AND ON THE ELECTEOMAGNETIC THEOEY OE LIGHT. 
651 
For let two oppositely electrified bodies A and B travel along the line joining them 
with equal velocities in the direction AB, then if either the potential or the attraction 
of the bodies at a given time is that due to their position at some former time (as these 
authors suppose), B, the foremost body, will attract A forwards more than A attracts B 
backwards. 
Now let A and B be kept asunder by a rigid rod. 
The combined system, if set in motion in the direction AB, will pull in that direction 
with a force which may either continually augment the velocity, or may be used as an 
inexhaustible source of energy. 
I think that these remarkable deductions from the latest developments of Weber and 
Neumann’s theory can only be avoided by recognizing the action of a medium in electrical 
phenomena. 
The statement of the electromagnetic theory of light in my former paper was con- 
nected with several other electromagnetic investigations, and was therefore not easily 
understood when taken by itself. I propose, therefore, to state it in what I think the 
simplest form, deducing it from admitted facts, and showing the connexion between the 
experiments already described and those which determine the velocity of light. 
The connexion of electromagnetic phenomena may be stated in the following manner. 
Theorem A. — If a closed curve be drawn embracing an electric current, then the 
integral of the magnetic intensity taken round the closed curve is equal to the current 
multiplied by 4 ir. 
The integral of the magnetic intensity may be otherwise defined as the work done on 
a unit magnetic pole carried completely round the closed curve. 
This well-known theorem gives us the means of discovering the position and magni- 
tude of electric currents, when we can ascertain the distribution of magnetic force in 
the field. It follows directly from the discovery of (Ersted. 
Theorem B. — If a conducting circuit embraces a number of lines of magnetic force, 
and if, from any cause whatever, the number of these lines is diminished, an electromotive 
force will act round the circuit, the total amount of which will be equal to the decre- 
ment of the number of lines of magnetic force in unit of time. 
The number of lines of magnetic force may be otherwise defined as the integral of the 
magnetic intensity resolved perpendicular to a surface, multiplied by the element of 
surface, and by the coefficient of magnetic induction, the integration being extended 
over any surface bounded by the conducting circuit. 
This theorem is due to Faraday, as the discoverer both of the facts and of this mode 
of expressing them, which I think the simplest and most comprehensive. 
Theorem C. — When a dielectric is acted on by electromotive force it experiences what 
we may call electric polarization. If the direction of the electromotive force is called 
positive, and if we suppose the dielectric bounded by two conductors, A on the negative, 
and B on the positive side, then the surface of the conductor A is positively electrified, 
and that of B negatively. If we admit that the energy of the system so electrified resides 
4 u 2 
