave travels up or down hill than when it follows a
level surface.
If the guiding surface is not a perfect conductor, which
is actually the case in practice, an electromotive force is
Fig. 93.— Propagation of grounded waves from an antenna over a
curved surface.
necessary to overcome the ohmic résistance and produce the
altemating currents in the surface. Hence the Faraday
tubes are distorted and strike the surface obliquely, giv-
ing up part of their energy to supply the ohmic losses in
the surface. Thus we see why the conductivity of the sur-
face has such an important influence on the atténuation of
the waves.
Where the conductivity is perfect the atténuation de-PROPAGATION OF GROUNDED WAVES.
175
pends simply upon the distance from the source. The
number of tubes in each wave remains constant as the wave
expands, but the cross-section of each tube continually in-
creases to fill the space at its disposai — not in a radial
direction, for the waves follow each other at constant
velocity and hence the distance between them is always the
same — but in a direction parallel to the wave-front. The
cross-section of a tube thus increases directly as the dis-
tance from the source. Now the “ displacement,” or induc-
tion per unit area, in the ether contained within a tube is
inversely proportional to the cross-section of the tube, for
the total induction, or electric flux, across any cross-section
is constant ; hence the electric force, which is proportional
to the displacement, varies inversely as the distance from
the source, and the energy of the field, inversely as the
square of the distance. This is the resuit given on page
129.
In a similar wav we may explain Hertz’s discovery that
most of the energy is concentrated near the équatorial
plane, instead of being distributed equally in ail direc-
tions as in the case of ordinary light.* Only a few of the
Faraday tubes stretch out to any considérable distance
from this plane — most of them bend around and reverse
their direction within a comparatively short distance, as
shown in Fig. 93. Hence those that do extend their cres-
cent-shaped loops toward the pôles hâve proportionately
more space to fill, and their energy is distributed and at-
tenuated. To reproduce the uniform distribution of ordi-
nary light, we should need an infinité number of oscillators,
oriented in ail directions.
Electric Waves, Eng. trans., p. 143.176
WIRELESS TELEGRAPHY.
Finally, we see that each of the three hypothèses we
hâve considered has some foundation in fact, though the
last is the only one that is adéquate to explain ail the phe-
nomena: in short, the grounded waves are really of the
saine essential character as free Hertzian waves, and they
are accompanied by alternating currents in the guiding
plane, but they are distinguished by their inséparable con-
nection with the surface over which they glide. If we
imagine the extreme case where the guiding surface is con-
tracted and bent into a long, narrow cylinder, we shall
hâve the propagation along a wire discussed in Chapter
YI of Part One. In this case the wrave becomes a plane
tr ans verse one, whose amplitude does not diminish with
increasing distance, except as the energy is wasted in
ohmic and other losses.
6. Effect of Daylight on Wave Transmission.—An in-
teresting phenomenon has been observed by Marconi in
his experiments with long-distance transmission.* It is
found that, under similar conditions, the signais trans-
mitted between two stations are much stronger at night
than in the daytime. The high-power station at Poldhu,
for example, may be sending out signais which are easily
received by a vessel in midocean, while it is night; but
as soon as the sun rises over England the intensitv of
the signais begins to wane, and in full daylight the différ-
ence is very marked.
This striking effect has not yet been thoroughly ex-
plained, but it calls up many interesting suggestions.
Hertz observed (p. 36) that a spark-gap breaks down
much more readily when exposed to ultra-violet light from
See paper before Royal Society, June 12, 1902.PROPAGATION OF GROUXDED WAVES.
177
another spark tlian it does when not so exposed. This
may be explained on the hypothesis, which is daily gain-
ing weight through the experiments of many noted in-
vestigators, that nltra-violet light has the power of
“ionizing” a gas, or splitting np some of its molécules
into smaller bodies, or “ ions.” The ions are not elec-
trically neutral, as normal molécules or atoms are, but
carry equal and opposite electrical chargée. These charges,
before their séparation, neutralized each other and so were
unobserved, but now they are free to move under the in-
fluence of any electrostatic stress.
An analogous effect occurs in an electrolytic cell. Pure
water does not conduct electricity; but if it contain in
solution a trace of hydrochloric acid, for example, it
conducts readily. In the act of dissolving, some of the
molécules of acid are supposed to split up into oppositely
charged ions, or électrons, and these, by their motion, con-
stitue a current through the solution.
If now we postulate a similar splitting up of the atoms
of air into oppositely charged ions,* under the influence
of sunlight, we may explain the anomalous atténuation
of electromagnetic waves. Under the influence of the
electrostatic stresses which accompany the waves, the op-
positely charged ions will move in opposite directions,
and by their motion will produce what is practically a
conduction current, or more strictly a convection cur-
rent, in the air, just as the ions of electrolysis are the
* J. J. Thomson has shown that the cathode discharge of a vacuum
tube consists in negatively-charged “ corpuscles,” whose mass is a
small fraction of that of a hydrogen atom, and similar bodies occur
in free gases. See J. J. Thomson, Proc. Roy. Inst., 1901, p. 574; also
PMI. Mag., March, 1903.
12178
WIRELESS TELEGRAPHY.
seat of the current in an electrolyte. Such currents will
fritter away the energy of the wave, in the same way as the
convection of heat from the warm compressed portions of a
Sound wave to the cooler rarified portions results in a dis-
sipation of energy, and an atténuation of the sound.
This explanation should not be taken as final, but it
is at least suggestive, and in harmony with the latest
views regarding the ionization of gases.CHAPTER Y.
THE RECEIVING APPARAJTTJS.
1.	Detectors Classified.— Since Chapter Y of Part One
was written by M. Poincaré, detectors of electric waves hâve
appeared in such a variety of forms that it is difficult to
select from among them those that may be described in one
short chapter. We shall hâve to be content with a classifi-
cation according to their principles of operation, and the
description of a few typical forms from each class.
The detectors now in use may be séparated roughly into
five groups :
lst. The Microphonie.
2d. The Mechanical.
3d. The Thermal.
4th. The Electrolytic.
5th. The Magnetic.
These groups are not always sharply defined, nor do they
include ail possiK'e forms of detectors, but the arrange-
ment is a convenient one, and covers ail the principal types
that are of nractical value.
2.	Microphonie Detectors.*— This Mass includes ail de-
tectors of the coherer type, which Marconi has broadly and
terselv defined as “ a sensitive imperfect contact.”!
It has long been known that two conduoting surfaces in
* The? word “Microphonie” is rather a niisnomer as applied to a
detector of electric waves; but as it suggests the principle involved,
it is a convenient term, in the absence of a better one.
t Marconi, Eng. pat. No. 12,039, June 2, 1896.
[179]180
WIRELE8S TELEGRAPHY.
light contact, such as a carbon point resting on a dise of the
same material, and forming part of an eleotrical circuit,
are remarkably sensitive to mechanical shocks, such as the
impact of sound waves. The slightest vibration of the
apparatus produces marked variations in the résistance of
the contact. Sir Oliver Lodge discovered that such a con-
Fio. 94.—Lodge’s knob coherer, applied to Syntonie jar experiment
(see p. 209). The knobs are shown at A, mounted on the same base as
the bell that gives the signal. The vibration of the bell is sufficient to de-
cohere the knobs.
tact, was also affected bj electric waves, and devised a de-
tector consisting in two knobs of métal, barely in contact
(Fig. 94), and forming part of the circuit through a bat-
tery and an electric bell.* Whenever this device was ex-
* Lodge, Jour. Instn. of Elec. Engrs., vol. 19, p. 352, 1890. Also
Signalïing through Space icithout Wires, 3d ed., p. 20.THE RECEIVING APPARATUS.
181
posed to a sudden electrical impulse, such as that produced
by the discharge of a Leyden jar, the contact résistance
suddenly decreased, and the bell began to ring.
Lodge considers that the thin film of oxide which sépa-
râtes the surfaces is broken down by the slight différence of
potential between them, an infinitésimal spark occurs, and
the partially fused surfaces are caused to cohere or weld
together. He shows also that the cohésion is aided, if in-
deed it may not be explained altogether, by the electrostatic
attraction between the surfaces : for example, he calculâtes
that the attraction between two surfaces separated by a film
of the thickness of 10'7 cm., and differing in potential by
one volt, would be equal to 4 x 107 dynes per sq. cm., or
about one-third of a ton per square inch — a very consid-
érable force, even where the area of the contact surfaces is
small.*
This explanation is quite generally accepted as applying,
in substance, to the varions forms of filings coherers, of
which Marconi’s coherer, described on p. 131, mav be taken
as typical. Here there are a large number of imperfect
contacts between the individual particles of métal, and so
the chance of the apparatus failing to work is greatly re-
duced, while its sensitiveness is correspondingly increased.
A great drawback to the use of such detectors is the fact
that their conductivity continues after the stimulus which
caused it is removed, and hence some sort of tapper or
mechanical decoherer is required to restore them to their
sensitive condition. This is disadvantageous, not only for
mechanical reasons, but also because the speed of signaling
is limited on account of the time required for the tapper to
* For a fuller exposition of Lodge’s ideas, see his Signalling
througli Space without Wires, pp. 86-87.182
WIRELESS TELEGRAPHY.
do its work. A speed of ten to fifteen words per minute is
considered quite good for such devices.
To obviate this difficulté, a number of self-restoring
“ auto-coherers ” hâve been devised, of which the “ Italian
Navy coherer,” invented by Sig. Castelli of the Royal
Italian Navy,* and used successfully by the Marconi Com-
Fig. 95.— Italian navy coherer. E, E' are iron or carbon électrodes,
and M a globule of mercury.
pany, is a specimen. (Fig. 95.) This consists in a glass
tube, similax to that of a filings coherer, plugged with iron
or carbon électrodes, between which is a globule of mer-
cury. In a more improved form the électrodes are both of
carbon, and embrace two drops of mercury separated by a
short iron cylinder. (Fig. 96.)
Fig. 96.— Italian navy coherer — second form. The électrodes, C C't
are both of carbon, and I is an iron cylinder separating two globules of
mercury.
The action here is entirely automatic. The cohésion
between the mercury globules and the électrodes exists only
while the stimulus is acting, and the apparat-us regains its
original sensitiveness as soon as the actuating cause is re-
moved.
See Angelo Banti, in Eîectrician, Lond., Julv 11. 1901, p. 477.THE RECEIVING APPARATUS.
183
This coherer is often used with a téléphoné in the local
battery circuit (Fig. 97), instead of the relay and recorder
shown in Fig. 69. When so operated it responds to very
rapid impulses, and each spark of the transmitting ap-
paratus produces its individual effect, instead of being
merged together with others into continuons dots and
dashes, as when a filings coherer is used. The resulting
Fig. 97.— Connections of auto-coherer with battery B and téléphoné T.
buzz which is heard in the téléphoné is easily read, and the
apparatus thus becomes quite effective, especially as the
téléphoné is much more sensitive than any relay and re-
cording device. This arrangement is one of those used by
Marconi in his mémorable transatlantic experiments,*
* Marconi before Roy. Instn. See Electrician, Lond., June 27, 1903,
p. 390.184
WIRELESS TELEGRAPHY.
when the first “ S ” was heard across two thousand miles of
océan.
It has one serions drawback, however; it insists some-
times on cohering permanently, and again, it will cease to
act in the middle of a message. Lodge has avoided this
irregularity by keeping the contact surfaces in constant
motion.* He replaces one of the fixed iron électrodes with
a dise of steel, which is revolved continuously by clockwork,
in light contact with a drop of mercury covered with a
Fig. 98.— Lodge’s auto-coherer. A Steel dise, a, rotâtes in light contact
with a globule of mercury, b, from which it is normally separated by a
thin film of oil.
thin film of oil. (Fig. 98.) The contact surfaces are thus
kept clean and fresh, and always in réceptive condition.
This device is used directly in connection with a siphon
recorder in the Lodge-Muirhead System, which will be dis-
cussed later. The recorder, though less sensitive than a
téléphoné, has the advantage of giving a permanent record
* H. C. Marillier on Lodge-Muirhead System, Electrician, Lond.,
March 27, 1903, pp. 930-934.
PlanTHE RECEIVIXG APPARATUS.
185
b'
where the impulses are too feeble to affect positively the
ordinary relay and recording apparatus.
Coherers may be made of
a great variety of materials.
In certain combinations, es-
pecially those involving sub-
stances that are easily oxi-
dized, the primary phenome-
non of cohérence, or decrease
in résistance, is followed by
the opposite effect, and the
device regains a condition of
high résistance while the im-
pulse is still acting. In an ex-
trême case, as when carbon
grains are used in place of me-
tallic filings, the former effect
disappears, and we find only
an increase in résistance when-
ever the stimulus occurs.
Such de vices are called anti-
coherers. The explanation of
the phenomenon is somewhat
obscure, though it seems prob-
able that the minute sparks
which occur between the
grains oxidize the surfaces,
and so destroy their con-
ductivity.*
3. Mechanical Betectors.— Hertz, in his experiments on
the propagation of waves in wires, found that a hoop of
Fig. 99. — Hertz’s inductive de-
tector. The hoop of aluminum
wire is shown with mirrow attached,
suspended within the box. The
wire abb'a' carries the oscillations,
which repel the currents that they
induce in the hoop.
* For a discussion of this subject, sec Hurmuzèscu, Annales Scient,
de VUniv. de Jassy, Repr. in L’Êcl. Elec., June 27, 1903.186
WIRELE88 TELEGRAPHY.
wire, suspended by a fibre in the neighborhood of a pair
of conductors carrying the oscillations, tended to take up a
position perpendicular to the plane of these wires. The
rapidly varying currents induced secondary oscillations in
the suspended hoop, and the mutual repulsion between
these two sets of currents caused the hoop to swing toward
the position where the action was a minimum. Hertz’s
apparatus is illustrated in Fig. 99.*
Similar arrangements hâve been employed by other
workers for the study of the currents set up in an antenna
exposed to radiations from a distance. Such devices are
useful.for quantitative measurements, and give quite ac-
curate results, but they are not applicable to ordinarv télé-
graphie work on account of their sluggish action and the
difficulty of making them sensitive.
4. Thermal Detectors.— The thermal detectors referred
to on p. 43 are subject to the same objections as the me-
chanical ones above mentioned: the wire whose heating is
measured, however fine it may be drawn by ordinary
methods, is still too coarse to be much affected by the feeble
currents induced in a receiving antenna; and, even were
the heating measurable, the rate of cooling would be too
slow to follow the rapid succession of signais required for
practical telegraphy.
Recently, however, by an ingenious method, wires hâve
been produced of such excessive fineness as to be free from
both these troubles. A rod of silver is cast with a piece of
platinum wire as a core, and the whole is drawn down to a
diameter of a few thousandths of an inch. In this process
the platinum is reduced to a diameter of .0001 inch or less,
which is amplv fine for ail practical purposes.
* See Hertz, Electric Waves, Eng. trans., p. 191.THE RECEIVING APPARATUS.
187
Of such material is the “ barretter ” of Prof. R. A. Fes-
senden.* A short loop of the platinum wire is exposed by
dissolving the silver casing in nitric acid, and then mounted
in a metallic box to protect it from air currents and from
stray electrical impulses. Connected in the circuit from
the antenna. to ground, the oscillations cause it to become
heated, the resulting increase in résistance causes a diminu-
tion of the current in a local circuit including the loop, and
a sound is produced in a téléphoné.
Such a fine wire loop is sensitive to very feeble currents,
and its heat is radiated so quickly that its température can
follow the most rapid succession of impulses required in
practice. Unfortunately, however, a loop that is fine
enough to be sensitive is extremely délicate, and is easily
burned out by atmospheric disturbances or by stray radia-*
tion from the transmitting apparatus.
5. Electrolytic Detectors.—■ Dr. Lee de Forest. and E. H.
S mythe hâve produced a detector which in volves an inter-
esting principle.f It dépends for its operation on the dis-
ruption of the minute metallic bridges or “ trees ” which
form, under suitable conditions, between the électrodes of
an electrolytic cell. The apparatus, which they call a “ re-
sponder,” consists in a glass tube similar to that of a co-
herer, in which are fitted two électrodes, preferably of tin,
though other metals will do. The space between the élec-
trodes — about l/64th inch — is filled wTith a viscous semi-
eonducting liquid, such as glycérine with a small admixture
of water, together with some peroxide of lead as a
depolarizer, to prevent the excessive évolution of gas.
When this cell is connected across a batterv of suitable
* See U. S. pat. 706,744, Au g. 12, 1902.
t See U. S. pat. 716,000, Dec. 16, 1902.188
W IRE LE 8 8 TELEGRAPHY.
voltage, a peculiar phenomenon occurs. The ordinary elec-
trolytic action of dissolving métal from the anode and de-
positing it on the cathode is accomplished through the
agency of ions, of atomic dimensions: here, however, the
particles torn from the anode are of pondérable size — in-
deed, they may easily be seen with the aid of a microscope,,
as they lie suspended in the liquid. Under the influence
(probably) of electrostatic attraction, they arrange them-
selves in bridges, or “ trees,” reaching across from cathode
to anode, in much the same way as iron filings distribu te
themselves between the pôles of a magnet. These metallic
bridges, in their normal condition, form a path of com-
paratively low résistance for the current; but when the
oscillations from an antenna are allowed to pass through
the cell, the bridges are broken down, the conductivity is.
destroyed, and a sound is produced in a téléphoné con-
neeted in the local battery circuit. The destruction of the
bridges is caused by the sudden évolution of gas in the
thin film of liquid which séparâtes the individual particles ;
this gas is promptly absorbed by the depolarizing agent in
the solution, and so the particles are allowed to corne to~
gether and complété the bridges as soon as the oscillations
cease. The apparatus is thus self-restoring and constantly
réceptive, and its action is said to be uniform and reliable.
This automatic responder and téléphoné, when used in
connection with the alternating-current transmitter of high-
frequency spark described on p. 151, constitutes a System
over which a high speed of signaling is possible.
Another electrolytic detector, embodying quite a dif-
ferent principle, was developed by the writer in the course
of a sériés of attempts to magnify the effect of the heating
of Fessenden’s “barretter” (see p. 187), by nnmersingTHE RECEIVIXG APPARATUS.
130
the wire in a liquid of high température coefficient and
low spécifie heat, which was made part of a local circuit.
The attempt was unsuccessful, but it led to the discovery
that a simple electrolytic cell, when polarized to the proper
critical point by the current from a local battery, is re-
markably sensitive to oscillatory impulses.* A simple
nitric acid solution, for example, with a minute anode of
platinum wire .0001 inch in diameter and a larger plati-
num cathode, gave a clearly readable signal when a coherer
was absolutely inoperative.
The effect is quite distinct from that involved in the de
Torest responder, for the platinum anode, minute thougli
it is, is not attacked by the electrolyte, and the electrolyte
itself is a good conductor. Mnreover, the apparent ré-
sistance of the cell falls, on the passage of the oscillations,
instead of rising.
The phenomenon is a rather complex one which we mav
not discuss here, except to note that an interesting reversai
of the jirocess occurs when the voltage of the local battery
is raised to a point where a considérable ébullition occurs
at the anode. In this case, the rapid évolution of gas which
occurs when the oscillations pass is sufficient to destrov the
contact between the liquid and the anode, and interrupt
the current. This effect is quite similar to that which
occurs in the Wehnelt interrupter, where the sudden évolu-
tion of gas from a small platinum anode is caused to break
the primary circuit of an induction coil at rapidly re-
* Since the above went into type an announcement bas been publisbed
of tbe independent discovery of tbis principle by Scbloemilcb, in Ger-
many. Tbe detector tbat be describes is almost identical witb tbe above.
See Elektrotechnische Zeitschrift, Nov. 19, 1903.190
WIRELESS TELEGRAPHY.
curring intervals, thus doing away with a mechanical
break.
This reverse action, though very strong, is rather irreg-
ular and uncertain, so it is neglected in favor of the direct
efïect, which is perfectly uniform
and reliable, and practically in-
stantaneous. The slightest ir-
regnlarity in the sending spark
has its efïect in altering the note
in the téléphoné,— indeed, a va-
riation in the température or
quality of the sending spark is
often observed by the receiver
when the sending operator him-
self cannot detect it,— and even
when a Wehnelt interrnpter is
nsed at the transmitting end, pro-
dncing sparks at the rate of a
thousand or more per second,
each impulse is separatelv de-
tected by the receiver, and the resulting musical note is
clear and strong. The speed of signaling is thus limited
onlv by the ability of the operator to handle his transmit-
ting apparatus.
The sensitiveness of this detector is extreme, far sur- .
passing the coherer, and even the délicate magnetic detector
of Marconi. It also lends itself readily to tuning.
The arrangement of connections for a simple untuned
receiver is shown in Fig. 100, where C is the electrolytic
cell, or detector proper, R is an inductive résistance, which
serves the double purpose of adjusting the voltage of the
local circuit and preventing the escape of the oscillations
Fig.	100.— Connections
of electrolytic detector in
an untuned circuit. A is
the antenna ; C, the detector
proper ; R, an adjustable
inductive résistance, and T,
a téléphoné.THE RECEIVING APPARATUS.
191
through the battery, and T is a téléphoné. The téléphoné
may be replaced by a siphon recorder when a permanent
record is desired.
6. Magnetic Detectors.— Over half acentury ago, Joseph
Henry, in the course of his monumental experiments on
the relation between electricity and magnetism, tried the
effect of discharging a Leyden jar through a coil of wire
surrounding a needle. Knowing that a discharge of static
electricity is équivalent to an electric current, he
naturally expected it to hâve a similar magnetic effect ; but
the results were quite the reverse. Not only was the needle
Fig. 101.— Rutherford’s magnetic detector. A magnetized Steel needle
in a coil of wire, with a magnetometer to observe the changes in mag-
netization.
not uniformly magnetized by the discharge, but after be-
ing magnetized in one direction it was often demagnetized
or even reversed. With a wonderful insight, he attributed
this anomaly to the fact, afterward demonstrated by Fed-
dersen (p. 22), that such a discharge is oscillatory. He
says :	“ The phenomenon requires us to admit the exis-
tence of a principal discharge in one direction, and then
several reflex actions backward and 'forward, each more
feeble than the preceding, until the equilibrium is ob-
tained.”*
Scientific Writings *>f Joseph Henry, Vol. I, p. 201, 1886.192
WIRELESS TELEGRAPHY.
This principle was applied by Rutherford to a detector
of electric waves.* He used Steel needles magnetized to
saturation and placed in a coil of wire through which the
oscillations passed (Fig. 101), and the changes in maghetic
State were detected by a magnet-
ometer. Unfortunately, however,
with this form of apparatus, a
newly magnetized needle is re-
quired for each experiment.
Other workers hâve improved
upon the idea, and Marconi in par-
ticular h as developed it into a very
serviceabledectector.f In one form
of his apparatus, he uses a bundle
of iron or Steel wires wound with
two coils of copper wire. (Fig.
102.) One of these coils is con-
nected between the antenna and
ground, the other, to a téléphoné
receiver. A permanent magnet
is rotated by clockwork close to
the core, in such a way that
its pôles, alternately approaching
and receding, keep up a continuons variation in the mag-
netic State of the iron. Ordinarily, these changes of mag-
netization are too slow to induce an appréciable electro-
motive force in the coil, and the téléphoné is silent; but
when oscillations pass through the other winding, the
smoothly-varying flux is jarred, as it were, into a sudden
* Philosophical Transactions, 1897.
f See paper before Roy. Instn. of Great Britain, June 13, 1903, in
Eîcclrician, Juse 27, 1903, p. 388.
A
A
J?
le.
Fig. 102.— Marconi's
magnetic detector. A is
the antenna coil, B the
téléphoné coil, C the core
of iron wires, and N S a
permanent magnet rotated
by clockwork.THE RECEIVIXG APPARATUS.
193
change, a momentary current flows, and a Sound is heard
iiï the téléphoné.
The efïect may be compared to the well-known fact that
a magnetized bar, when struck or jarred, loses part of its
magnetism ; and it would seem that the rapidly varying im-
pulses caused by the waves hâve a similar disturbing efïect
on the molecular State of the iron. It is evidently a hys-
térésis efïect. We know that, when a piece of iron is alter-
nately magnetized and demagnetized, the magnetization
Fio. 103.— Mareoni’s magnetic detector— improved form. W is a con-
tinuons band of iron wires, passing over the pulleys P P', and threading
the coils A and B. M and M' ara fixed permanent magnets.
always lags behind the magnetizing force, as if the molé-
cules of the métal were restrained by some viscous résist-
ance from arranging themselves in the way the force impels
them. With soft iron the hystérésis is not great, but Steel,
especiallv when hardened, is very reluctant to change its
13WIRELESS TELEGRAPHY.
m
magnetic State. Under the influence of the oscillations,
however, the hystérésis is diminished, and the molécules,
freed from this restraint, jump suddenly into place and a
signal is produced.
The Sound is loudest when the magnet is approaching
the core and less when it is receding, so the sensitiveness
of the detector is constantly varying. This is a great dis-
advantage in practical signal ing, and to obviate the diffi-
culté, Mr. Marconi has devised another form of apparatus,
(Fig. 103.) Here the magnet is fixed and the core moves
— not transversely across the pôles of the magnet, but in
the direction of its own length. In short, the core is a con-
tinuons band of iron wires, passing over two pulleys, and.
threading a double coil of wire placed between the pôles of
a magnet. In this way a fresh mass of iron is continuously
exposed to the action of the oscillations, and the per-
formance of the apparatus is made uniform and regular.
This apparatus has been so perfected that its sensitive-
ness is very great, and its action thoroughly reliable.
7. Arrangement of Receiving Apparatus.— Open-circuit
detectors of the coherer type are characterized by the fact
that a différence of potential between the terminais is neces-
sary to operate them. In its normal sensitive condition no
appréciable current flows through the coherer, and it is
not until the différence of potential reaches a certain
critical point that the insulation breaks down and the
particles cohere. It matters little whether this potential
be oscillating, slowly alternating, or direct — indeed the
voltage of the local battery, if raised above a volt or so, will
cause the apparatus to block and become inoperative.
This feature makes the coherer ill adapted to the ordi-THE RECEIVIXG APPARATUS.
195
nary connection in sériés, between the antenna and ground.
The base of the antenna is, as we know, a loop of the oscil-
lation, where the current is a
maximum, and the variation
of potential is normally zéro ;
hence a coherer located at this
point works to very poor ad-
vantage.
Various arrangements hâve
been devised to avoid the dif-
ficulty, for it is obviously im-
possible to operate the coherer
at the nodal point at the top
of the wire. One method, due
V777777777777777777777&777777Z
to Prof. Slaby* is
Fig. 104.— Slaby’s method
of operating a coherer. A hori-
zontal wire B is attached near
the base of the antenna A and
the coherer T is connected at
its outer end. N. N are nodes
of the oscillation, L a loop.
Figs.
104 to 106.
I.
Fig. 105.— Slaby’s method
(2). An inductance coil I is in-
serted at the base of the an-
tenna, to stiffen the coupling be-
tween the wire B and the an-
tenna.
k
shown in
If a straight horizontal wire of the
proper length be attached to
the antenna near its foot (Fig.
104), it will vibrate in unison
with the latter with a node at
its outer end, and a coherer
may then be connected be-
tween this end and the ground.
In order that the horizontal
wire may be set in vibration,
there must be some variation
of potential at its inner end,
so it is connected to the
antenna at a little dis-
tance above the ground; or, better still, a coil having a
small self-induction is inserted in the ground wire. (Fig.
Slaby, Die Funkentelegraphie, p. 110.190
WIRELESS TELEGRAPHY.
105.) In practice, there is no need of stretching the aux-
iliary wire out straight, for it may as well be wound into a
coil of suitable length, as in Fig. 106. In this form the
apparatns is called by Slaby
a “ multiplier/’	(Multipli-
Tcator) and we shall consider it
further in the next chapter.
Another method of connect-
ing the coherer is through a
transformer, or induction coil.
(Fig. 107.) This has the ad-
vantage, not only of providing
an uninterrupted path for the
oscillations in the antenna and
of establishing a convenient
node for the coherer, but, by
a suitable arrangement of
winding, the voltage may be
stepped up so as to increase its effect on the coherer.
It is not sufficient to simply multiply the turns of the
secondary winding, as in the case of an ordinary induc-
tion coil ; for here we are dealing no longer with alterna-
tions of such low frequency that the current may be con-
sidered the same in ail parts of the circuit, but with oscil-
lations of such rapidity that the time required for an im-
pulse to travel from one end of the coil to the other is
considérable, compared to the period of the oscillation;
and the distributal capacity of the wire itself, acting like
a sériés of condensers strung along the route, stores up
energy where it is not wanted, and gives rise to certain
peculiar effects. We may compare the induction coil to
a pipe line of rigid métal, in which the flow is constant
Fig. 106.—If a coil M be
substituted for the wire B, Fig.
105, we hâve Slaby’s “ Multi-
plier.”THE RECEIVIXG APPARATUS.
197
fl.
throughout its length ; while the high-frequency coil is like
a tube of elastic rubber, which swells and contracts witb
variations of pressure, and it is even possible to hâve the
water flowing in opposite direc-
tions in different parts of the
tube at the same time. This
property may be turned to use-
ful account, however: for in-
stance, we may so proportion
the tube for a given frequency
of flow of the water that a con-
sidérable velocity may occur at
the middle of the tube, while
the ends, which are closed, ex-
périence only variations of pres-
sure.
This resuit, for electrical os-
cillations, lias been accom-
plished by Marconi in his
“ jigger,”* one form of which
is shown in Fig. 108. The
primary consists in a single
layer of, say 100 turns of in-
sulated copper wire, while the
secondary has, say 1,000 turns
of finer wire, wound in a
peculiar manner. The coil is divided into two equal sec-
tions, each of which is wound so as to hâve a triangular
cross-section, the inner layer having the greatest number
of turns, and the succeeding layers being tapered off gradu-
ally, so that the outer layer has only two or three turns.
*77777.
Fig. 107.— A coherer T con-
nected to an antenna A through
a transformer, P S. The con-
denser C prevents short cir-
cuiting the battery through
the coil.
See Eng. pat. No. 12,320, June 1, 1898.198
WIRELESS TELEGRAPHY.
The inner ends of the coils are connected together and the
outer ends go to the coherer. In the figure, the zigzag
lines represent diagrammatically successive layers of the
Fig. 108.—Marconi’s jigger — half section. G is a glass tube on
which are wound the primary P and secondary S. Each of the zigzag
lines represents a layer of winding. The terminais A E of the primary
are connected to the antenna and to earth, and the terminais C J of the
secondary go to the coherer and the recording apparatus.
winding, the individual wires being perpendicular to the
plane of the paper.
A more improved form of the apparatus* is shown in
Fig. 109, and its connections to the coherer and receiving
apparatus in Fig. 110. The inner terminais of the second-
Fig. 109.— Marconi’s jigger — improved arrangement. The secondary
winding j2 is divided in the middle and the ends are connected to a con-
denser, j3. For connections, see Fig. 110.
ary winding are connected to a condenser, and thence,
through choke coils which confine the oscillations to the
jigger circuit, to the relav and local battery.
Eng. pat. No. 25,186, Dec. 19, 1899.THE RECEIVING APPARATUS.
190
In this arrangement the coherer is plaeed at a decided
node of the secondary oscillation, whose voltage is much
higher than that of the primary. The effectiveness of the
device is shown by the fact that
it enables signais to be received at
ten times the distance that is pos-
sible when the coherer is simply
inserted in the antenna circuit.
Conversely, it makes it possible to
use less sensitive coherers, which
are mudh more reliable and easily
handled than the very sensitive
ones.
The “ jigger ” is to some extent
a syntonie device, as it works best
when designed with reference to
the height of the antenna and the
frequency of the oscillations. It
will be considered further in the
next chapter, together with a va-
riety of other syntonie receivers.
8. Current Multiplying Devices.— Many detectors differ
from the coherer in the fact that their operation does not
dépend upon the voltage of the oscillation, but on the cur-
rent. A thermal detector, for instance, is dépendent di-
rectly upon the heating effect of the current. Such detect-
ors should be plaeed, not at a node of the oscillation, but
at a loop. For this reason they operate effectively when
simply connected between the antenna and ground. If it
is desired to multiply the effect by a transformer, it should
be designed to step-down the voltage and thus increase the
current — a process quite the reverse of that which occurs
in Marconi’s “ jigger.” To accomplish this successfully, it
Fig. 110.— Connections
of jigger, Fig. 109. T is
the coherer, across the sec-
ondary terminais. The
battery B and relay R are
connected across the con-
denser js through choke
coils Ci, Cj. The primary
jx goes to the antenna A
and earth E.200
W IRE LE 8 S TELEGRAPHY.
is usually necessary to tune the circuits in unison, and we
shall consider, in the next chapter, the means by which this
Fig. 111.—A cur-
rent-multiplying coil
for detectors of the
closed circuit type.
B and C are two
coils of unequal
numbers of turns,
wound in closely in-
ductive relation, A
is the antenna ter-
minal, and D the
detector. The cur-
rent in the longer
coil B is opposed to
that in the antenna,
and the current in
C is equal to the
arithmetical sum of
the other two.
may be done.
Another arrangement, which is net
necessarily syntonie in its action, has
been used successfully by the writer.
It is based on the principle that rapidly
oscillating currents in a System of con-
ductors tend to distribute themselves
in such a way as to make the kinetic
energy a minimum.* It consists in a
simple annular coil, made up of two
wires wound side by side, as close to-
gether as possible, so that their mutual
induction shall be large. The two cir-
cuits are similar, except that one has
more turns than the other, say in the
ratio of three to two, and they are
connected in parallel between the an-
tenna and ground. (Fig. 111.)
jSTow, if the currents in the antenna
were continuons or slowly alternating,
they would distribute themselves be-
tween the two coils according to the
résistance; but for rapidly oscillating
currents this is not the case. Here
the self-induction is the controlling
factor. If the currents in the two
coils were approximately equal, they would induce a
magnetic field of considérable kinetic energy, and the
tendency is to reduce this energy to a minimum. This is
accomplished when the two currents flow in opposite direc-
tions, and are numerically proportional inversely to the
See J. J. Thomson, Recent Researches in Elec. and Mag., Chap. VI.TUE RECEIVING APPARATUS.
201
number of tums. Their magnetic effects then neutralizé
each other, and the kinetic energy of the System, as a whole.
is a minimum. The current in the antenna, which is eqnal
to the algebraic sum of the currents in the two coils, is
thus smaller than either of the latter: for example, in the
case cited, if the current in the antenna be unity, the cur-
rent in coil C will be + 3 units, and that in B will be
— 2 units.
Looking at the matter from another point of view, sup-
pose for the moment that the currents in the two coils are
in the same direction : the resulting magnetic field will in-
duce an electromotive force in each coil ; but the EMF of
B will be greater than that of C, and will cause a local cur-
rent to flow through the two coils in sériés until its mag-
netic effect balances that of the main current, and the re-
sulting flux is zéro. Unless the différence in the number
of turns of the coils is great, the local current will be
greater than that in the main circuit, and a detector con-
nected in either branch will expérience a correspondingly
magnified effect.
It should be remembered, however, that this is not a
perpetual-motion machine, and so cannot create energy.
Increased current in the detector means an increased ex-
penditure of energy, and this can never be greater than that
received by the antenna. Before this limit is reached, the
ohmic résistance has its effect, and cuts down the current
in the antenna ; so that the multiplving effect of the coil
goes for naught. It is possible, however, with suitably
designed coils, to amplify the signal considerably ; thus,
with a certain coil having a ratio of tums of 3 : 4, the in-
tensity of the signal was increased nearly fourfold — the
full theoretical value.
Usuallv, a similar resuit may be accomplished by tun-
ing, and we shall now tum to that phase of the subject.CHAPTER VI.
SELECTIVE SIGNALING.
1. The Problem.—* Thus far we hâve considered mainly
two phases of the subject: First, the means of producing
powerful vibrations, capable of traveling over long dis-
tances; and, second, the construction of a receiving ap-
paratus, sufficiently sensitive to detect these radiations,
even when greatly attenuated. Given a powerful trans-
mitter and a sensitive receiver, a new problem présents
itself:	How can we control and direct these radiations
so that they shall reach the receiver for which they are
intended, and not make havoc with ail others within their
sphere of influence? The radiations from an antenna
spread out uniformly in ail directions, like light from a
beacon, and the receivers which we hâve been consider-
ing, like so many human eyes, respond almost equally
well to the signais from ail transmitters within their
range. For use on shipboard, and between ships and shore,
this is a distinct advantage; for it is important that a
ship be able to communicate with any other ship, and
that ail vessels may connect with the shore stations. Be-
sides, where vessels are scattered over the high seas, the
çhances of interférence are small, and even where two or
three ships are “ talking ” at once, it is not difficult to dis-
tinguish between them. Especially where téléphonie re-
ceivers are used, the different qualities of sound due to
the peculiarities of different transmitters make it quite
easy to hear and read any one of several messages coming
.at the same time, just as several conversations may be
[202]SELECTIVE 8WXALISG.
203
carried on in the same room without interférence; but
when the requirements of commercial work multiply
stations and magnify their power, we shall hâve a vérita-
ble stock exchange, in which sélective signaling will be
indispensable. The demand is not so much for secrecy,
for this may always be secured by means of codes. There
is not the least difficulty in tapping a land telegraph line,
and eavesdropping over the téléphoné is easier still,—
vet the telegraph and téléphoné are among our most valu-
able instruments of commerce. The main requirement
is to communicate at will with any one of several stations,
without interfering with messages passing between other
stations at the same time.
Three methods hâve been proposed for accomplishing
this :
First. To direct the radiation of the transmitter, like
the beam of a searchlight, so that it shall go only where
it is wanted.
Second. To tune the oscillations of transmitter and re-
ceiver so that only those stations which hâve the same
frequency or “tune ” can communicate: this is équivalent
to providing lighthouses with lights of different colors, and
eausing the observer to look through colored spectacles
which eut off ail light but that for which he is looking.
Third. To give the radiations a distinctive character,
aside from their wave-frequency, e. g., by varving the fre-
quency of interruption of the spark: thus, lighthouses
which are otherwise alike are caused to émit flashes at
stated intervals, by which thev may be distinguished.
Ail three of these methods hâve been used, and each
has its peculiar advantages.
2. Directed Signais.— The first and most obvious method
is to send the signais only in the direction where they204
WIRELESS TELEGRAPHY.
are needed. Marconi, in sonie of his earliest experiments,
nsed this metliod, foiiowing the lead of Hertz in concen-
trating the radiations into a bundle of parallel rays, bv
means of a parabolic reflector (see p. 132). This arrange-
ment is entirely feasible and quite successful where
small oscillators are nsed ; but the mirrors, in order to be
effective, must be at least comparable in dimensions to
the wave-length of the oscillator, and, for the best results,
they should be much larger. When long antennaï are
nsed, emitting radiations with a wave-length of a thousand
feet or so, reflectors are quite out of the question.
It is unfortunate that some such device as this lias not
been made practicable, for it would not only resuit in a
great saving of energy, due to its concentration in the
one direction where it can be made useful, but the range
of signaling would be greatly extended, on account of
the diminished atténuation of the waves,— just as the
beam of a searchlight is more efficient and more pene-
trating tlian the light of an ordinary beacon. Lacking
this, we must hâve recourse to one or other of the meth-
ods of tuning.
3. Syntonie Signaling.— The second method is that of
electrical résonance, or syntony. We hâve leamed a good
deal about this in Part One, and now we need only review
the principles involved, and see how they may be applied
in practice.
We hâve seen that, in order to hâve strong résonance
between two circuits, both must be persistent vibrators.
If the oscillator sends out waves whose amplitude de-
cavs rapidly, the resonator, whose vibration is built up as
the summation of ail the impulses received, will not be
fairly started before the impulses die out. On the other
hand, if the oscillation of the resonator is strongly dampedSELECTIVE SLGNALISG.
205
a point is soon reached where the energy is dissipated as
fast as it is received, and no amount of persistence on
the part of incoming waves will increase the amplitude
of its vibration.
The phenomenon of multiple résonance shows that the
strongly damped vibrations of a Hertzian oscillator may
excite oscillations in resonators of widely differing periods,
just as a beam of white light appears red when viewed
through a red glass, and green when viewTed through a
green glass, whereas a beam of pure color should be visi-
ble only through a glass of the proper tint.
So with strongly damped oscillations, strong résonance
and sharp tuning are both impossible, and we miss the
two great advantages of a good syntonie System: strong
response to a feeble signal, and the ability to distinguish
one set of signais from another.
îsTow, a grounded antenna is simply half of a Hertzian
oscillator, on a large scale; and we know that the latter
is a very poor vibrator — its oscillations decay to one-
tenth of their initial amplitude in about nine vibrations.
(See p. 67.) Increasing the size of an oscillator does
not affect its damping, if the proportions be kept the
same — a large oscillator radiâtes more energy than a
small one, but it also conta ins more, and the proportion
of the total energy sent ofï during each oscillation is the
same, whatever the size of the oscillator.* So we would
naturally expect the oscillation of a simple antenna to
hâve a high rate of decay, and this is indeed the case.
The reason for this will be apparent, if we refer again
to the diagrams, Figs. 89-92. It is readilv seen that the
fraction of the total energy sent off in the detached wave
during each oscillation dépends upon the number of Fara-
* Sre Macdonald. Electric Waves, p. 77.206
WIRELE88 TELEGRAPEY.
day tubes which go to make up the wave, and on the
relative energies of the detached part of a tube and of
the part that goes back to the oseillator. The process
of separating off the waves is very much like the boyish
game of “ snap-the-whip,” where each boy represents a
tube. The chances of a certain boy being snapped off
the line dépend upon his distance from the end of the
line; and the number of boys snapped off, and the violence
with which they go, dépend upon the suddenness of the
snap.
So with our oseillator. The chances of a tube being
snapped off dépend upon the wideness of the circuit that
it makes; the tubes that stretch nearly straight from
pôle to pôle of the oseillator, or from the antenna to
ground, simply shrink back into the conductor; while those
that make a wide circuit into space are snapped off. Again,
the number of tubes snapped off and their relative energy
dépend upon the suddenness of the snap; i. e., the fre-
quency of the oscillations. If the oseillator be allowed
to discharge slowly through a high résistance, ail the
tubes will hâve time to shrink back and be absorbed, and
there will be no radiation; but if the discharge be sudden
and rapid, many of the tubes will be snapped off, and
the number and relative size of the detached loops will
dépend upon the suddenness of the snap.
This suggests one method of prolonging the oscilla-
tions. If we can diminish the frequency of the oscillation
without altering the electrostatic field about the antenna,
the rate of decay will be decreased. This may be ac-
complished by inserting a self-induction coil between the
antenna and the ground, thereby increasing the “ inertia ”
of the circuit and slowing down the oscillations. It isSELECTIVE SI GN ALI N G.
207
analogous to hanging a weight at the middle of a vibrating
string.
The curve, Fig. 112, shows the resuit of sueh an experi-
ment made by the writer. The transmitter was a plain
multiple-wired antenna, in sériés with which was a coil
of twenty tums of coarse wire having an inductance of
Fig. 112.— Curve of résonance between two distant antennœ with in-
ductance in sériés. Lx is the constant inductance of transmitter, L2 the
variable inductance of receiver, D the detector and measuring apparatus.
.043 millihenry — or about three times that of the an-
tenna itself. The receiver was a similar antenna with
an inductance that could be varied. The abscissas of
the curve represent the values of this inductance, and
the ordinates, the corresponding intensities of the cur-
rent in the receiver. By this means a fairly sharp réso-
nance point is found, whereas the same antennæ, without208
WIRELESS TELEGRAPHY.
the extra inductance, are so nearly dead-beat tliat réso-
nance between them is difficült to detect at ail, and the
receiver responds almost equally well to oscillations of
widely v-arying frequencies.
It should be noted, however, that this method of pro-
longing the oscillations does so at the expense of intensity
of the signais. The total energy of the oscillation is the
same, whether the inductance be used or not; but in the
latter case the energy is radiated with a sudden rush,
making a strong evanescent wave like the sound of a
drum, while in the former the same energy is distrib-
uted over a longer interval, as when a bell is struck, and
the amplitude of the wave is correspondingly reduced.
This method is thus of limited utility, unless means be em-
ployed for increasing the initial energy of the oscillation.
Another method of prolonging the oscillations, which
possesses certain adv-antages, is by increasing the capacity,
as Marconi did with his early antennæ by attaching a
plate at the top. (See p. 141.) This increases the total
energy of the wave-train, and at the same time prolongs
the oscillations by reducing the frequency. Looking at
it from the Faraday tube point of view, the total number
of tubes proceeding from the antenna is increased, owing
to the increased charge, but as the frequency is dimin-
ished, a relatively smaller number of tubes are “ snapped
off ” at eacli oscillation. The oscillations are thus pro-
longed without a corresponding decrease of intensity.
4. Lodge’s Syntonie Cônes and Leyden Jars.— Sir Oliver
Lodge and Dr. Alex. Muirhead devised a System of syn-
tonie signaling, combining both these principles.* The
transmitting apparatus is the ungrounded oscillator re-
See English pat. No. 3 8,644, July 16, 1898.SELECTIVE 8IGNALIXG*
209
ferred to on p. 136, with an inductance coil inserted be-
tween the two large capacity-cones. (Fig. 113.) The
receiving apparatus is similar, with a coherer substituted
for the spark gap — or rather connected in shunt across
the inductance, thereby getting the full différence of po-
tential in the coil without interfering with the oscillations.
Fio. 113.— Lodge’s syntonie cônes with inductance coils (cf. Fig. 71,
p. 136); Trans. is the transmitter. Rec. the receiver, with coherer T,
battery B and rela.y R. S is a non-inductive shunt to eliminate the
choking effect of thé highly inductive receiving apparatus R.
Even this combination method, though it is operative
within certain limitations, falls short of what is required
in a practicable working System.
Lodge also devised an experiment which illustrâtes
beautifullv the principle of résonance.* A pair of Ley-
den jars whose opposite coatings are connected, each pair
* Lodge, Hignallinç ihrmigh Space vithoui IVt’res, 3d ed., p. 6.
14210
WIRELESS TELEGRAPH Y.
by a loop of wire, are placed a short distance apart, as
shown in Fig. 114. The circuit of one of the jars is
interrupted by a spark gap, so that oscillations may be
set up when the jar is charged by means of an electrical
machine, and the other is provided with a slider, by which
the length of the circuit may be varied until its period
is the same as that of the oscillator. When this is done,
Fig. 114.— Lodge's syntonie Leyden jars. The two circuits are tuned
to résonance by moving the slider S until sparks appear in the “ overflow ”
air-gap.
résonance occurs and sparks may be drawn f roiû the second
jar.
In this case, nearly ail the Faraday tubes run straight
across, through the glass, from coating to coating of the
jars — very few of them reach out into space — conse-SELECTIVE SIGNALING.
211
quently the oscillations are very much prolonged and
extremely sharp résonance is possible. Only a small
movement of the slider is required to throw the jars out
of tune. Unfortunately, how-
ever, such a System is a very
poor radiator, and the effect
can be observed only at a
short distance — indeed the
properties of good radiation
and persistent oscillation are
directlv opposed to each other,
and the conditions which im-
prove the one impair the
other. Some sort of a com-
promise must be effected.
5. Marconi’s Concentric Cylin-
ders.—Marconi devised a Sys-
tem embodying the principle
of the syntonie jars, but he
made the “ jars” very long
and narrow, so that they
would hâve a considerablé ra-
diating surface. In other
words, he used two vertical
concentric cylinders of zinc,
separated by an air space,
which formed a condenser of
considérable capacity and at the same time played the
part of an antenna.* The cylinders at the sending sta-
tion were connected together through an inductance and
a spark gap, and the inner cylinder was grounded. (Fig.
trie cylinders, Ci, C2» combine
the functions of condenser and
antenna, and the frequency may
be adjusted by varying the in-
ductance L.
See Eng. pat. No. 5,387, March 21, 1900.212
WI RE LE SS TELEGRAPUY.
115.) At the receiving station the arrangement was
similar, but without the spark gap and with a coherer and
recording apparatus connected across the inductance.
This System was used with considérable
success for a time, and was found capable
of working over a distance of thirty miles,
with cylinders seven meters liigh and 1.5
meters in diameter, but it was finally sup-
planted by a better arrangement.
6. The Closed Oscillating Circuit.— The
great desideratum is a transmitter wliose
oscillations are prolonged as much as
possible, and yet whose radiation is not
correspondingly diminished in intensity;
in other words, one which is capable of
giving off energy at a good rate for a
considérable time. This means that the
initial supply of energy must be large.
The closed oscillating circuit of Lodge’s
syntonie jars possesses two of these quali-
fies; its oscillations are prolonged, and the supply of
energy is limited	only by	the	capacity of the	jars
and the \oltage	to	which	they are charged.	But
this arrangement	is	a poor	radiator. Is it	not
possible to combine	such	an	oscillating System of
high power with a good radiator in such a way that its
energy may be made available for signaling? This is
what is done by the physicist when it is desired to increase
the Sound of a tuning fork: the fork itself is a persistent
vibrator but a poor radiator, so it is mounted on a reso-
tenna with closed
oscillating cir-
cuit. The re-
son a t i n g box
takes energy
from the tun-
ing fork and ra-
diâtes lt as
sound.
nating box, tuned to the same pitch as the fork, and thus
the energy of the fork is made available as sound. (Fig.
116.)SELECTIVE SWXALIXG.
213
The same thing may be done with electrical oscillators,
by coupling a closed oscillating circuit (a persistent vi-
brator) to an ordinary antenna (a good radiator). The
coupling may be accomplished in a number of ways, ail
of which are modifications or combinations of the three
methods illustrated in Figs. 117 to 119 : the first con-
ductive, the second electrostatic, the third inductive.
Figs. 117, 118, 119.— Three methods of coupling a closed oscillating
circuit, C G L, to an antenna A ; the first, a rigid electrical connection
across a résistance R (Fig. 117) ; the second, an elastic connection across
a condenser C' (Fig. 118) ; the third, an inductive coupling through a
transformer, M (Fig. 119).
These three methods may be illustrated by their me-
chanical analogues, Figs. 120 to 122, which show three
methods of communicating the vibrations of a tuning fork
to a stretched string. The first is a rigid mechanical con-
nection between the fork and the string; the second is
an elastic connection, through a spring; the third is
through a transversely vibrating rod or lever carrying
at its center of vibration a mass, whose inertia serves the
purpose of a fulcrum.
So, in coupling the oscillating circuit to the antenna,
we may hâve a rigid electrical connection across a dead
résistance (Fig. 117) ; an elastic connection across a con-214
WIRELESS TELEGRAPHY.
denser (Fig. 118);* or an inductive coupling through a
transformer (Fig. 119). In ail these cases the essential
element is the création of a variation of potential at the
base of the antenna, as in thd mechanical analogues we
must hâve a variation of pressure on the string, if it is
to be set in vibration. In the first case, this is accom-
plished by the ohmic drop in the dead résistance; in the
second, by the elastic reaction of the dielectric in the
condenser; in the third, by the inertia reaction of the
Fig. 120.	Fig. 121.	Fig. 122.
Figs. 120, 121, 122.— Three methods of coupling a tuning fork F to a
vibrating string S, tuned in unison with it ; the first, a rigid mechanical
connection through the reciprocating rod R (Fig. 120) ; the second, an
elastic connection through a spring C (Fig. 121) ; the third, an inertia
coupling through the mass M (Fig. 122), which acts as a fulcrum to the
vibrating rod A B.
ether surrounding the coil. The first is obviously a
wasteful method (at least in its simple form), as the
ohmic drop is accomplished at the expense of energy con-
verted into heat, with a conséquent damping of the os-
* A more strictly analogous case is that of Leclier and of Hertz,
Fig. 34.SELECTIVE S1GNALING.
215
dilations. The second is more. efficient, and has been
advocated by some workers. The third is the method
which, with its modifications, is almost universally used,
and we shall now consider it more carefully.
7. Inductively Interlinked Circuits.— Where we are
dealing with very rapid oscillations, the inductive effects
of the currents are so powerful that no spécial apparatus
is required to produce them — two wires lying side by
side affect each other strongly, and even a stovepipe
erected near an antenna may be the source of disagree-
able shocks. In the case of Lodge’s syntonie jars, the ef-
fect is one of almost pure induction — there is no true
radiation worth mentioning. So, to connect the closed
oscillating circuit to the antenna, a transformer of very
few turns is sufficient. It is usually a coil of stout wire
or cable inclosing a similar secondary coil, without iron,
and the whole immersed in a vessel of oil. (cf. Fig. 77,
p. 147.)
Now let us see what occurs when such a System is put
in operation. To get -a clear idea of the phenomena we
should put aside ail preconceived ideas as to the opera-
tion of a transformer, and look at the matter afresh from
Maxwell’s point of view. We shall thus be in a position
to see some notable apparent exceptions to the theory as
it is usually applied to ordinary alternating circuits.
The oscillating currents in the closed primary circuit
create a variable magnetic field in the space inclosed by
the coil. The “ inertia ” of this field constitutes the self-
induction of the primary, and there must be an electro-
motive force across the terminais of the coil to overcome
this inertia. But this field does not affect the primary
alone; at least part of it is embraced by the secondary216
WIRELEtiS TE LEO RAP H F.
also, and a similar electromotive force is induced across
the secondary terminais. In like manner (referring to
Fig. 122) the vibrating tuning fork tends to set in mo-
tion the mass M, attached to the rod AB which con-
nects it with the string. If the end B, of this rod, be
held immovable, the mass will vibrate with the fork, re-
qniring a force to keep it in motion, and so retarding
the vibration of the fork as if it were attached to the
prong. But, at the same time, the inertia reacts at the
fixed end, B, of the lever, and so a force is exerted which
tends to set this end in motion. This force will impose
forced vibrations on the string, whatever the period of
the fork may be, and the intensity of the force dépends
upon the lengths of the lever arms, AM and BM. So
our transformer will impose forced oscillations on the
antenna, whatever the period of the primary circuit, and
the electromotive force which produces these oscillations
is approximately proportional to the ratio of turns. Thus
far no exceptions.
Now let the frequency of the primary circuit be
changed, by altering either the capacity, C, or the in-
ductance, L.* As the frequency of the oscillation ap-
proaches the natural frequency of the antenna there is
a marked increase in the amplitude of the secondary os-
cillation, until the point is reached where the two cir-
cuits are “ in tune,” and the amplitude of the oscillation
reaches a maximum. When this occurs, the current in
the secondary of the transformer is too great to be
* Remembering that the frequency is N =--=. L and C being
2* |/LC
botli expressed in the same System of units, and N being in cycles
per second..SELECTIVE 8IGXALIXG.
217
neglected, and being in opposition to the primary cur-
rent, it tends to neutralize the effect of the iatter in proi
ducing a magnetic field. In the idéal case (never at-
tainable in practice) where both circuits are perfectly
free oscillators exactly in tune, and where the two coils
of the transformer are perfectly interlinked, without
magnetic leakage, we should hâve no flux wbatever in
the transformer, no EMF. across either primary or
secondary, and the ratio of primary to secondary currents
would be equal to the inverse ratio of turns.
This is analogous to the case where the tuning fork
and its string are exactly in tune, and the amplitudes of
their respective vibrations are proportional to the lever
arms, AM and BM. The mass, M, will then be motion-
less, and there will be no force acting on either the string
or the fork. Any change in the amplitude of either vibra-
tion will set the mass M in motion, and the inertia reactions
which resuit will tend to retard one vibration and aug-
ment the other, until the stable condition is again
attained.
This leads to the conclusion that the amount of step-
ping-up of the voltage from the primary condenser to
the antenna is not directly proportional to the ratio of
turns of the transformer — indeed, in our idéal case,
the reverse is true; L ethe greater the ratio of second-
ary to primary turns, the less the secondary current, and
consequentlv the potential to which the antenna is charged.
With a simple 1 : 1 transformer the two currents are equal
and the potential dépends simply upon the ratio of the
capacities of primary and antenna. We may thus step up
the voltage to any desired extent (within limits), bv in-
creasing the capacity and decreasing the inductance of the
primary circuit.218
WIRELESS TELEGRAPHY.
In actuâl practice these principles are modified to sonie
extent by the fact that the antenna is a good radiator,
and so there must be a transfer of energy to it from
Fig. 123.— Two
masses Mlt M2,
supported by sp rings
Sj, S2, and ad-
justed to vibrate up
and down in syn-
chronism.
Fig.	124.— The
same masses con-
nected by a light
rod R. The vibra-
tion remains un-
changed.
Fig. 125.— The
masses Mi and Ms
of Figs. 123 and
124 divided, and
part of each moved
to the middle of the
rod. If the ratio
Mp : Ms is equal to
: M2, the System
vibrâtes about Mm
as a fulcrum with
an increased fre-
quency.
the primary. Hence, to use the mechanical figure, the
mass M is never really motionless ; and there must alwavs
be a flux in the transformer and a corresponding départ lire
from the idéal conditions.
Another corollary is that two circuits which are turned
separately to the same period will not necessarily be in
tune when brought into inductive relation ; or, if they are
in tune, their frequency will generally be altered.
This may be illustrated by another mechanical model.
Suppose two masses, Mi and M2,(Fig. 123) to be supported,
each by a spring, Si and S2, in such a manner they may
vibrate freely up and down with a frequency depending
upon the mass and on the elasticity of the spring. Sup-
pose further that each mass is so proportioned to the elas-
ticity of its spring that their periods of vibration are the
same.SELECTIVE SIGNALIXG.
219
Now let the masses be connected by a rod of negligible
mass (Fig. 124). They will vibrate in unison, as before,
with the frequency unchanged.
Fig. 126.— Two closed oscillating circuits with their capacities Ci, C2,
and inductances, Li, L2, so adjusted that the product L1C1 — L2C2. The two
circuits are then in tune.
Suppose now that the masses be divided, and a portion
of each moved out to the center of the rod (Fig. 125). The
masses will no longer vibrate together, moving up and
down in unison, for this is an unstable condition; but
each will tend to vibrate with a new frequency of its own
depending upon its mass, but restrained by the rod. If
the masses Mpand Msbe properly selected with reference
to their springs these frequencies may be made the same,
and the masses will
then vibrate in unison,
but not in 'phase: Mp
will move up when Ms
its moving down, in
sew-saw fashion, while
M m remains motionless.
The frequency of the
System will be higher
than the original fre-
quency, according to the
proportion of the individual masses made inactive by mov-
ing them to the middle.
i-m.
Fig. 127.— The same two circuits brought
into inductive relation. The mutual in-
duction, Lm, partly neutralizes the self-
inductions Lj and L2. leaving Lp and La to
control the frequency.220
W IRE LE 8 S TELEGRAPHY.
Thus we may hâve two electrical oscillating circuits
(Fig. 126), each with its inductance, L, and capacity, C,
corresponding to the mass and elasticity of the mechanical
vibrators, and tuned to oscillate in unison. When these
circuits are brought into inductive relation, however, the
conditions are changed (Fig. 127). The magnetic fields of
the two inductance coils now overlap, and may be separated
into three parts: First, the part whose flux is interlinked
with the primary coil only, and constitùtes the effective
primary self-induction, Lp; second, the part whose flux is
interlinked with the secondary alone, and constitùtes the
effective secondary self-induction, Ls ; and, third, the part
whose flux is interlinked with both, and constitùtes the
mutual induction, Lm. This mutual flux tends to become
zéro,* and so reduce the kinetic energy of the field to a
minimum, leaving the fluxes of Lp and Ls alone to control
the frequency of the oscillation ; just as the mass Mm tends
to remain motionless, and let Mp and Ms control the
vibration.
In practice, however, this state of affairs is only ap-
proximated, as the mutual flux is necessarv to transfer
energy from the primary to the secondary. This may be
illustrated in the mechanical model by supposing the mass
Mg to be retarded by a friction, which dissipâtes energy in
heat as the antenna does in radiation. The mass Mm
will then not remain motionless, but will vibrate sufRciently
to transfer energy from Mp to supply that lost by Mg.
The departure from the idéal conditions is usuallv not
very great. For example, Fig. 128 shows a résonance curve
* It affects the two oscillations in opposite senses, augmenting
the one and opposing the other, while it, in turn, owes its existence
to a différence between the opposing magnetic effects of the currents*
It thus tends to its own destruction.SELECTIVE SIGXALING.
221
between the two antennæ referred to on p. 207, taken
nnder the same conditions, except that the sending antenna
was conpled inductively to a closed oscillating circuit.
The same coil L was connected in sériés with the sending
antenna, but in the one case (Fig. 112) this coil was used
as a simple self-induction, wliile in the other (Fig. 128)
Current in receiver—Scale arbitrary. O* CA «e £		L;	i t.007				—I—				—i—					
	B							At				A,				
	r															
																
		L														
		\										D Rec.				
		A						; Li			U					
			\ 0		T	•ans										
													‘M			
																
				\												
					\											
						N										
																
																
																
																
																
																
A																
JO .02 .01 .06 .08 .10 .12 .14 J8																
Receiver inductance, L2 — millihenry.
Fig. 128.— Curve of résonance between a transmitting antenna AjLlf
v/itli closed oscillating circuit CPC'G, and a receiving antenna A2, witn
variable inductance L2 and detector D. The secondary coil Li is the
same as 1., in Fig. 112.
it was placed within a second coil, which constituted the
primary of a transformer of which the antenna coil was
the secondary. These two circuits were tuned to résonance
by adjusting the primary capacity. At the receiving end
the same variable inductance was used in the simple an-222
WIRELESS TELEGRAPHY.
tenna circuit, and the curve is plotted on the same scale,
with values of this inductance as abscissas and intensities
of signais as ordinales. The spark length was decreased
for the compound oscillator, so that the maxima hâve ap-
proximately the same value, and the curves may thus be
compared directly.
The différence is striking. In the curve, Fig. 112, the
inductance at the résonance point is .042 millihenry — al-
most exactly the same as that in the sending circuit. In
Fig. 128, the inductance is reduced to .007 millihenry, or
one-sixth of this value. In other words, five-sixths, or 83$,
of the self-induction Li is neutralized by the mutual induc-
tion, and the frequency is thus brought so near to the nat-
ural frequency of the antenna that the maximum occurs al-
most on the axis. And this with a transformer that was
very loosely wound, so that there was a large amount of
leakage. With closely interlinked coils the approximation
to the idéal conditions may become very close.
The résonance curves are not sharp, owing to the use of
the simple antenna circuit for receiving—a poor resonator.
To recapitulate, we find that the use of a closed oscil-
lating circuit, with a condenser of large capacity compared
to that of the antenna, not only prolongs the oscillations,
by virtue of its property of storing energy, but it may also
increase the intensity of the radiation by increasing the
potential to which the antenna is charged; and that the
intensity of the radiation, as well as the rate of decay, may
be varied within wide limits by changing the windings of
the transformer and the amount of additional self-induc-
tion in the primary or secondary circuits. Thus the ap-
paratus may be adapted at will to use with tuned re-
ceivers, or with the simple “ responsive ” or untuned re-
ceivers previously described. The frequency also may be8 ELECTIVE SIGNAUX G.
223
varied within wide limite, without changing the propor-
tions of the antenna — an important feature where several
stations must intercommunicate at will. In such cases,
each station may hâve several sets of apparatns, each
tuned to a different frequency, which may be connected
to the antenna as needed. Indeed it is possible to work
two or more transmitters at once from the same antenna,
each sending signais in ite own tune, just as the belly of a
violin — an aperiodic radiator — may resound to the notes
of two or more strings at once.
8. Tuned Receiving Apparatus.— Given a transmitter
capable of emitting radiations with a small rate of decay,
the next requisite of a syntonie System is a strongly-reso-
nant receiver.
Many of the factors which go to make a good transmitter
apply equally well to the receiver. A good radiator is, in
general, a good absorber ; so an antenna which works well
at the sending end may be used with good results for receiv-
ing. So also, a persistent oscillator usually makes a good
resonator, and a correspondingly poor absorber. Hence
we must resort to the same kind of compromise at the re-
ceiving end as was neeessary in the transmitter, coupling
the antenna to some sort of résonant circuit, in which the
detector is placed. This résonant circuit may be similar
to the closed oscillating circuit of the transmitter, though
very muçh reduced in size, as the currents which it is called
upon to carry are extremely feeble and the correspond-
ingly low voltages require no spécial care in insulating.
The spécifie arrangement, however, dépends upon the form
of detector to be used, and its ohmic résistance. In this
respect there is the greatest variety, with the coherer,
which is normally open-circuited, at one end of the scale,
and the low-resistance thermal and magnetic detectors at224
W IRE LE 8 S TELEGRAPHY.
the other ; and each requires a different mode of treatment
to obtain the best results.
Putting aside the coherers for the moment, let us con-
sider the closed-circuit detectors. These may be connected
simply in sériés, in the résonant circuit, as indicated
diagrammatically in Fig. 129.
îfow the damping of such a circuit is governed, not bv
radiation, but by the dissipation of its energy in ehmie
and other losses; hence it would seem
at first glance that the lower the ré-
sistance of the detector the better.
But this is not necessarily the case.
In the first place, low résistance usu-
ally involves lack of sensitiveness, as
when a magnetic detector is wound
with few turns of coarse wire; and,
in the second place, it is not the ab-
solute value of the résistance, but the
ratio of résistance to inductance,
that détermines the damping fac-
tor, or “ logarithmic décrément.”
Hence, it is usually désirable to design
the résonant circuit with a large in-
ductance, and the proportionately
small capacity necessary to give the proper frequencv.
Here, however, enters the objection that small ca-
pacity means small current, and we encounter the di-
lemma of damped oscillations on the one hand and weak
impulses on the other, just as we found in considering the
transmitter that prolonged oscillations are opposed to good
radiation. Fortunately, the solutions in the two cases are
similar. Most detectors of the type which we are now
Fig. 129.— Receiver
with closed circuit de-
tector D in a local
résonant circuit,
DCML. The induct-
ance Jj is made vari-
able for the purpose
of tuning.SELECTIVE SIGNALING.
225
eonsidering are cumulative in their action, and the inten-
sity of the signal dépends upon the total energy received
from the wave-train—not on its intensity. As in the trans-
mitter we strive to increase the energy of the wave-train,
even at the expense of intensity, so in the receiver our aim
must be to apply the energy to the detector in the most
efficient manner, sacrificing the intensity of the current, if
need be, in the interest of résonance. In the one case this
resulted in large capaeity and small inductance, while in
the other the tendency is to small capaeity and large in-
ductance.
As an extreme case, we see the “ jigger ” of Marconi
(p. 197), where the coil is long and fine and the capaeity
is reduced to that of the wire itself and the coherer con-
nected to its terminais. In this case, the small capaeity in-
volves high voltage, which is just what is wanted to ope-
rate the coherer to best advantage. The capaeity of a
coherer is a rather uncertain quantity; hence, to obtain
sharp and definite tuning, it is often désirable to connect
a condenser of larger capaeity (C', Fig. 131), as a shunt
across the coherer terminais, thus making its variations
negligible. Unfortunately, however, this has the effect of
reducing the potential, and so diminishing the sensi-
tiveness.
Having considered some of the general principles which
govem the operation of syntonie Systems, let us now look
at a few spécifie forms of apparatus, and see how the prin-
ciples hâve been applied.
9. Marconi Coherer System.*—A System which is used
very successfully by the Marconi company is illustrated in
* Marconi, before Roy. Instn. Great Britain, June 13, 1903.
15226
WI RELE 88 TELEGRAPHY.
Figs. 130 and 131, the former showing the transmitting ap-
paratus and the latter the receiver. Both embody the
closed oscillating circuit which we hâve been considering,
coupled to the antenna through a transformer.
At the sending end of the line is a condenser, C, con-
sisting in a battery of Leyden Jars, charged by an induc-
tion coil, I, operated by a telegraph key in sériés with the
automatic break. This condenser discharges through the
is an additional condenser sometimes
used to improve the résonance.
primarv, P, of a transformer, which is made up of a
few turns of stout stranded cable immersed in oil. The
condenser C, primary coïl P, and spark gap B, make up the
closed oscillating circuit, which has no separate inductance
coil — the self-induction of the primary, whose loosely
wouiid tums permit a good deal of magnetic leakage, being
sufficient. The secondarv is connected to the antenna
through a coil whose inductance may be varied at will, toSELECTIVE SIGXALING.
227
facilitate tuning. The frequency of the primary circuit
may also be varied by changing the capacity of the con-
denser, so that the two circuits may be adjusted, not only
to resonate with each other and thus secure the most effi-
cient radiation, but also to vibrato in unison with the re-
ceiving apparatus.
The receiver (Fig. 131) is similar in principle to the
transmitter, though very different in construction. The
antenna is connected through a variable inductance, L, to
the transformer, J, which, in this case, is the peeuliarly-
wound “ jigger ” described on page 197. The coherer is
connected to the outer terminais of the secondary, while
Fig. 132.— Marconi transmitters arrangée! for multiplex working on a
single antenna.
the inner terminais go to a condenser, C, across which is
connected the local battery and the sensitive relav wkich
opérâtes the recorder. The “ jigger ” itself, as we hâve
already seen, is to a considérable degree syntonie in its
action, but where sharper tuning is desired a condenser, C',
is sometimes connected in shunt across the coherer. This,228
WIRELESS TELEGRAPHY.
by virtue of its larger capacity, buries any irregularities
in the capacity of the coherer, and makes the apparatus
more perfectly sélective, though less sensitive.
Figs. 132 and 133 show this apparatus arranged for
multiplex working, with two transmitters at one station
and two receivers at the other, both sets connected to the
same pair of antennæ, though tuned to different fre-
quencies.
The coherer at best lends itself reluctantly to tuning;
hence, where the most perfect selectivity is desired, Mar-
coni often uses the magnetic detector, with a speciallv con-
structed jigger designed to place the detector at a loop of
the oscillation, where the current is a maximum and the
voltage small—just the reverse of that used with the
coherer.
10. Slaby-Arco System.*—In this svstem the coupling
between the closed oscillating circuit and the antenna is
*See Slaby, D\ie Funkentclegraphie, pp. 114, 115; also C. Ardt, Die
Funkentelegraphie, Leipzig, 1903, p. 39.SELECTIVE SIGXALIXG.
220
partly inductive and partly by direct metallic connection.
Instead of a transformer with separate windings, there is
a single coil of wire which plays the part of both primary
and secondary, on the principle of the “ auto-transformer ”
of ordinary alternating-current practice. The condenser
circuit is connected across a portion of the coi], and the
antenna circuit across another portion (see Fig. 134),
and the ratio of turns of these portions is ma de variable,
to facilitate tuning. The capacity of the condenser, Ci,
may also be varied, so that the condenser circuit may be
made to oscillate in résonance with the antenna circuit,
and both be tuned to the frequency of the receiver.
The receiver is similar in principle, but modified in
form. The coil, L2 (Fig. 135), is retained in the antenna
circuit, to déterminé its frequency and to act as an auto-
transformer in coupling the two circuits together, but there
is an additional coil, M, in the receiver circuit, whose func-230
WIRELESS TELEGRÂPHY.
tion is that of the “ multiplier/’ described on page 196.
It is wound with fine wire of such a length that îts own
period of oscillation is the same as that of the antenna,
and a .node is produced at its outer end, where the coherer
is connected, similar to that which occurs at the top of
the antenna.
We should keep clearly before us the essential différence
between the oscillating circuits of transmitter and receiver,
as it applies to this and to most other coherer Systems. In
the case of the transmitter, the inductance is usually small,
and the total length of the circuit is small compared to
the wave-length. It is thus sufficient to assume that the
current is the same in ail portions of the circuit, and
the frequency is determined by the simple formula
N — -----* —.* In the case of the receiver, however, the
2-|/LC
coil is so long that an appréciable time is required for an
impulse to travel from one end to the other, and, in the
spécial case where this time is equal to a quarter period, a
stationary wave will be produced with a node at the outer
end and a loop at the inner end. The current flows in and
ont at the latter, while only variations of potential occur
at the former. It is the potential at the node that is
utilized to operate the coherer.
The case of the transmitter may be illustrated by a cord
of small mass stretched by springs and carrying a weight
in the middle. (Fig. 136.) The System vibrâtes as a whole
* Remembering that L, in this case, is the effective value of the
self-induction, after deducting from the true self-induction the effect
of the mutual induction in the transformer or auto-transformer,
Seep.219. The effect of résistance is usually small, and is here neg-
lected.SELECTIVE SIGNALING.
281
with a frequency depending simplyon the ratio
We need not consider the form of the weight nor the loca-
tion of the springs. But if the mass and elasticity be dis-
c*
Fig. 136.— Model illustrating the action of an oscillating circuit with
concentrated inductance Ij and capacity Cj, C2, which are analogous to
the mass M, and the elasticity concentrated in the springs Sn eg.
tributed uniformly along the cord (Fig. 137), an impulse
appjied at the middle takes an appréciable time to travel to
Fig. 137.— Model illustrating the operation of Marconi’s “ jigger,”
whose distri bu ted capacity and inductance (elasticity and mass) are so
proportioned to the frequency that the current (velocity) imparted
through the mutual induction (mass M) to the secondary (welghted
spring) is transformed into potential (tension) at the free terminais of
the coil (ends of spring).
the end. In the spécial case where this time is equal to a
quarter period of the impressed impulses, the string will
* Where the “ elasticity,” or ratio of force to displacement, is equal
to 4 X tension -*• lengtli of cord. To complété the analogy, note
that the capacity of a condenser, or ratio of its charge to potential,
is analogous to the reciprocal of an elasticity.232
WIRELE8S TELEGRAPH Y.
vibrate with a loop where the force is applied and a node at
each end. Moreover, if the damping of the System be
small, the forces occurring at the node may be much
greater than the impressed force, and the System will act
Fig. 138.— Model illustrating the operation of Slaby’s “ Multiplier.”
The current (velocity) imparted directly to the multiplying coil M
(weighted spring) by the antenna (reciproeating rod R) is converted
into potential (tension) at the outer end of the coil (spring). The
potential at this point may be much greater than that applied at the
other end of the coil.
as a “ multiplier,” as in the case of Slaby’s coil, M (Fig.
138), or Marconi’s jigger, J (Fig. 137). Indeed, in many
respects the multiplier is the équivalent of half of the
jigger, though in the one case the oscillation is impressed
on the coil by induction from a primary, and in the other
the current is fed directly into one end of the coil from the
antenna.
Th' similarity mav be illustrated by comparing the
models, Figs. 137 and 138. In the former, the oscillation
is impressed on the weighted spring at its middle point,
through the freelv-suspended mass M (cf. page 214), and
the spring vibrâtes like a piano cord with a loop in theSELECTIVE SIGNAUX G.
233
middle and a node at each end. In the latter, the motion
is forcibly applied, directly at one end of the weighted
spring, which vibrâtes with a loop at this end and a node
at the other.
The increase in voltage at the outer end of the coil is
analogous to the well-known “ Ferranti ” effect (so-called),
which is observed when one end of a two-conductor cable
of suitable proportions is connected to the terminais of an
alternator. When the length of the cable bears the proper
relation to its distributed capacity and inductance and
to the frequency of the alternator, electrical résonance
occurs, and the voltage at the free end of the cable may
become much greater than that of the generator. The
energy which leaves the generator as a current flowing into
the cable is transformed into potential energy, stored in the
cable as a condenser, and measured by the increased poten-
tial at the distant end.
11. Braun’s System.*— Prof. Ferdinand Braun has pro-
duced a System whose distinctive feature is the absence of
a ground connection. The sending apparatus (Fig. 139)
comprises a closed oscillating circuit coupled to the an-
tenna circuit through a transformer, P, S. The antenna,
A, is conAected to one terminal of the secondary, and the
other terminal, instead of being grounded, is attached to a
capacity area in the form of a cylinder of métal, made in
two parts which telescope one over the other so that the
area of the surface may be varied to facilitate tuning.
The antenna circuit thus approximates a Hertzian oscil-
lator in which one conductor is greatly attenuated and the
* F. Braun, Über drahtlose Télégraphié, Physikalische Zeitschrift,
1902, p. 143. Also A. Voiler, Elektrische Wellentelegraphie, Ham-
burg, 1903.234
WIRELESS TELEGRAPH Y.
other shortened and expanded, though it is doubtless true
that the capacity area, acting as one coating of a condenser
of which the earth is the other coating, perforais to some
extent the funetion of a gronnd connection, thus extending
the range of the apparatus beyond the limits of free
Hertzian radiation.
The condenser, Ci C2, consists in a number of small
tubular Leyden jars, and the transformer comprises a
primary winding of a few turns of heavy copper cable or
tube, with a secondary winding with a larger number of
turns of smaller conductor. Tuning is accomplished by
Fig. 139.— Braun System — transmitter. The antenna, instead of being
grounded, is connected through the secondary S of a transformer to an
adjustable capacity area K.
changing the number of tubes in the primary condenser
and by adjusting the telescopic capacity.
The receiver, Fig. 140, combines the elastic and inductive
methods of coupling. The antenna circuit, with its adjust-
able capacity area, contains a condenser, Ci C2, across
whose terminais is shunted the primary, P, of a trans-
A
CiSELECTIVE SI GN ALIX G.
235.
former, which, with the condenser, constitutes the primary
résonant circuit. The coherer, T, is placed in a secondary
circuit, inductively coupled with the first, and thus a con-
sidérable irregularity in the capacity of the coherer may
occur without destroving the résonance of the primary
circuit.
The coherer itself is somewhat different from those pre-
viously described. It consists in a hard rubber tube with
Fig. 140.— Braun System — receiver. The primary résonant circuit
C1C2P is coupled elasticaily ” to the antenna through the agency of
the condensers Ci, C2, and inductively to the coherer circuit through the
transformer P, S.
électrodes of Steel, between which is a quantitv of liard-
ened steel filings. One of the électrodes is provided with
an adjusting screw, whereby the distance between the élec-
trodes, and hence the sensitiveness, may be regulated.
12. Lodge-Muirhead System.*—Lodge has described a
number of forms of syntonie apparatus — commencing
* See Electrician, Lond., March 27, 1903, pp. 930-934.236
WIRELESS TELEGRAPHY.
with his syntonie Leyden jars, then the syntonie cônes con-
nected by an inductance (p. 208), then this inductance was
made the primary (or secondary) of a transformer, the
other winding of which was connected to the coherer (or
to the condenser and spark-gap). With the advent of the
grounded antenna, this arrangement evolved itself into
that shown in Figs. 141 and 142, the former representing
the transmitting and the latter the receiving apparatus.
The connections are so similar to others that we hâve
considered that we need not discuss them in detail, except
to note that a condenser, Ci, and sometimes also another,
C2? is connected in sériés in the antenna circuit. It is
convenient to make these condensers adjustable, to allow
of tuning the antenna circuit without changing the induc-
tance. The antenna circuit is connected with the oscillat-
Fig. 141. — Lodge-Muirhead
System — transmitter. A closed
oscillating circuit, C3BC4P,
coupled to the antenna
through a transformer PS. An
adjustable condenser Ci in
sériés in the antenna circuit.
System — receiver. Condensers
Ci and C2 in sériés with the
antenna and primary P of trans-
former. Mercury-steel coherer
D, in secondary circuit, operating
syphon recorder R.
Fig. 142.— Lodge-MuirheadSELECTIVE SIGNAUX G.
237
ing circuit of the transmitter or with tlie résonant circuit of
the receiver througb a transformer, and each . of these cir-
•cuits has its own condenser, arranged in much the same
way as in other Systems that we hâve considered.
Another arrangement of receiv-
ing apparatus is shown in Fig.
143. Here the transformer is dis-
pensed with, and the coherer circuit
is connected in shunt across the ré-
sonant circuit, which has a direct
electrical connection with the
antenna.
The detector used with this Sys-
tem is the mercury-steel coherer,
having a steel dise rotating continu-
ously in light contact with a
globule of mercury (see page 184),
and opérât ing a siphon recorder.
13. Limitations of Syntonie Sig-
Ttaling.— The Systems above de-
scribed are ail designed to use detectors of the coherer type,
whose characteristic is a normally open circuit which be-
comes closed under the influence of the oscillations. Its
action is a sort of trigger effect: the oscillation in the
résonant circuit increases in amplitude until it is able
to break down the insulation of the coherer, and the sig-
nal is received. While the résonant rise of the oscil-
lation is going on, the coherer is, in effect, a condenser
of small capacity, and must be so considered in cal-
culating the period of the résonant circuit. This capacity
is not a definite quantity, but is constantly varying as the
blows of the tapper change the arrangement of the filings in
the tube ; hence the problem of tuning is a difflcult one, and
Fig.	143.— Another
form of Lodge-Muirhead
receiver. Inductance L,
capacity C2. and coherer
circuit C3D, ali in mul-
tiple.238
WIRELESS TELEGRAPHY.
leads to the adoption of the various expédients which we
hâve considered, for making the coherer capaeitv a mafc-
ter of minor importance not vitally afFecting the résonance
of the apparatus.
With detectors of the closed-circuit type the problem is
simpler. Here the oscillatory currents pass freely through
the detector, which usually has little effect on the period
of the circuit. Hence the design of the receiver is governed
by the simpler principles which apply to the case of the
transmitter, and the main concem is to so proportion the
resonating circuit that the current shall hâve the maximum
effect on the détector compatible with the conditions of
good résonance. The principles involved hâve been con-
sidered already, and we need not go farther into details
which interest only the specialist.
We should note, however, one fundamental fact. Any
detector is primarily a device for translating energy.* It
reçoives the energy of rapidly oscillating currents and
transforms it into some form — mechanical, thermal,
Chemical, or otherwise — in which it can be utilized.
Now the criterion of a good resonator is that its damping—
which is only another way of saying its rate of dissipation
of energy — be small. The very fact of a receiver being
operative involves an expenditure of energy and so im-
poses a limit on the résonance of the circuit.
* The coherer is an apparent exception, as it seems to act more as
a relay than as an energy-transformer ; but recent experiments
show that the degree of cohérence, and hence the intensity of the
signal, dépends not only upon the intensity of the impulse, but also
on its duration. There is thus an important sense in which even
the coherer may be accepted as conforming to the general rule.
See Hurmuzescu in Annales Scientifiques de L’Université de Ja£sy>
reprinted in L’Eclairage Électrique, June 27, 1903.SELECTIVE SIGNALING.
230
At first glance it would seem a» if a strongly résonant
receiver must. necessarily be an insensitive one, but for-
tunately this is not the case. Most detectors of the closed-
circuit type are cumulative in their action : it is not only
the strength of current flowing through the detector that
détermines the intensity of the signal, but the duration of
the current must be considered as well. A resonator which
perforais a hundred oscillations and gives up (on an aver-
age) a hundredth part of its energy at each swing may
give as strong a signal as one which affects the detector at
ten times that rate, but only perforais ten vibrations. The
important thing is to see that the energy is utilized in
the detector and not wasted in ohmic résistance of the
eonductors and dielectric losses in the condenser. This is
simply a question of careful design of the apparatus,
which has been solved successfully by a number of work-
ers. The resuit is a receiving apparatus whose damping
is small enough to make it a good resonator, yet a fair
proportion of the energy received from the ether is made
effective in producing a signal. Its characteristics ap-
proach, to a fair degree of approximation, those of a piano
string — a vibrator which, growing out of the crude, tink-
ling harpsichord, has been so perfected that its note is
quite persistent, considering that it is set in vibration by
a single blow.
And this brings us to the next point: we know that a
strongly résonant receiver alone does not make a good
syntonie System, but we must look at the limitations im-
posed by the transmitter. The vibration of the receiver
dépends upon the energy which it absorbs, whether this
be given by a sudden impulse or distributed over a longer
period. If we depress the loud pedal of a piano, so as to
lift ail the dampers from the strings, and then beat a240
WIRELESS TELEGRAPHY.
drum in the room, ail the strings will respond — feebly, it
is trtte, but ail equally well. Sa the strongly-damped
radiations of a simple antenna will, if they hâve sufficient
energy, affect even a good tuned receiver to some extent,
though ordinarily the effect is trifling. If necessary, as
in the case of a malicious attempt to block a receiver, the
interférence may be eut out by making the inductive
coupling between antenna and résonant circuit very loose
— thus, if our piano cord be protected by a felt pad, it may
receive quite a hard blow without being set in vibration,
yet it will respond to a prolonged, though feeble, synchron-
ous vibration.
Such précautions are usually unnecessary, and it is bet-
ter to avoid interférence by using only properly con-
structed transmitters. If a person may be restrained from
disturbing the peace by making unnecessary noises in the
Street, why should not the same principle apply to etheric
disturbances ?
Ilowever this may be, prolonged oscillations hâve othèr
advantages than those which concern the question of selec-
tivity. Returning again to the piano, with the loud
pedal depressed, suppose a note to be sung into the instru-
ment. The string tuned to this note will respond loudly,
even though the note sung be quite feeble. If the same
note be struck on another piano, the same response will
occur, though less loudly. If the note be struck re-
peatedly, the response will be louder. ÏTow, the damp-
ing of even a good tuned transmitter may be considerably
gréa ter than that of a piano string, hence the energy rep-
resented by its radiations is small, considering their inten-
sity. Suppose, for example, a transmitter makes 100
oscillations before their amplitude is greatly reduced. IfSELECTIVE SIGNALING.
241
the wave frequency be 1,000,000 per second — correspond-
ing to a wave-length of about 1,000 feet — the oscillation
persists for	second. If now the spark- or group-
frequency be 100 per second — a rather large figure for
mechanical interrupters — the oscillation endures for only
one hundredth of the interval between sparks, and for
99$ of the time the transmitter is absolutely inactive. No
resonator is persistent enough to bridge over this interval,
so répétitions of the impulse, as when the same note was
struck repeatedly on the piano, are useless for increasing
the intensity of the signal. It is possible that a compound
oscillâting System, like that of Prof. Fleming (p. 153), may
be so designed as to charge the sma.ll secondary condenser
several times for each charge of the primary, and thus
bring the wave-trains close enough together to hâve a
cumulative effect on the receiver, though to what extent
this may be done has not been made public by the inventor.
If it were possible to generate an absolutely undamped
radiation, analogous to the Sound of the voice in our piano
experiment, the intensity of the signal would be greatly
increased, even though the amplitude of the waves were
very small. Under such circonstances, a simple Hertzian
oscillator would radiate energy at the rate of over twenty
horse-power.*
Such a generator has never yet been made practicable.
Mechanical generators will not do, on account of their
low frequency. To feed energy into an electrical oscil-
lator at each reversai, to replace that lost by radiation,
seems possible from a theoretical point of view, and
there is experimental ground for encouragement in this
* See Hertz, Electric Waves, Eng. trans., p. 150.
16242
WIRELESS TELEGRAPHY.
direction; but as we are dealing with facts, not possibili-
tés, we must pass that by until such an apparatus is put
in operation.
However perfect the résonance between transmitter and
receiver may be made, there will always be a practical
limit to the number of stations that may be tuned to selec-
tivity, as there is a limit to the number of strings in a
piano. To escape this restriction we may hâve recourse
to the third and last method of securing selectivity.
14. Other Means of Securing Selectivity.— Where the
number of stations witliin the sphere of each other’s in-
fluence is so great that it is impracticable to differen-
tiate them by their wave-frequencies, some other char-
acteristics must be added to their radiations, by which
they may be distinguished. There are several ways of
doing this.
Mr. Tesla has proposed the plan of having each station
émit two wave-trains of different frequencies, either
simultaneously or in succession, and providing the re-
ceiver with two separate tuned circuits, so arranged that
résonance in either alone will not give a signal, but both
must respond together.* The number of combinations is
thus increased, after the fashion of a combination lôck,
as the square of the number of individual frequencies.
Another method is to give to each station a definite
rate of récurrence of the sparks, or group-frequency, and
to tune some part of the receiving apparatus to respond
to this frequency. For example, if the signais are re-
ceived on a téléphoné, the ordinary diaphragm may be
replaced by a weighted tongue, which vibrâtes at a defi-
* Tesla, U. S. pats. Nos. 723,188, Mardi 17, 1903, and 725,605, April
14, 1903. Application filed July 16, 1900.SELECTIVE SIGNALING.
243
nite frequency. The téléphoné will then respond only
to signais whose group-frequencies correspond to the
natural period of the tongue, even thongh the detector
may be in active operation.
This method was proposed by Professor Rathenau, of
Berlin,* for use in connection with a conductive System
of wireless telegraphy (see p. 112, footnote).
M. Blondel applied the same principle to Hertz-wave
telegraphy,f and proposed several methods — mechani-
Fig. 144.— Blondel’s mechanical method of tuning to group fre-
quencies. T,, T2, T3 are mono-téléphonés, each tnned to respond to a
different spark-frequency, D is an auto-coherer or other self-restoring
detector, A the antenna, and B a battery.
cal and electrical — for tuning the receiver to the com-
paratively low frequencies involved. He prefers to use
* Professor Eathenau, Elektrotechnische Zeitschrift, v. 15, p. 616,,
1894.
f See note addressed to the Frencli Academy in 1898, Sur la
Syntonie dans la Télégraphie sans fil, Comptes Rendus, 21 Mai, 1900,,
p. 1383; also Eng. pat. No. 21,909, May 3, .1900. Accepted Nov. 9^
1901.244
WIRELE88 TELEGRAPHY.
Mercadier’s “ mono-telephones,”* winch were designed
by their inventor for use in multiplex wire-telegraphy
and are so constructed as to hâve a very definite period
of vibration. A number of these téléphonés, each tuned
to a different note, may be connected in the local re-
ceiver circuit (Fig. 144), and several messages may thus
be received at the same time. It is, of course, necessarv
to use a self-restoring detector.
Fig. 145.— Blondel’s method of electrical tuning to group frequencies,
showing two receivers. The local circuits Ci lMT, and C2L2V, with
capacities C,, C2 and inductances 1.2< are tuned to their respective
soark frequencies. On the right is shown a mono-telephone T, for re-
cèiving by ear, and on the left a vibrating relay V, with recording ap-
paratus R.
M. Blondel also applies the principle of electrical
résonance, as in Fig. 145, which shows two receivers con-
nected to the same antenna. Each receiver circuit, in-
cluding battery and téléphoné (or relay), has its capacity
and inductance so adjusted that its natural period is the
* See FAéktrotech. ZcitschrApril 27, 1899, p. 305 ; also Annales
Télégraphiquesy 1898, p. 287.
fl
?;?777ss7”
R.SELECTIVE SIGNALING.
245
same as that of the interrupter at the sending station to
vvhich it is to respond.
By these means the selectivity of the System is made
independent of the wave-frequency of the signais, and
even a strongly damped radiation is incapable of affecting
a receiver which is not tuned to reçoive it.
15. Conclusion.— We hâve traced briefly the develop-
ment of the art of wireless telegraphy from its germ in
Maxwell’s epoch-making conceptions regarding the elec-
tromagnetic field to its fruition in a practical working
System, containing ail the essential éléments of a suc-
cessful enterprise. Let ns now glance at the subject
from a commercial point of view, and see what has been
practioally accomplished, and what are the prospects of
future progress.
To say that ail the problems hâve been solved, and that
the System now stands complété, would be idle prattle;
but much has been donc. The problem has passed through
the hands of the prophet, the mathematician, the physi-
cist, and the practical expérimenter; it is now in the
hands of the engineer and financier, to perfect and cor-
relate the achievements of ail these workers, and fuse
them into the complex organism which a commercial en-
terprise of this kind must eventually become. From
the vision of Maxwell the seer to the fulfilment of Hertz
was twenty years; from Hertz’s discovery to Marconi’s
first démonstration in England was nine years,* from
this exhibition of a working, though crude, Hertz-wave
telegraph to the présent day is little more than seven
years — yet what progress has been made !
* Marconi applied for his first English patent, No. 12,039, June 2,
1896, and the démonstration referred to took place shortly after.246
WIRELESS TELEGRAPHY.
To-day, commercial wireless telegraphy is an estab-
lished fact. The traveler on an océan liner can commu-
nicate at will with his friends on shore, and in some cases
can read a morning paper from the. steamer’s press, con-
taining the latest shore news. The navies of most of
the great nations hâve equipped, or are equipping, their
fighting ships with what they consider an indispensable
instrument of modem warfare. Snow-bound Alaska and
remote islands of the sea are put into communication with
the outer world. Land stations hâve been set up and ope-
rated, paralleling wire lines. Transatlantic wireless tel-
egraphy has been demonstrated by the actual transmission
of messages between England and America, while smaller
stations hâve carried on their business without interfér-
ence from the powerful “thunder houses” nearby. It
is needless to multiply instances showing what has been
actually done : enough has been said to show that wire-
less telegraphy is no longer a laboratory experiment, but
an accomplished fact, and several companies are now
carrying on business for ail those who care to employ
their services.
To make prédictions as to the future is not the pur-
pose of this paragraph. Rather let the reader look for
himself at what has already been achieved, and draw
his own conclusions. The writer is not one of those
enthusiasts who believe that wireless telegraphy is des-
tined to tum the cable into a rusting relie of the past
and to relegate the wire lines to the scrap heap. The
téléphoné did not supplant the telegraph, nor did the
trolley-car make the steam railroad obsolète; but each
has found its own sphere of usefulness, and the estab-
lished Systems, far from suffering from the innova-
tions, hâve gone on increasing their trafic. Is it notSELECTIVE SIGNALING.
247
reasonable to think that our twentieth-century commerce
will supply enough business to occupy ail the Systems that
may be put in the field? The future of wireless teleg-
raphy is now îh the hands of the practical man, and it
remains for him to show how he will occupy the territory
thrown open by the pioneers who hâve preceded him.
Many of the world’s best thinkers and workers hâve pre-
pared the way — let him now go in and possess the land.INDEX
PAGE.
Alternating Currents, Tend to Sur-
face of Conductor... 49, 83, 145
----, Do not Explain Transmission. 159
----, Distribution in Parallel Cir-
cuits ..............................200
---- in Guiding Surface. . . 157, 159, 173
---- for Transmitters .............. 151
Ampère on Mutual Action of Cur-
rents .......................... 13
Antenna, the........................138
----, Character of Radiations from. 156
----, Dumping of Oscillations in... 205
----, Development of............... 141
----, Importance of Low Résistance
in .........................145
----, Multiple-Wired............... 143
----, Value of Inductance in....... 207
----, Wave-Length of Radiations... 143
---- with Closed Oscillating Cir-
cuit ...........................213
Anti-Coherers...................... 185
Arco-Slaby — Wireless Telegraphy.. 228
Atténuation of Waves, Effect of
Light on...........  .	176
----, --- Guiding Surface on. 162, 174
----, Law of......... 129, 159, 161, 175
Attraction, Electrodynamic........... 9
----, Electrostatic .............4,	166
----, Mutual, of Faraday Tubes ... 166
Auto-Coherers...................... 182
“ Barretter ” ................ 187,	188
Birkeland and Pérot, Détermination
of Wave-Form..................... 42
Bjerknes, Détermination of Wave-
Form ....................... 69
----, Measurement of Damping. 45,	67
----, Proof of Skin-Effect....50,	83
——, Use of Electrometer .... 44,	45
Blondel, Sélective Signaling........243
Blondlot, Apparatus for Waves in
Wires...................49,	69
----, Measurement of Velocity of...	55
---- on Inductive Capacity of Ice..	82
----, Oscillator................34,	85
----, Resonator.................34,	80
Bolometric Method of Measuring Os-
cillations ..................... 43
Bose.......... 35, 47, 83, 85, 91, 99, 100
----, Detector...................... 89
----, Diffraction Gratings.......... 97
----, Oscillator............... 88,	91
PAGE.
Branly’s Coherer........... 45, 89, 131
----, on Opacity of Metals......... 83
Braun, Multiple Antenna ........... 144
----, Wireless Telegraph System... 233
Capacity, Analogous to Elasticity. .
----, Distributed.............. 193,
Castelli Coherer ...................
Charge, Moving, Equivalent to Cur-
rent........................ 173,
Circuits, Closed Oscillating .... 22,
151, 212 et
----------, Coupling to Antenna ...
----, Inductively lnterlinked ......
----, Unclosed ................ 18,
Closed Oscillating Circuit ......22,
151, 212 et
Clouds Transparent to Electromag-
netic Waves......................
Coherer, Anti-......................
----, Auto-.........................
----, Capacity of.............. 227,
----, Carbon Grain..................
---- Inactive when Buried...........
---- is Potential-Operated..........
----, Lodge’s Theory of.............
----, Mercury.............. 182, 184,
----, Methods of Using .............
----, Principle of .................
----, Self-Restoring................
---- Works Best at Node--------195,
----, Bose’s........................
----, Branly’s...................45,
----, Castelli’s....................
----, “ Italian Navy ” .............
----, Lodge’s Knob .................
----, Lodge’s Mercury.......... 184,
----, Marconi’s ........... 131, 181,
----, Popoff’s......................
Cohn on Inductive Capacity of
Water ...........................
Compound Oscillating System.. 153,
Condenser, Discharge of..........22,
----, Seat of Charge in ............
Conduction, Wireless Telegraphy
by.......................... 112,
Conductivity of Electrolytes. 83, J57,
---- of Guiding Surface........162,
Conductors Opaque to Electromag-
netic Waves ................. 82,
Convection Currents ........... 173,
15
231
231
182
177
148
seq.
213
215
20
148
seq.
158
1S5
182
237
185
161
194
181
237
194
179
182
199
89
89
182
182
180
237
226
138
82
241
191
18
243
177
174
157
177
[249]250
INDEX.
PAGE.
Currents, Alternating, Tend to Sur-
face ....................... 49, 83, 145
----, ---- Distribution of, in Paral-
lel Circuits ................200
----, Closed or Unclosed......... 18,	20
----, Diffusion of............... 51,	54
----, Displacement. 14, 118,	124,	163,	173
---- in Guiding Surface... 157, 159, 173
---- in Dielectrics.................... 15
----, Kinetic Energy of...........10,	113
----, Moving Charge	Equivalent
to...................173,	177
---- -multiplying Devices ............199
----, Mutual Action of ............. 7
----, Velocity of, in Wires. 14, 50 et seq.
Curvature of Earth ............ 156, 173
Cylinders, Marconi’s Concentric.... 211
Damped Vibration Appears Complex.
Damping of Oscillations...........
---- in Antenna .............. 205,
---- in Receiving Apparatus........
----, Bjerknes’ Measurement of.. 45,
----, Cause of Multiple Résonance.
----, Décombe’s Proof of...........
----, Effect on Résonance, 38, 204,
Daylight, Effect on Wave Transmis-
sion ..............................
Decay of Oscillations (see Damping).
Décombe, Experiments on Wave-
Form.............................
Décrément, Logarithmic.........66,
See also Damping.
De Forest, Responder ..............
----, Transmitter .................
De la Rive and Sarasin, Discovery
of Multiple Résonance.......
----, Experiments on Waves in
Space ...........................
Detectors of Oscillation (see also
Coherers).. 38 et seq., 179 et
---- Classified ...................
----, Closed-Circuit.......... 224,
----, Current-Operated........199,
----, Electrolytic .a..............
----, Magnetic ............... 191,
----, Mechanical .............. 44,
----, Microphonie (see Coherers) ..
----, Thermal.................. 43,
----, Bose’s ......................
----, De Forest’s..................
----, Fessenden’s.................
----, Marconi’s Magnetic......192,
----, Righi’s .....................
----, Rutherford’s............
----, Schloemilch’s ..........
----, Vreeland’s .............
Dichroism.....................
Dielectrics, Currents in......
----, Inductive Capacity of ..
----, Propagation of Waves in
----, Properties of ..........
----, Reflection of Waves by ,
Diffraction.........•.........
Diffusion of Current...........
Directed Signais .............
96,
51,
66
27
240
224
66
63
69
240
176
69
224
187
151
62
74
seq.
179
238
224
187
228
185
179
180
89
187
186
228
87
192
189
188
100
15
79
79
14
95
158
54
203
PAGE.
Discharge of Condenser (see also
Oscillations)................ 22
----, Compared to	Pendulum.......	24
----, Compared to	Tuning	Fork, etc. 26
----, Henry on..................... 191
----, Lord Kelvin on..............;	23
Displacement Currents..........14,	118
--------,	about Oscillator . . . 163, 173
--------,	in Moving Waves ........ 173
--------,	Magnetic Effect of ......124
----, Relation of Force to.... 163, 175
Distributed Capacity and Induct-
ance ......................... 196,	230
Dolbear’s Wireless Telegraph Sys-
tem ...........................119,	126
Double Refraction	................. 100'
Edison and Gilliland, Train Teleg-
raphy............................  120
Elastic Reaction of Dielectrics.....	15
Electrical Phenomena, Generaliza-
tions..................... 1
---------,	Mechanical Explanation of. 1
---- Waves (see Waves).
Electrodynamic Attraction............. 9
----, Model Illustrating............. 10
Electrolytes, Conductivity of... 83, 157
177
Electrolytic Detectors ............. 187
Electromagnetic Waves (see Waves).
Electrostatic Field, Energy of.. 17, 118
166
---- Method of Signaling_______ 117, 127
---------,	Dolbear’s ................119
---------,	Edison’s................. 119
---- Phenomena, Hydraulic Analo-
gies of .............................   3
Energy of Faraday Tubes .............166
---- of Electromagnetic Waves (see
also Atténuation).. 129, 171, 175
---- of Electrostatic Field.. 17, 118, 166
---- of Magnetic Field. 10, 113, 127, 166
---- ----- Tends to Become Mini-
mum ...................... 200.
----, Lost in Résistance......18, 145
Ether, Properties of........... 9, 20
----, The Seat of Inductive Effects. 10
Experimentum Crucis............... 71
Faraday on Dielectrics ............. 14
Faraday Tubes.......................164
---- in Leyden Jars................. 210
---- in Wave Propagation........... 167
----, Magnetic Field from Motion
of...................... 166,	170
----, Normal to Perfect Conductors. 173
----, Properties of................ 165
----, Self-closed..............165, 167
----, “ Snapping Off ” of..... 206, 208
---- Terminate in Electrical
Charges.......................164,	173
Feddersen on Discharge of Leyden
Jar....................... 22, 148, 191
“ Ferranti Effect ” ................233
Fessenden Thermal Detector... 187, 188
Fizeau and Gounelle’s Experiments. 52INDEX.
251
rAUK.
Fleming High-Power Transmitter... 153
241
Fog Transparent to Electromagnetic
Waves.......................... 158
Foucault’s Experiment ............. 6
Free-Wave Hypothesis ............ 157
----, Propagation of............. 163
---- (see also Waves, Electromag-
netic).
Frequency of Oscillation .........
—- Altered by Mutual Induction. 219
----, Effect of, on Absorption .... 127
---- ----- on Conductivity of Elec-
trolytes ........................83,	157
---- ----- on Radiation ..........125
----in Wireless Telegraphy ...... 157
----, of Blondlot’s Oscillator ....... 85
----, of Bose’s Oscillator ........... 89
----, of Closed Circuit Oscillator . .	23
216, 230
----, of Hertz’s Oscillator .... 37, 85
----, of Light Vibration ...... 19,	89
----, of Righi’s Oscillator .......... 87
---- (see also Wave-Length).
Fresnel, Theory of Light....... 98, 104
----, Interférence Experiments. . 93, 94
---- on the Ether................13,	20
Garbasso, Experiments on Disper-
sion ............................... 65
----, Experiments on Secondary
Waves............................. 106
Generators, Alternating Current, in
Transmitters.................151
----, High Power .................... 151
Geneva, Experiments at............!.	74
Gilliland and Edison, Train Teleg-
raphy ............................. 120
Gordon’s Method of Measuring In-
ductive Capacity............... 81,	82
Gounelle and Fizeau’s Experiments. 52
Gouy on Diffraction of Light.. 96, 104
158
---- on Inductive	Capacity of
Water.............................. 82
Group Frequency, Tuning to.. . 203, 242
Grounded Oscillator ..............  141
---- Waves....................156, 172
Guarini’s Antenna .................   142
Guided Waves ................. 156, 172
rAUB.
Hertz, Mechanical Detector........ 185
----, Oscillators ........ 34, 85, 160
----, Sketch of Life .............. 31
----, Theory of Wave Propagation. 168
Hertzian Waves (see Waves).
----, Propagation of ............. 163
----, Telegraphy by .............. 130
High-Power Generators..............151
Hurmuzescu on Coherers........ 185, 238
Hystérésis in Magnetic Detector... 193
Imperfect Contact .................. 179
Index of Refraction of Dielectrics. 79
Inductance, Effective, Mutual Induc-
tion Diminishes......................220
----, Value of, in Antenna .........206
---- (see also Self-Induction.)
Induction Coil, Requirements of... 149
----, Electromagnetic, Signaling by. 113
---- ----, Phelps’ Method .......... 116
---------, Preece’s Method.... 115, 127
----, Electrostatic, Signaling by.. 117
127
---------, Dolbear’s and Edison’s
Methods.....................119
----, Self and Mutual ................ 7
See also Self-Induction; Mu-
tual Induction; Inductance.
----, Tubes of (see Faraday Tubes).
Inductive Capacity of Dielectrics...	79
---- and Index of Refraction........	79
----, Measurement of ................ 80
Inductively Interlinked Circuits____215
Inertia of Faraday Tubes............ 166
----, Self-Induction Analogous to..	7
25, 206, 215
Interférence of Secondary and Di-
rect Waves .......................... 95
---- of Waves in Air............72, 92
---------Intersecting Obliquely ...	93
---------Moving in Same Direction. 93
---------in Wires.................... 59
Ions in Electrolysis............84, 177
----in Gases ....................... 177
Iron Wire, Velocity of Current in.. 55
Italian Navy Coherer................ 182
‘ Jigger,” Marconi’s.....197, 225, 227
----, Theory of ................. 230
Jones’ Experiments on Wave-Form. 69
Heating Effect of Current.... 18,	19
---- of Oscillations ..... 43. 145, 186
Henry, Joseph, on Magnetic Effect
of Oscillations	............... 191
Hertz, Apparatus	for	Waves	in
Wires.....................48,	142
----, Distribution	of	Energy	in
Waves.................... 175
----, Effect of Light	on	Spark. .	36,	176
----, Experiments	at	Karlsruhe	...	73
----, Experiments with Small Oscil-
lator ...............,.... 74, 130, 204
■---, Measurement of Wave Length. 59
73
Karlsruhe, Hertz’s Experiments at. 73
Kelvin, Lord, Hypothesis Regarding
Ether...................... 12
----, on Discharge of Condenser ...	23
Kirchhoff on Velocity of Current. 14, 50
Klemencic and Trouton on Polar-
ization............................. 98
Lecher, Waves in Wires .............214
Leyden Jar, Discharge of (see Dis-
charge of Condenser)... 22, 191
----, Lodge’s Syntonie .............209
Light and Electricity, Relations Be-
tween...............................
13252
INDEX.
PAGE.
Light and Electricity, Effect of, on
Wave Transmission..........176
----, Nature of .......... 19, 81, 102
----, Reflection by Minute Particles. 158
----, Synthesis of ................ 101
----, Ultra-Violet, Effect on Spark. 36
----, Velocity of ............. 13,	79
Lightning, Oscillatory Nature of... 138
Lines of Force, Magnetic.. .114, 124, 170
----, Electrostatic (see Faraday
Tubes)..................... 117,	163
Lippmann’s Interférence Striæ ....	92
Liquids, Conductivity of........83,	157
Lodge’s Conical Capacity Areas. 136, 208
---- Early Experiments ............ 130
---- Knob Coherer ................. 180
---- Mercury Coherer.......... 184,	237
---- -Muirhead System ..............235
----, Naming of Coherer ............ 45
---- on Signaling by Induction_____127
----Oscillator..................35,	86
---- Syntonie Leyden Jars......180. 208
212, 215
---- Theory of Coherer ............ 181
Logarithmic Décrément ......... 66,	224
Loops and Nodes, Définition........ 60
See Nodes; Stationary Waves.
Magnetic Detectors........ 190, 191, 228
----Field, About Oscillator .. 124, 170
---------Accompanies Moving Fara-
day Tubes .................... 166,	170
---------, Kinetic Energy of... 10, 113
127, 160
Marconi Coherer ............... 130, 1S1
---- Coherer System ............... 225
---- Concentric Cylinder Appara-
tus........................211
----, Définition of Coherer ........179
---- Early Apparatus______ 132, 204, 245
----, Effect of Daylight on Trans-
mission ........................... 176
---- “ Jigger ” .......... 197, 225, 227
---- ----, Theory of ..............232
---- Magnetic Detectors ........... 192
---- Multiple Antenna ............. 144
---- Multiplex Working .............227
---- Transatlantic Telegraphy. 155, 183
----, Use of Antenna -----:........ 139
----, Use of Capacity Areas.. . 141, 208
Maxwell’s Early Conceptions........	2
---- Method of Measuring Induc-
tive Capacity....................... 81
---- Relation ...................... 79
---- Theory of Displacement Cur-
rents...................... 14
---- Theory, the Experimentum
Crucis..................... 71
---- Theory of Light......19, 81, 101
Mechanical Detectors ..........44, 185
---- Explanation of Electrical Phe-
nomena............................ 1
Mercadier, Mono-Telephones..........244
Mercury Coherer s......... 182. 184, 237
Methods of Observing Waves. 38 et seq.
Michelson’s Interférence Apparatus. 94
PAGE.
Micrometer, Spark........... 33, 39, 42
Microphonie Detectors................ 179
Mono-Telephones, Mercadier’s .........244
Moving Charge Equivalent to Cur-
rent........................ 173,	177
Muirhead and Lodge, Syntonie Ap-
paratus ....................... 208,	235
Multiple Résonance.......... 61, 73, 205
——, Explanation of ................... 62
----, Experiments of Sarasin and de
la Rive....................... 62
----, Experiments of Strindberg...	68
Multiplex Signaling........... 223,	227
Multiplier, Slaby's .......... 195,	232
Multiplying Devices ................. 199
Mutual Induction....................... 7
----in Oscillation Transformer... 220
----, Model Illustrating .............. 8
---- Tends to Become Zéro.............220
Newton’s Rings, Electrical Imitation
of............................94,	99
Nodes and Loops, Définition........... 60
---- in Air........................ 73
----in Antenna................ 160,	195
---- in Wires ..................... 60
Obstacles, Absorption of Waves by. 134
156
----, Waves May Pass, in Four
Ways................................157
Oil in Spark-Gap ........... 37, 86, 135
Optical Phenomena, Imitation of...	91
Organ Pipe, Resonator Compared to. 39
95
----, Oscillator Compared	to......161
Oscillation Transformer ...... 148,	213
----. Theory of ......................215
Oscillations:
---- Before Hertz .................... 22
---- Confined to Skin	of Conduc-
tor.................... 50,	145
----, Detectors of (see Detectors).
---- Hertzian......................... 31
---------, Character of ........... 62
---------------, Bjerknes’ Experi-
ments on...........................	67
--------------,	Décombe’s ditto ...	69
--------------,	Garbasso’s ditto ...	65
--------------,	Pérot’s and Jones’
ditto............... 69
--------------,	Strindberg’s ditto ..	68
--------------,	Zehnder’s ditto ....	65
----, Means of Prolonging.. 206 et seq.
----, Undamped .......................241
Oscillator, Bose’s ............ 88,	91
----, Blondlot’s ..................... 34
----, Feddersen’s..................... 22
----, Hertz’s Large .................. 34
----, Hertz’s Small ........... 35,	74
----, Lodges....................35,	86
----, Mode of Vibration	of.......... 160
----, Principle of'............ 32,	163
----, Righi’s......................... 85
----, the Grounded ...................141INDEX.
253
PAGE.
Oscillating Circuit, Closed....22, 148
151, 212 et seq.
----, Coupling to Antenna.............213
---- of Receiver and Transmitter
Contrasted......................230
Oscillatory Discharge ............. 22
----, Henry on ...................... 191
Parabolic Reflectors ... 74, 87, 132, 204
Period of Vibration (see Frequency).
Pérot and Birkeland, Détermination
of Wave-Form ...................... 42
Pérot on Inductive Capacity of lce. 82
----, Experiments on Wave-Form. .	69
----, Method of Measuring Induc-
tive Capacity ...................... 81
Phelps’ System of Train Telegraphy. 116
Polarizing Grid ...................... 97
Polarization by Reflection ........... 98
----, Circular and Elliptic .......... 98
---- of Electric Waves ............... 97
----, Plane of ................... 77,	98
---- ----, Rotation of ............ 97
Poldhu, High-Power Station at...... 176
Popoff, Researches on Lightning . . . 138
----, Use of Antenna................. 138
Preece’s Method of Signaling.. 115, 127
Propagation of Grounded Waves... 156
172
---- of Hertzian	Waves	..... 71,	168
---- of Inductive	Effects,	Finite	Ve-
locity of ................... 71
---- of Waves in	Wires........... 48
---- of Waves, Different	Modes	of.	20
See also Waves, Propagation
of.
Radio-Conductors (see Coherers)____	45
Radiation a Cause of Damping.......	30
----, Means of Increasing......141,	146
----, Hertzian (see Waves).
---- ----, Emitted by Sun ......... 47
---- ----, Nature of .......... 76, 163
Rathenau, Sélective Signaling......243
Receiving Apparatus ................. 179
----, Arrangement of................. 194
----, Tuued ..........................223
Reflection :
---- at End of Wire................... 59
---- by Conductors'................... 72
---- by Dielectrics................... 95
---- by Rarified Air..................158
---- by Small Bodies ................ 158
----, Polarization	by ............... 98
----, Total .......................... 99
Reflectors, Parabolic____ 74, 87, 132, 204
Refraction, Atmospheric ............. 159
----, Double......................... 100
----, Index of ....................... 79
---- of Electromagnetic Waves ...	98
Résistance, Analogous to Friction. 6
28
---- a Cause of Damping........28, 145
---- a Cause of Diffusion.......... 51
----, Effect of, in Antenna.......... 145
PAGE.
Résistance, Effect of, in Guiding
Surface .................... 162, 174
Résonance, as Means to Selectivity. 204
---- Between Distant Antennæ. 207, 221
---- Between Antenna and Closed
Circuit ....................216
----, Curves of ............... 207, 221
----,	Electric . ................... 38
----, Multiple ............ 61, 73, 205
Resonators.....................38,	87
----,	Principle of ................. 38
Respondcr, de Forest’s ............. 187
liighi............... 35, 91, 98, 105, 130
----,	Interférence Experiments.....	93
---- Oscillator................85, 133
---- Resonator....................... 87
----, Study of Secondary Waves.. 94
103, 106
Rowland, Moving Charge Equivalent
to Current ...................... 173
Rutherford, Magnetic Detector.......192
Sarasin and de la Rive, Discovery
of Multiple Résonance______; 62
----,	Measurement of Waves in
Space ............................ 74
Schloemilch, Electrolytic Detector. 189
Secondary Waves............ 94, 103, 106
Sélective Signaling..................202
Selectivity, Other Methods of Se-
curing............................242
Self-Induction, Analogous to Inertia. 7
25, 206, 215
---- Neutralized by Mutual Induc-
tion ................................220
----,	Seat of, in the Ether.......... 9
Self-Restoring Coherers............. 182
“ Skin-Effect ” ...........49, 83, 145
Sky, Color of the................... 158
Slaby-Arco System....................228
----Multiple Antenna.................144
----Multiplier................. 195,	229
Smythe, Electrolytic Detector.......187
Spark, Function of, in Oscillator... 33
35, 128
----, Function of, in Resonator_____	40
----, Influence of Light on .... 36, 176
----, Length of............ 134, 147, 149
----, as Means of Measurement....	42
----, Quality of..... 35, 134, 147, 152
Spark-Frequency, Tuning to.. . . 203, 242
Spark-Gap, Oil in.......... 37, 86, 135
Spécifie Inductive Capacity ......... 79
Stationary Waves ............... 59,	171
----, False ......................... 63
------ in Space ................ 72,	92
Stchegtiœf, Inductive Capacity of
Alcohol........................... 82
Striæ, Interférence.................. 92
Strindberg, Researches in Multiple
Résonance......................... 68
Synthesis of Light.................. 101
Syntonie Signaling ................. 204
----, Limitations of ................237254
INDEX.
PAGE.
Tapper for Coherers........... 132,	181
Téléphoné for Receiving.. 183, 188, 190
192, 244
---- Mono-.........................244
“ Tesla Coll” ...................... 148
Tesla Sélective System ............. 242
Thermal Detectors ............. 43,	186
Thin Films.........................  94
Thomson, Elihu, Oscillation Trans-
former ...........................147
Thomson, J. J., on Cathode Dis-
charge ............................ 177
---- Conductivity of Electrolytes. 157
---- on Faraday Tubes ..............166
Total Reflection ................... 99
Train Telegraphy ...........   116,	119
Transatlantic Wireless Telegraphy. 155
183, 246
Transformer, the Oscillation---148, 215
----, Theory of .................... 215
Transmission, Effect of Sunlight on. 1<6
---- Guiding Surface on.. 159, 162, 174
Transmitters (see also Wireless Te-
legraphy) ..........................141
----, High Power ...................151
Trouton and Klemencic on Polariza-
tion............................... 98
Tubes of Induction, Faraday ........164
---- in Leyden Jar..................210
----, Magnetic Field from Motion of. 166
170
----, Normal to Perfect Conductor. 173
----, Properties of ............... 165
----, Self-Closed ............. 165, 167
----, “ Snapping Off ” of..... 206, 208
----, Termination of, in Electrical
Charges..................... 164,	173
Tuned Receiving Apparatus...........223
Tuning, as Means to Selectivity... 203
204
---- of Interlinked Circuits... 149, 216
---- to Group-Frequencies ..........242
----, Mechanical................... 242
Turpain’s Experiments ... ........... 65
Ultra-Violet Light, Effect on Spark. 36
176
Units, Ratio of Absolute............. 13
----, C. G. S. System ..............164
Velocity, of Light..............13,	79
---- ----, Blondlot’s Measurement
of..................... 55
---- of Current in Wire........14,	50
---- ---- Dépends upon Material of
Wire..................  54
---- ----, Fizeau and Gounelle’s
Measurement of........	52
---- ----, Kirchhoff’s Computation
of..................... 50
---- of Propagation of Inductive
Effects..................... 71
---- of Waves in Space.............. 71
---- of Waves in Dielectrics ....... 79
PAGE.
Vibrations, Modes of Propagation of. 20
See also Waves.
Viscous Reaction of Conductors. 16, 83
Vortex Motion, in Ether............ 12
----, Analogy to Faraday Tubes... 165
Vreeland, Electrolytic Detector____188
---- Current-Multiplying Coil .....200
Waves, Electromagnetic :
----, Absorption of, by Obstacles... 132
----, Atténuation of, with Distance. 129
161, 175
---- ----, Anomalous ...............176
----, Concentration of, by Mirrors. 74
87, 132, 204
---- ----, Around a Wire .... 78, 176
-----, Detectors of. 38 et seq., 179 et seq.
----,. Diffraction of............... 96
----, Double Refraction of..........100
----, 'Effect of Daylight on .......176
----, Effect of Guiding Surface on. 162
174
----, Free, Propagation of ........163
----, Grounded................. 156, 172
---- ----, Character of ............176
---- ----, Propagation of ..........172
----, Intensity of, Greatest at Equa-
tor................ 77. 103, 175
----, Interférence of...... 59, 72, 92
---- in Space ...................... 72
----, Nature of ........... 76, 124, 163
----, Plane of Polarization of.. 77, 98
----, Polarization of .............. 97
----, Propagation of, Along a Wire. 48
59, 78, 176
---------in Air ............... 71, 163
---- ----, in Dielectrics .......... 79
---- ----, Over Conducting Surface. 172
----, Reflection of (see Reflection). 59
72, 95, 158
----, Refraction of ................ 98
----, Signaling by ................ 121
----, Total Reflection of........... 99
----, Velocity of Propagation of. .	55
71,	79
----, Very Short ................... 85
Waves, Hertzian (see Waves, Elec-
tromagnetic).
---------, Signaling by..........	130
---- in Water ..................... 121
----, Longitudinal and Transverse. 20
----, Secondary............ 94, 103, 106
----, Stationary, About Oscillator.. 171
---- ----, in Space ........... 72, 92
---- ----, in Wires ................ 59
Wave-Form of Hertzian Radia-
tions ....................... 62 et seq.
Wave-Length, of Hertz’s Oscillators. 37
----, Measurement	of ............ 59
---- of Grounded	Antenna....... 143
---- of Waves in	Space........... 72
Wehnelt Interrupter............ 189, 190
Wiener, Interférence Experiments..	92INDEX.
255
Wires, Wave-Propagation Along______ 48
59, 78, 176
Wireless Telegraphy ................111
---- by Grounded Waves............ . . 141
---- by Hertzian Waves ............130
----, Classification of Systems .... lil
----, Comparison of Methods ........126
----, Conductive	Method .... 112,	243
----, Detectors ....................179
----, Electromagnetic Method .......113
----, Electrostatic Method..........117
----, High Power Transmitters .... 151
----, Limitations of ...............237
----, Multiplex............... 223,	227
----, Receiving Apparatus ..........179
----, Sélective .............. 202,	242
----, Svntonic......................204
----, Transatlantic....... 155, 183, 246
PAGE.
Wireless Telegraphy, The Commer-
cial Situation ............ 245
----, Wave Method ..................121
----, Blondel’s Sélective Method ... 243
----, Braun’s System ...............233
----, de Forest’s Apparatus .. 151, 187
----, Dolbear’s Method .............119
----, Edison and Gilliland’s Method. 120
----, Lodge’s Early Apparatus .... 136
----, Lodge-Muirhead System ........235
----, Marconi’s Early Apparatus ... 132
----, Marconi Coherer System .... 225
----, PopofTs Apparatus............ 138
----, Preece’s Method ..............115
----, Slaby-Arco System ............228
Zehnder, Expérimenta on Waves_____ 65