284 ELECTROSTATICS. 



If the apparatus is symmetrical 



10 rf 

 OO 



[PT. II. CH. VI. 



In the quadrant electrometer, the box AB is a flat circular 

 box divided into four quadrantal sectors, 

 connected alternately to form the two 

 conductors A and B. 



The conductor C, called the needle, 

 is of the shape shown in the figure, and 

 rotates about its center, the angle of 

 rotation being the coordinate 6. The 

 couple tending to produce rotation is 



FIG. 61. 



where 



c^F^+itoas + fe-cOflJF, 8 



r i F 2 + (a 13 - c.0) F! F 3 + (a 23 + c 2 6>) F 2 F 3 , 

 giving 



<T) JLf>~F7"2 l/iT 7 '2_4_jL//> /^M^2 />T7"T7"_i_rl7"T/ r 



^ 2^1 r l 2 L/ 2'2i2 V^l t/ '3 Kl r l r *iVB'i'8 



If the electrometer is correctly constructed and adjusted Cj = c 2 

 and 



The needle is usually suspended either by a torsion fibre, or 

 by a bifilar suspension, so that the force of restitution <I> is 

 proportional to the deflexion, the factor of proportionality being 

 denoted by A. 



In the usual method of use, the potential F 3 of the needle 

 is made large in comparison with V 1 and F 2 . We may then 

 neglect the second term in the brackets, and the deflection is 

 proportional to F 2 Fj. This is called the heterostatic method 

 of use, the needle being charged by an extraneous source of 

 potential. 



In the idiostatic method, the needle is put into connection 

 with one pair of quadrants, which are put at the potential to 

 be measured, the second pair of quadrants being to earth. Then 



