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the gap, and the oscillations at every discharge disturb the
circuit of B, exciting in it feebler oscillations of the same
period. By tuning the two circuits to unison by moving
the slider, the oscillations in B become sufficiently violent
to make it overflow through the tin-foil strip c, which
comes over from the inner coating and nearly touches the
border of the outer one. This provides an easy overflow
path, so that when the jars are near together and the two
discharge circuits are parallel, every discharge of A is
accompanied by a bright spark at the air gap c.

372. Electromagnetic Waves. When a current is
established through a conductor a magnetic field is set up
about it, and when the current is increased the magnetic
field is increased also ; the magnetic lines enlarge and
new ones push out from the conductor. When the circuit
is opened or reversed, these lines close in on the conduc-
tor and restore to it the energy stored in the ether
through an E.M.F. of self-induction. But when the cur-
rent oscillates with extreme rapidity, part of the energy
radiates into space, or electromagnetic waves are set up in
the surrounding medium. With the slow alternations
employed commercially the loss by electromagnetic radia-
tion is inappreciable, but such is no longer the case when
the rate equals a million or more a second, as in the oscil-
lations of a Leyden jar.

Joseph Henry appears to have been the first to detect
electromagnetic waves radiating from a circuit running
round a room when an inch spark from the prime con-



426 ELECTRICITY AND MAGNETISM.

ductor of a frictional machine was thrown on to the end
of the circuit. Sewing-needles were magnetized in a
parallel circuit thirty feet below, with two floors and
ceilings intervening. He says : " The diffusion of motion
in this case is almost comparable with that of a spark
from a flint and steel in the case of light." Thanks to
the remarkable researches of Hertz, we now know it to be
the same. The magnetic field produced by the discharge
through the one conductor spread with the velocity of
light to the closed circuit below, where a part of its
energy was absorbed by cutting through the circuit, and
produced an electric flow sufficient to magnetize the
needles placed in the helix.

The energy stored in a Leyden jar is not all dissipated
in the heat of the spark, but some of it is radiated into
space in the form of electric and magnetic waves.

373. Maxwell's Electromagnetic Theory of Light. -
The foundation of Maxwell's theory was laid by the
experiments of Faraday in electrostatic and electromag-
netic induction. These premise a medium as the agency
through which induction takes place. When, therefore,
a periodic disturbance, like the discharge of a Leyden jar,
induces similar disturbances in conductors about it, they
do not occur simultaneously with the initial one, but later
and later in proportion as the conductors in which they
are induced are more and more remote from the source.
In other words, the inductive action proceeds from the
source in the form of electric and magnetic waves.

Maxwell saw that it is not philosophical to fill all space
with a new medium whenever any new phenomenon is to
be explained, and that the evidence for the existence of
such a medium will be strengthened if it can be shown



ELECTRIC OSCILLATIONS AND WAVES. 427

that the properties which must be attributed to it to
account for electromagnetic phenomena are of the same
kind as those which we attribute to the luminiferous
ether. He therefore proposed the theory that waves of
light are not mere mechanical motions, but consist of
undulations partly electrical and partly magnetic ; oscillat-
ing electric displacements are accompanied by oscillating
magnetic forces at right angles to them ; both lie in the
plane of the wave, or are at right angles to its direction of
propagation.

Maxwell arrived at the conclusion that the propagation
of an electromagnetic disturbance through the ether takes
place in accordance with the laws governing the transfer
of motion through an elastic solid, and that the speed of
propagation is

v

where JJL and K are the permeability and the dielectric
constant respectively. For all transparent media /-t is
nearly unity. Hence the speed of light through two
transparent media should be inversely as the square roots
of their dielectric constants. If the velocity of light in a
vacuum be taken as unity, the absolute index of refraction
(I., 187) equals \/v. Therefore the square of the index
of refraction of any substance should equal Jf, if the
electromagnetic theory is true. The agreement between
the two is not very close except for waves of great length.
The index of refraction corresponding to waves of longest
period should be selected, because these are the only ones
whose motion can be compared with the slow processes by
which the capacity of the dielectric is determined.

According to the same theory the velocity of propa-
gation should be the number of electrostatic units of
quantity in one electromagnetic unit. Michelson's value



428



ELECTRICITY AND MAGNETISM.



for the speed of light (1882) is 2.9985 x 10', and Row-
land's determination of the ratio v is 2.9815 x 10 10 , both
in centimetres per second. So far, therefore, the prob-
abilities favored Maxwell's theory, but no decisive test
had been applied.

374. Hertz's Researches. 1 To Hertz belongs the
credit of having put the theory of electromagnetic waves
to the test of experiment, and of demonstrating the truth





Fig. 224.

of Maxwell's theory of light. The simplicity of his appli-
ances is no less remarkable than the magnitude of the
results derived from them. With the insight of genius he
seized on the only available means of producing electric
waves short enough to be measurable, viz., the disturb-
ances propagated outward from the discharge of a con-
denser of small capacity.

Hertz's apparatus to serve as the source of the waves he
called an oscillator (Fig. 224). It consisted of two metal-
lic plates A and B 40 cms. square and mounted 60 cms.

1 Hertz's Electric Waves, Trans, by D. E. Jones,



ELECTRIC OSCILLATIOXS A\D WAVES. 429

apart. The balls at the spark gap were kept brightly
polished. The receiver, or resonator, was a circle 70 cms.
in diameter, and its spark gap was adjustable by means
of a micrometer screw. The oscillator was connected to
the induction coil. The plates formed a condenser of
small capacity with air as the dielectric, and the discharge
across from ball to ball was oscillatory. This oscillation
had a definite period, and hence a succession of electro-
static and electromagnetic waves of equal period were
emitted by it. The half period was 1/100,000,000 of a
second.

The finite speed of the wave was demonstrated by plac-
ing a large sheet of zinc on a distant wall of the room and
observing the sparks produced at the small break in the
resonator in different positions along the dotted base line.
The metal acted as a reflector, so that stationary waves
were produced by interference between the direct and
reflected waves precisely as in Sound. The nodes and
antinodes were detected with considerable precision. The
distance between them determined the wave-length, and
the product of the wave-length and the frequency of the
oscillation gave the velocity. This was found to be of the
same order of magnitude as the known velocity of light,
though the data for calculating the period are somewhat
uncertain. Professor Trowb ridge has since measured the
velocity of electric waves by a direct method, with a result
agreeing very well with the velocity of light.

By the aid of large parabolic zinc reflectors Hertz
demonstrated that electric waves are reflected to a focus
in the same .manner as light. He also constructed a huge
prism of asphaltum and measured its index of refraction.
Gratings consisting of parallel conducting bars exhibited
polarization effects.



430 ELECTRICITY AND MAGNETISM.

Thus Hertz demonstrated that the waves radiating from
an oscillatory discharge spark and the associated condenser
are capable of reflection, refraction, and polarization the
same as light. They possess all the characteristics of light,
and are light except in point of wave-length. Maxwell's
theory does not replace the undulatory theory of light, but
supplies the mechanism of the undulations.

375. Faraday's Magneto-optic Rotations. The first
definite relation between light and magnetism was estab-
lished by Faraday in 1845. A beam of plane polarized
light is transmitted through a transparent diamagnetic
medium. When a magnetic force is made to act in the
direction of the rays of light -within the medium, the
plane of polarization is rotated in the direction in which
the current must circulate around the beam to produce
the given magnetic field.

Let a beam of light, polarized by transmission through
a Nicol's prism (I., 229), pass through a prism of heavy
glass (borosilicate of lead), with parallel polished ends
and placed in a powerful magnetic field, whose direc-
tion coincides with that of the beam of light. A second
Nicol's prism as an analyzer receives the beam, and is
turned so as to cut off all the light. The glass can be
conveniently placed in the magnetic field by boring holes
through the pole pieces attached to a large electromagnet.
The holes and the glass prism are all arranged in line for
the transmission of the polarized light.

When the magnet is excited light passes through the
analyzer. It may be extinguished by rotating jt through a
small angle, but it will not be possible to produce complete
extinction; colors will appear, showing that the angle of
rotation is a function of the wave-length. It is nearly



ELECTRIC OSCILLATIONS AND WAVES. 431

inversely as the square of the wave-length. If the elec-
tromagnet is large, it will be evident that time is required
to magnetize it, inasmuch as the transmitted light grows
sensibly in intensity for a second or more after closing the
circuit through the coils. On the other hand, Professor
Lodge has shown that the rotation of the beam of light,
first in one direction and then in the other, follows the
oscillations of the discharge of a Leyden jar through the
coils producing the field without iron.

376. Verdet's Constant. The angle through which
the plane of polarization is turned depends on the fol-
lowing:

(1) It is proportional to the distance which the beam
travels within the medium. The direction of the plane of
polarization therefore changes continuously from incidence
to emergence.

(2) It depends on the nature of the medium. In
some paramagnetic substances it is opposite in direction
to the current producing the magnetization.

(3) It is proportional to the resolved part of the mag-
netic field in the direction of the beam.

This last fact was discovered by Verdet. The three
laws may be combined in one formula,

6 wlBS cos a,

where w is Verdet's constant determined by the nature
of the substance. 08 cos a is the component of the field
in the direction of the beam, and I is the distance between
the points of incidence and emergence. The expression
os a is the difference in magnetic potential between
the point where the beam of light enters and leaves the
medium. Lord Rayleigh found for carbon bisulphide at



432 ELECTRICITY AND MAGNETISM.

18 C. the constant 0.04202 in minutes of arc for a mag-
netic potential difference of one C.G.S. unit.

377. Explanation of Magneto-optic Rotation. A
ray of plane polarized light may be resolved into two cir-
cularly polarized rays of the same period, each of half the
amplitude of the plane rectilinear vibration, and with the
motions in opposite directions round the circles (I., 32).
If now one of these circular vibrations be accelerated the
plane of the resultant rectilinear harmonic motion will be
rotated in the direction of the accelerated circular com-
ponent, since the resulting motion always lies in the plane
of symmetry. The circular vibration in the direction of
the rotation performs a larger number of vibrations within
the transparent medium than the other one. This mode
of stating what has taken place is independent of any
theory of light, and depends only on facts ascertained by
experiment.

The direction of the rotation in space is the same
whether the light passes_one way or the other through the
magnetic field. Hence the effect may be increased by
passing the same beam back and forth by reflection along
the same magnetic field.

Magnetism consists of something in the ether analogous
to a whirl. This whirl apparently increases one of the
circular components of the plane polarized beam and so
rotates the plane of polarization.



APPENDIX.



TABLE I.

Absolute Dilatation of Mercury (8., 51).



Temp, by
air ther-
mometer.


Dilatation from
to t C.


Mean coefficient
between
and t C.


Coefficient
referred to vol.
at 0.


True
coefficient.









.00017905


.00017905


10


.001792


.00017925


.00017950


.00017922


20


.003590


,00017951


.00018001


.00017938


30


.005393


.00017976


.00018051


.00017955


40


.007201


.00018002


.00018102


.00017972


50


.009013


.00018027


.00018152


.00017989


60


.010831


.00018052


.00018203


.00018006


70


.012655


.00018078


.00018253


.OQ018024


80


.014482


.00018102


.00018304


.00018041


90


.016315


.00018128


.00018354


.00018059


100


.018153


.00018153


.00018405


.00018076


110 .019996


.00018178


.00018455


.00018092


120 .021844


.00018203


.00018505


.00018109


130 .023697


.00018228


.00018556


.00018125


140 .02550.3


.00018254


.00018606


.00018142


150 .027419


.00018279


.00018657


.00018159


160


.029287


.00018304


.00018707


.00018175


170


.031160


.00018329


.00018758


.00018190


180


.033039


.00018355


.00018808


.00018206


190


.034922


.00018380


.00018859


.00018221


200


.036811


.00018405


.00018909


.00018237


210


.038704


.00018430


.00018959


.00018252


>0


.040603


.00018456


.00019010


.00018267


230


.1)42506


.00018481


.00019061


.00018282


240


.044415


.00018506


.00019111


.00018297


250


.046329


.00018531


.00019161


.00018313


260


.048247


.00018557


.00019212


.00018327


270


.050171


.00018582


.00019262


.00018341


280


.052100


.00018607


.00019313


.00018855


290


.054034


.00018632


.00019363


.00018370


300


.055973


.00018658


.00019413


.00018384


310


.057917


.00018683


.00019464


.00018398


320


.059866


.00018708


.00019515


.00018412


330


.061820


.00018733


.00019565


.00018426


340


.063778


.00018758


.00019616


.00018440


350


.065743


.00018784


.00019666


.00018453



(433)



434



ELECTRICITY AND MAGNETISM.



TABLE II.

Volume and Density of Distilled Water after Rosetti (S., 54).



Tempera-
ture.


Volume.


Density.


Tempera-
ture.


Volume.


Density.


- lu


1.001858


.998145


14


1.000701


.999299


9


1.001575


.998427


15


1.000841


.999160


g


1.001317


.998685


16


1.000999


.999002


7


1.001089


.998911


17


1.001160


.998841


6


1.000883


.999118


18


1.001348


.998654


5


1.000702


.999298


19


1.001542


.998460


4


1.000545


.999455


20


1.001744


.998259


3


1.000410


.999590


21


1.001957


.998047


2


1.000297


.999703


22


1.002177


.997826


1


1.000203


.999797


23


1.002405


.997601





1 .000129


.999871


24


1.002641


.997367


1


1.000072


.999928


25


1.002888


.997120


2


1.000031


.999969


26


1.003144


.996866


3


1.000009


.999991


27


1.003408


.996603


4


1.000000


1.000000


28


1.003682


.996331


5


1.000010


.999990


29


1.003965


.996051


6


1.000030


.999970


30


1.004253


.995765


7


1.000067


.999933


40


1.00770


.99235


8


1.000114


.999886


50


1.01195


.98820


9


1.000176


.999824


60


1.01691


.98338


10


1.000253


.999747


70


1.02256


.97794


11


1.000345


.999655


80


1.02887


.97194


12


1.000451


.999549


90


1.03567


.96556


13


1.000570


.999430


100


1.04312


.95865



APPENDIX.



435



TABLE III.
Pressure of Aqueous Vapor in Mms. of Mercury (G., 130).



f C. Mms.


tC.


Mm-.


fC.


Mms.


tC.


Atmos.


10 2.08


16


13.54


90


525.39


100


1.0


2.26 17


14.42


95


633.69


110


1.4


8


2.46


18


15.36


9


733.21


120


1.96


-


2.67


19


16.35


99.1


735.85


130


2.67


- 6


2.89


20


17.39


99.2


738.50


140


3.57


5


3.13


21


18.50


99.3


741.16


150


4.7


- 4


3.39


22


19.66


99.4


743.83


160


6.1


- 3


3.66


23


20.89


99.5


746.50


170


7.8


-2


3.96


24


22.18


99.6


749.18


180


9.9


1


4. "27


25


23.55


99.7


751.87


190


12.4





4.60


26


24.99


99.8


T54JW


200


15.4


1


4.94


87


26.51


99.9


757.28


210


18.8


2


5.30


28


28.10


100


760.00


220


22.9


3 5.69


29


29.78


100.1


762.73


230


27.5


4 6.10


30


31.55


100.2


765.46






5 6.53


35


41.83


100.3


768.20






6 7.00


40


54.91


100.4


771.95






7 7.49


45


71.39


100.5


773.71






8


8.02


50


91.98


100.6


776.48






9


8.57


55


117.48


100.7


779.26






10


9.17


60


148.79


100.8


782.04






11


9.79


65


186.94


100.9


784.83






12


10.46


70


233.08


101


787.59






13


11.16


75


288.50


105


906.41






14


11.91


80


354.62


110


1075.37






Ifi


12.70


85


433.00











436



ELECTRICITY AND MAGNETISM.



TABLE IV.

Specific Kesistances in C.G.S. Units at C- 1



Metals.


Spec. Resist.


Temp. Coef. between
and 100 C.




10 917


00367


Gold ,


2 197


0.00377


Palladium


10 19


00354




1 468


00400




1 501


0.00428




2 563


00423




9 065


00625


Nickel


12,323


0.00622


Tin .


13 048


00440




4 355


00381


Zinc




00406




10 023


00419


Lead


20 380


00411


Thallium . . ...


17 633


OOSQS








Alloys.


Spec. Resist.


Temp. Coef. at 15<> C.


Platinum Silver


31 58 ->


000 9 43


Pt, 33; Ag,66.


30 896


000822


Pt, 80; Ir,20.


21 142


00143


Pt, 90; Rd, 10.
Gold Silver


6 280


001' 7 4


Au,90; Ag, 10.


4 641


00238


Al, 94; Ag, 6.


2 904


00381


Al, 94; Cu, 6.


8 847


000897


Cu, 97; Al, 3.


46 678


0000


Cu, 84; Mn, 12; Ni, 4.


29 982


000273


Platinoid .


41 731


00031









Dewar and Fleming, Phil. Mag., Vol. XXXVI., p. 271.



INDEX.



Numbers refer to images.



Absorption, of radiation, 112; two
characteristics of, 113.

Accumulator, Kelvin's water-drop-
ping, 179.

Adiabatic lines, I. ,.").

Agonic lines, 325.

Air thermometer, constant volume,
39 ; method of measuring poten-
tial of the, 230.

Alcohol thermometer, 18.

Alternators, 410.

Amalgam, 247.

Ampere, 3:14: the, 341.

Ampere's rule, 330; stand, 345,
363.

Andrews, 76, 85.

Anions, 255.

Arago, 353 ; rotations, 379.

Arc, electric, 291.

Armature, 354; drum, 400; the
(irumine, 403.

Arts, electrolysis in the, 270.

Astatic, pair, Xobili's, 337; mirror
galvanometer, 337.

Athermanous, 112.

Atomic heat, 50.

Attraction, and repulsion, 152;
due to induction, 173.

Aurora, the, 232.

Ay rton- Mather, 339.

Bacon. 2.



Balance, Coulomb's torsion, 161:

Kelvin, 349.
Barlow's wheel, 347.
Battery, Grove's gas, 266; voltaic,

236.

Bichromate cell, 246.
Bidwell, 314, 321.
Boiling point, 15, 69; effect of

pressure on, 69.
Bosanquet, 363.
Bottomley's experiment on rege-

lation, 60.
Boutigny, 72.
Boyle, 2; thermometer, 19; law,

37 ; and Charles' laws combined,

38; laws, deduction of, 145, 153.
Bridge, Wheatstone's, 279.
Budde, 72.
Bunsen, 59; cell, 245.

Caloric, 2.

Calorie, 42.

Calorimetry, 42.

Capacity, definition of, 201; of

insulated sphere, 201 ; of two

concentric spheres, 204 ; of two

parallel plates, 205.
Carbon, filament, 292; specific

heat of, 48.
Carnot's cycle, 136; reversibility

of engine, 139.
Cathode, 255 ; .rays, 394.



438



INDEX.



Cations, 255.

Cautery, electric, 290.

Cavendish, 218.

Cell, bichromate, 246; Bunsen,
245 ; chemical action in Daniell,
242; Clark standard, 251; cop-
per oxide, 249; Daniell, 241,
253 ; data relating to, 252 ; grav-
ity, 243; Leclanche, 248;
Plante's storage, 267 ; reversi-
bility of Daniell, 262; silver
chloride, 250; effect of heat on,
253.

Cells, in multiple series, 283; in
parallel, 282 ; in series, 280.

Celsius, 16.

Change of volume during fusion,
56.

Charge, distribution of, 159; ex-
ternal, 158; redistribution of,
160; residual of Leyden jar,
207.

Charged sphere, force outside of,
165.

Charges, equal and of opposite
sign, 156.

Charles, law of, 20.

Chemical action in relation to
energy, 244.

Choking coils, 414, 416.

Circular coil, intensity of field at
centre of, 333.

Clark standard cell, 252.

Clausius, 141, 147, 268.

Coefficient, of elasticity of a gas,
131; of thermal conductivity,
94.

Coefficients, of dilatation and
pressure, table of, 37 ; of length
and volume, relation between,
26.



Coil, the induction, 388.

Coils, choking, 414, 416.

Cold due to evaporation, 76.

Comparator, interferential, 29.

Concentric spheres, capacity of
two, 204.

Condensation, effect of electrifica-
tion on, 225.

Condensers, 202 ; capacity of, 202 ;
connected in series, energy of,
211 ; energy expended in charg-
ing, 209.

Conduction, by gases, 98; by
liquids, 99; by solids, 91; in
wood and crystals, 97.

Conductivities, comparison of
thermal and electrical, 95 ; table
of, 96.

Conductivity, coefficient of ther-
mal, 94; electrical, 276.

Conductor, equilibrium of a, 191.

Conductors and insulators, 154;
distinction between, 214.

Consequent poles, 312.

Constant, Verdet's, 431.

Convection, by hydrogen, 102 ; cur-
rents, 350; electric, of heat,
302 ; electrolytic, 269 ; in gases,
100; in liquids, 99.

Cooling, Newton's law of, 124.

Copper, oxide cell, 249; voltam-
eter, 262.

Cores, 354.

Coulomb, 160; the, 341.

Coulomb's law, 168; torsion bal-
ance, 161.

Counter E.M.F. in a circuit, 288.

Critical temperature, 85.

Crookes tubes, 392.

Cryolite, 271.

Cubical dilatation of solids, 23.



INDEX.



439



Cuneus, 206.

Current, electromagnetic unit of,
334 ; heating eftect of, 290 ; in-
tensity of, 233; lag of, behind
E.M.F., 411; magnetic relations
of, 329 : through a circular con-
ductor, magnetic field about,
332.

Currents, convection, 350; poly-
phase, 417; steady, 233; theory
of production of, 243; value of
alternating, 412.

Curves of magnetization, 359.

Cycle, Caruot's, 136.

D'Alibard, 224.

Daniell cell, 224 ; chemical action
in, 242.

D'Arsonval galvanometer, 338,
349; Ayrton-Mather form of,
339.

Davy, experiment, 5, 291, 353.

Declination, magnetic, 325 ; varia-
tions in, 326.

Definition of capacity, 201.

Definitions, 118.

Deflections, magnetic forces by
method of, 317.

De la Tour, 84.

Depolarization by chemical means,
241.

Depretz, 56.

Dew, 80; point, 79.

Dewar, -S6, 297, 314.

Diathermancy, of gases, 115; of
liquids, 114.

Diathermanous, 112.

Dielectric, eflect on electric in-
tensity, 221 ; on the forces be-
tween the plates, 221.

Dielectric polarization, 213.



Dielectrics, 155.

Dilatation, of gases, 35; of liquids,
30 ; of solids, the cubical, 23 ; of
water, 33.

Dip, magnetic, 327.

Dipping needle, 327.

Discharge, with impulsive rush,
228 ; with steady strain, 227.

Discharges, in high vacua, 390 ;
oscillatory, 422.

Discovery, Faraday's, 372.

Displacement, electric, 215.

Distribution of charge, 159.

Dry pile, 234.

Dulong and Petit, 50, 124 ; experi-
ments, table of, 47.

Dynamo, and motor, reactions in
field of, 403; compound- wound,
401; ideal simple, 397; over-
compounded, 402.

Earth a magnet, 323.

Ebullition, 65, 67.

Effect, Hall, 351; Peltier, 299;
Thomson, 301.

Efficiency, electrical, of a motor,
408 ; of transmission, 409.

Electric, arc, 291; cautery, 290;


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