CHAP, vi.] ELECTRICITY AND MAGNETISM. 



327 



which we at once see is of the nature of a velocity. This 

 velocity occurs in every case in the ratio of the electrostatic to 

 the electromagnetic measure of every unit. It is a definite 

 concrete velocity, and represents that velocity at which two 

 electrified particles must travel along side by side in order that 

 their mutual electromagnetic attraction (considered as equivalent 

 in moving to two parallel currents) shall just equal their mutual 

 electrostatic repulsion, see Art. 337. This velocity, "z/," which 

 is of enormous importance in the electromagnetic theory of light 

 (Art. 390), has been measured in several ways. 



(a) Weber and Kohlrausch measured the electrostatic unit of 

 quantity and compared it with the electromagnetic unit of 

 quantity, and found the ratio v to be = 3*1074 x io 10 centims. 

 per second. 



(b] Sir W. Thomson compared the two units of potential 



and found v = 2-825 x io 10 , 



and later, =2-93 x io 10 . 



(c) Professor Clerk Maxwell balanced a force of electrostatic 

 attraction against one of electromagnetic repulsion, and found 



v =2-88 x io 10 . 



(d] Professors Ayrton and Perry measured the capacity of a 

 condenser electromagnetically by discharging it into a ballistic 

 galvanometer, and electrostatically by calculations from its size, 

 and found 



v 2-980 x io 10 , 



The velocity of light is believed to be 



= 2-9992 x io 10 ; 



or, according to G. Forbes's latest determination, 

 the velocity of red light is =2-9826 x io 10 . 



