466 PROFESSOR CLERK MAXWELL OX THE ELECTROMAGNETIC FIELD. 
as those of Weber, which expresses the number of electrostatic units of electricity 
which are contained in one electromagnetic unit. 
This velocity is so nearly that of light, that it seems we have strong reason to con- 
clude that light itself (including radiant heat, and other radiations if any) is an electro- 
magnetic disturbance in the form of waves propagated through the electromagnetic field 
according to electromagnetic laws. If so, the agreement between the elasticity of the 
medium as calculated from the rapid alternations of luminous vibrations, and as found 
by the slow processes of electrical experiments, shows how perfect and regular the 
elastic properties of the medium must be when not encumbered with any matter denser 
than air. If the same character of the elasticity is retained in dense transparent bodies, 
it appears that the square of the index of refraction is equal to the product of the 
specific dielectric capacity and the specific magnetic capacity. Conducting media are 
shown to absorb such radiations rapidly, and therefore to be generally opaque. 
The conception of the propagation of transverse magnetic disturbances to the exclu- 
sion of normal ones is distinctly set forth by Professor Faraday * in his “ Thoughts on 
Kay Vibrations.” The electromagnetic theory of light, as proposed by him, is the same 
in substance as that which I have begun to develope in this paper, except that in 1846 
there were no data to calculate the velocity of propagation. 
(21) The general equations are then applied to the calculation of the coefficients of 
mutual induction of two circular currents and the coefficient of self-induction in a coil. 
The want of uniformity of the current in the different parts of the section of a wire at 
the commencement of the current is investigated, I believe for the first time, and the 
consequent correction of the coefficient of self-induction is found. 
These results are applied to the calculation of the self-induction of the coil used in 
the experiments of the Committee of the British Association on Standards of Electric 
Resistance, and the value compared with that deduced from the experiments. 
PART II. — OX ELECTROMAGNETIC INDUCTION. 
Electromagnetic Momentum of a Current. 
(22) We may begin by considering the state of the field in the neighbourhood of an 
electric current. We know that magnetic forces are excited in the field, their direction 
and magnitude depending according to known laws upon the form of the conductor 
carrying the current. When the strength of the current is increased, all the magnetic 
effects are increased in the same proportion. Now, if the magnetic state of the field 
depends on motions of the medium, a certain force must be exerted in order to increase 
or diminish these motions, and when the motions are excited they continue, so that the 
effect of the connexion between the current and the electromagnetic field surrounding 
it, is to endow the current with a kind of momentum, just as the connexion between 
the driving-point of a machine and a fly-wheel endows the driving-point with an addi- 
* Philosophical Magazine, May 1846, or Experimental Researches, iii. p. 447. 
