# Page:A Dynamical Theory of the Electromagnetic Field.pdf/41

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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.

This wave consists entirely of magnetic disturbances, the direction of magnetization being in the plane of the wave. No magnetic disturbance whose direction of magnetization is not in the plane of the wave can be propagated as a plane wave at all.

Hence magnetic disturbances propagated through the electromagnetic field agree with light in this, that the disturbance at any point is transverse to the direction of propagation, and such waves may have all the properties of polarized light.

(96) The only medium in which experiments have been made to determine the value of ${\displaystyle k}$ is air, in which ${\displaystyle \mu =1}$, and therefore, by equation (46),

 ${\displaystyle V=v\,}$ (72)

By the electromagnetic experiments of MM. Weber and Kohlrausch[1],

${\displaystyle v}$ = 310,740,000 metres per second

is the number of electrostatic units in one electromagnetic unit of electricity, and this, according to our result, should be equal to the velocity of light in air or vacuum.

The velocity of light in air, by M. Fizeau's[2] experiments, is

V = 314,858,000;

according to the more accurate experiments of M. Foucault[3]

V = 298,000,000.

The velocity of light in the space surrounding the earth, deduced from the coefficient of aberration and the received value of the radius of the earth's orbit, is

V = 308,000,000.

(97) Hence the velocity of light deduced from experiment agrees sufficiently well with the value of ${\displaystyle v}$ deduced from the only set of experiments we as yet possess. The value of ${\displaystyle v}$ was determined by measuring the electromotive force with which a condenser of known capacity was charged, and then discharging the condenser through a galvanometer, so as to measure the quantity of electricity in it in electromagnetic measure. The only use made of light in the experiment was to see the instruments. The value of V found by M. Foucault was obtained by determining the angle through which a revolving mirror turned, while the light reflected from it went and returned along a measured course. No use whatever was made of electricity or magnetism.

The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.

(98) Let us now go back upon the equations in (94), in which the quantities J and ${\displaystyle \Psi }$ occur, to see whether any other kind of disturbance can be propagated through the medium depending on these quantities which disappeared from the final equations.

1. Leipzig Transactions, vol. v. (1857), p. 260, or Poggendorff's 'Annalen,' Aug. 1856, p. 10.
2. Comptes Rendus, vol. xxix. (1849), p. 90.
3. Ibid. vol. lv. (1862), pp. 501, 792.