A DIRKCT COMPARISON OF ELECTROSTATIC 



In this expression the only quantities which must be determined in 

 absolute measure are the resistances. The other quantities which must be 

 measured are the ratios of the radius of the disk to its distance from the 

 fixed disk, and the ratio of the radius of the coils to the distance between 

 them. These ratios and the number of windings in the coils are of course 

 abstract numbers. 



In the experiments, 



n = 144 n' = 121 



a = 2-0977 inches. a = I '934 inch. 



To determine a', the circumference of every layer of the coils was measured 

 with watch-spring, the thickness of which was '008 inch. 



One turn of the micrometer-screw was found by Mr Hockin to be equal 

 to "0202 inch. If TO is the micrometer-reading in terms of the screw, 



b = m -1270, &' = w + 26-31. 



In terms of the micrometer measure we have for a and a', 



a=103'85 turns, a' = 95 75 turns. 



The resistances were determined by Mr Hockin as follows : 



R=l 102 000 Ohms. 

 G= 46 220 



The experiments were made for two days, using a small battery charged 

 with bichromate of potash. The current due to this battery was found to 

 diminish so rapidly that a set of Grove's cells was used on the third day, 

 which was found to be more constant than the great battery. A proper com- 

 bination of the two batteries would perhaps produce a current which would 

 diminish according to the same law as that of the great battery. Another 

 difficulty arose from the fact that when the connexions were made, but before 

 the key was pressed, if the micrometer was touched by the hand the disk was 

 attracted. This I have not been able satisfactorily to account for, except by 

 leakage of electricity from the great battery through the floor. When the 

 micrometer was not touched, the disk remained at its proper zero. In certain 

 experiments I kept my hand always on the micrometer in order to be able 

 to adjust it more accurately. These experiments gave a value of v much too 

 small, on account of the additional attraction. When I discovered the attraction, 



