M'Clelland — Comparison of Capacities in Electrical Work. 175 



(2) To find the small change in capacity a when cd has been 

 moved into its second position. 

 In first position : — 



100 divisions in 59-0" 

 100 divisions in 59"5" 

 100 divisions in 59-1" 

 Mean, 59-20" 

 In second position : — 



100 divisions in 60-5" 

 100 divisions in 59-8" 

 100 divisions in 60-0" 

 Mean, 60-10" 



Therefore, — ^— = 7^, 



and C = 201 micro-microfarads ; 



a = 3*05 micro-microfarads. 

 Calculating a from the formula 



I 



a = r"' 



21ogli 



r 2 



where I = 8-01 cms., 



Vi = 3-95 cms., 



r 2 = -97 cm., 

 we get a = 2'85 electrostatic units 



= 3*16 micro-microfarads. 

 We get, therefore, 3 -16 by calculation, 



and 3*05 by experiment. 



The method is therefore quite capable of detecting^and measur- 

 ing with considerable accuracy a capacity of 1 micro-microfarad or 

 even less. 



Discussion of the Advantages of the Method. 



It is not necessary to compare this method in detail with the 

 many other methods used in comparing capacities ; it will be 

 sufficient to point out a few leading facts. 



