M'Clelland — Comparison of Capacities in Electrical Work. 369 



increases at first as the potential difference between the plates 

 increases ; but when this potential difference is made sufficiently- 

 great, the current attains a maximum, and does not further increase 

 for further increase of potential difference between the plates. If 

 then B is kept at a sufficiently high potential, small changes in 

 this potential, due to the potential of the battery falling, will pro- 

 duce no effect ; and, again, in making an observation, the potential 

 of A need never change by more than one volt, so that there is no 

 trouble in keeping a constant current to the plate A. The con- 

 stancy of the current in the above arrangement only depends on 

 the constancy of the radiation from the uranium ; and numbers 

 will be given to show how very constant this radiation is. The 

 potential difference required to produce the maximum current to 

 A will depend on the distance between A and B ; but it is well to 

 have 200 volts available. 



C represents one of the condensers being compared ; and E is a 

 quadrant electrometer. D is an insulating block of paraffin, con- 

 taining two mercury cups, a and b, one of which is connected to 

 earth. A connecting piece L is shown, joining the mercury cups, 

 and, by means of a string arrangement, L can be lifted out of the 

 mercury cups and lowered again as desired, from a distance, 

 so as to avoid induction effects produced by movements of the 

 observer. 



As soon as L is lifted out of the mercury cups, the plate A, 

 the condenser C, and the electrometer begin to charge up ; and 

 the time is observed during which the spot of light moves over, 

 say, 100 scale-divisions. An exactly similar experiment is done 

 with a second condenser C in the place of C. If the intervals of 

 time are respectively t and t r , we have 



C + c t 



c + c r 



where c is the capacity of the electrometer, the condenser AB 

 and the connecting wires. The capacity c can be determined in 

 terms of, say, C, by taking an observation with C joined up as 

 shown, and then an observation with C disconnected, so that only 

 c is charged ; or the capacity c may be determined once for all by 

 comparing it in this way with a known capacity. We thus get 

 the ratio C/C. 



