134 HEXEY A. EOWLAXD 



where fc = ^-i^jL^, and F(Jc) and E(k) are complete elliptic 

 V c? + ( C? + 0) 



integrals of the second and first orders respectively. 



The determination of the potential was by means of the spark which 

 Thomson has experimented on in absolute measure. For sparks of 

 length I between two surfaces nearly plane, we have on the centimetre, 

 gram, second system, from Thomson's experiments, 



V- V = 117-5 (1 + . 0135), 



and for two balls of finite radius, we find, by considering the distribu- 

 tion on the two sheets of an hyperboloid of revolution, 



V-V' = 117-5 (I + -0135) 



where r is the ratio of the length of spark to diameter of balls and had 

 in these experiments a value of about 8. In this case 



V V = 109-6 (I + -0135) . (6) 



A battery of nine large jars, each 48- cm. high, contained the store 

 of electricity supplied to the disc, and the difference of potential was 

 determined before and after the experiment by charging a small jar and 

 testing its length of spark. Two determinations were made before and 

 two after each experiment, and the mean taken as representing the 

 potential during the experiment. 



The velocity of the disc was kept constant by observing a governor. 

 The number of revolutions was the same, nearly, as determined by the 

 sizes of the pulleys or the sound of a Seebeck siren attached to the 

 axis of the disc; the secret of this agreement was that the driving cords 

 were well supplied with rosin. The number of revolutions was 61- per 

 second. 



In such a delicate experiment, the disturbing causes, such as the 

 changes of the earth's magnetism, the changing temperature of the 

 room, &c., were so numerous that only on few days could numerical 

 results be obtained, and even then the accuracy could not be great. 

 The centimetre, gram, second system, was used. 



First Series, a = 2-05, & = 9-08, w=-697, Z> = 110-, H -182 

 nearly, 5 = 1-68, /?=-50, (7 = 10-55, N 61-, v = 28,800,000,000-, 

 7Z ' =-0533, C" = 10. 



