ELECTROMETERS. 



245 



reading." Let D be the distance of A from c for this adjust- 

 ment. The conductor to be tested is then connected with 

 c, and by moving the plate A the hair is again brought 

 to the sighted position. Let D' be the new distance. If v' 



FIG. 10. 



and v be the potentials of the earth and of the tested 

 conductor, we see at once that 



V'-V=(D'-D) 



or if v, the potential of the earth be taken as zero, which 



