244 



ELECTRICAL MEASUREMENTS 



A Sightiug 

 U Device 



of the system of conductors, the relative potentials of its parts 

 and the dielectric coefficient of the medium separating the 

 attracting bodies. 



The Attracted-disc Electrometer. The first suggestion of the 

 attracted-disc electrometer was due to Sir William Snow-Harris, 

 who used an instrument of this sort. Its essential members were 

 a fixed circular plate electrode supported by an insulating stan- 

 dard, and a movable plate electrode hung directly over the fixed 

 plate from the arm of a gravity balance. By putting weights in 

 the scale pan, a balance could be secured and a measure of the 

 electrostatic attraction and consequently of the P.D. between 

 the electrodes obtained. A defect of any such simple arrange- 

 ment, which renders it useless as an ab- 

 solute instrument, is that owing to the 

 influence of the edges of the plates the 

 distribution of the charges over the 

 surfaces will not be uniform. This 

 renders inexact the application of a 

 simple formula for the attraction be- 

 tween the two plates, based on the 

 assumption of a uniform density of dis- 

 tribution of the charge. 



The distribution of the charges over 



FIG. 134. Elements of the central portions of two parallel 

 the Kelvin absolute elec- i . i ,. 



trometer. plates, whose dimensions are large com- 



pared with their distance apart, will be 



practically uniform. Therefore, if the force exerted on the cen- 

 tral portion of one of the plates is measured, the use of a for- 

 mula which assumes a uniform distribution will be legitimate. 



Lord Kelvin secured a practically uniform distribution by the 

 use of the guard ring. This is a broad ring closely surrounding, 

 but not touching, the movable member and in electrical connec- 

 tion with it. The stationary, or attracting plate, has the same 

 diameter as the guard ring. 



Absolute electrostatic instruments are not of industrial import- 

 ance; however, the application of the guard-ring principle will be 

 illustrated by the Kelvin absolute electrometer, the elements of 

 which are shown in Fig. 134. 



In this instrument the attraction on the movable member is 



