POTENTIAL AND CAPACITY 



93 



V = d 



(2). 



It is very difficult to measure d accurately, and it is better to 

 make the results depend on the differences of the distance in two 

 successive experiments, that is, on the distance the lower plate is 

 moved. To do this let the lower plate be connected to a source of 



uparvd 



metfrscrtw. 



FIG. 67. 



potential V, the upper plate being connected to earth ; then 

 equation (2) holds. 



Now connect the upper plate to a source at potential V', the 

 lower plate being connected as before. If the distance, after ad- 

 justment, is now d\ 



V- V" 



(3). 



Subtracting (3) from (2), 



V =(<*-<*') 



which gives V in terms of measurable quantities. It may be noted 

 that in obtaining (4) we suppose V either less than V or opposite 

 in >ign. 



I another form of the instrument the attracted disc is 

 hung from one arm of a delicate balance and counterpoised so that 

 when there is no electrification it hangs exactly in the plane of the 

 guard-ring. A stop is then arranged so that the disc cannot move 

 upwards and an extra weight P is put on the other arm. The 

 subsequent working is as with the arrangement just described. 



