AND ON THE ELECTROMAGNETIC THEORY OF LIGHT. 143 



the current and by the number of lines which it crosses per unit of distance 

 through which it moves, or, in symbols, 



I . (24). 



=^~ b yy 



If the two wires instead of being straight are circular, of radius a', and 

 if V the distance between them is very small compared with the radius, the 

 attraction will be the same as if they were straight, and will be 



C)~rr,' 



F_ ^iTTll , 

 = 2p-y-yy (25). 



When // is not very small compared with a', we must use the equation 



2A 



(3) to calculate the value of -g- by elliptic integrals. 



Making X= Y and comparing with equation (6), we find 



but, by (19), P = I^- 



Hence v = pV (27), 



where v is the electromagnetic ratio and V is the velocity of light. 



But since all the experiments are made in air, for which fi is assumed equal 

 to unity, as the standard medium with which all others are compared, we have 

 finally 



v=V (28), 



or the number of electrostatic units in one electromagnetic unit of electricity is 

 numerically equal to the velocity of light. 



