148] ELECTROMETERS. 285 



F 2 =F 3 = F, F 1 = 0, 



< = -F 2 



2 V ' 



and the deflection is proportional to the square, instead of to the 

 first power of the potential. This method does not show the sign 

 of V like the former. 



If the electrometer is not in perfect adjustment, we use the 

 more general form 



<> = A0 = ic^ 2 - ic 2 F 2 2 + i(d - c 2 )F 3 2 - c 1 F 1 F 3 + c 2 F 2 F 3 . 



In order to be able to adjust c x and c 2 to equality, two of the 

 quadrants should be capable of motion toward or from the center, 

 one roughly, the other micrometrically, so that the amount of 

 surface of the needle covered by them may be varied. In order 

 to make the adjustment, we may first put both pairs of quadrants 

 to earth and observe the deflection when the needle, originally to 

 earth, is charged. Calling this Q , we have 



which shows whether d or c 2 is greater. We may then adjust 

 until there is no deflection, however the needle is charged. 



If a high potential is not available for F 3 , we may conveniently 

 proceed as follows : 



By means of a voltaic battery and two commutators, we may 

 charge either of the quadrant-pairs to a given potential F either 

 positive or negative, the other quadrant-pair being to earth. We 

 thus have four combinations, as follows : 



e v, F 2 



reverse commutator A 

 -V Oh D 



reverse B 



_ v 



e t o F j reveree 



The deflections are given by 



i - c 2 )F 3 2 + Cl FF 3 , 

 l - c 2 )F 3 2 - c 2 FF 3 , 

 I - c 2 )F 3 2 + c 2 FF 3 ; 



