﻿384 Theory of the Behaviour of the Quadrant Electrometer. 



the left, but when Y 3 was = 4-148 volts it was 15 millimetres 

 in the same direction. The instrument was surrounded by 

 an earthed metallic box to screen it from external influences. 

 In the experiment, the result of which is given in fig. 2, the 

 deflexions, which are measured from the resting point in 

 each case, were not exactly equal but always differed by a 

 small quantity. When, for example, V 3 =+100 volts the 

 deflexion was 150 mm. to the left, but when V 3 = —100 volts 

 it was 148 mm. to the right. The first result is plainly in- 

 consistent with the formula 



6 = 



2«V 3 (V 3 -^)-2 7 *V 3 2 



which, when Y 2 vanishes, should give the same value of 6 

 whether V 3 is positive or negative, provided its numerical 

 value is not altered. 



As a matter of fact, there is another effect quite as im- 

 portant as the existence of 7 which, I believe, is always 

 present. It is certainly present in the Dolezalek electro- 

 meters in my possession, and in one of them it is very 

 marked. This effect is due to the existence of a contact- 

 difference of potential between the two pairs of quadrants. 

 In other words, when the needle and both pairs of quadrants 

 are earthed there is still a difference of potential between the 

 air in contact with one pair of quadrants and the air in 

 contact with the other pair. Suppose this difference to be r. 

 Then, denoting by V 3 the potential of the air in contact with 

 the needle, we have 



0a = 



(v 3 -0-2 7 «V 3 * 



for the deflexion when both quadrants are earthed. Here 6 

 is measured from the position which the needle would occupy 

 if both V 3 and r were zero. If V 3 be made negative without 

 changing its numerical value, which it may be remarked 

 cannot be done by connecting the needle with the negative 

 pole of the battery instead of with the positive, as there is 

 always a difference of potential between the needle and the 

 air in contact with it, we get for the deflexion 



2a ^_V 3 -|)-2 y ^ 



0o- 



