ELECTROMETERS. 169 



By substituting figures for the letters in the second 

 member of this equation, the difference of potential, of 

 any two bodies we wish to test, may be expressed 

 arithmetically. , 



I O 7t W C/ 



The expression J ^- is constant ; since it rep- 

 resents the attraction of the disc B for (7, when the 

 Ley den jar is at the constant potential, V: while the 

 expression (d A) is variable; representing the differ- 

 ence of distance, required by the variable difference of 

 potential, expressed by x z. 



THOMSON'S QUADRANT ELECTROMETER. This in- 

 strument, invented by Sir William Thomson, is highly 

 esteemed for its great sensitiveness. It is represented 

 by Fig. 55, and consists of a frame supporting a Leyden 

 jar, which resembles an inverted glass shade, with a 

 brass cover, to which the principal parts are. attached. 



These consist of the idiostatic gauge and replenisher, 

 already described, and the quadrants and needle, and 

 parts connected with them. 



The jar contains strong sulphuric acid : which forms 

 the inner coating, keeps the interior free from moist- 

 ure, and forms a perfect connection with the needle, 

 without friction. The outer coating consists of strips 

 of tin-foil, connected with the cover and supporting 

 frame. The upper part of the jar incloses the quad- 

 rants and needle ; protecting the needle from currents 

 of air, and permitting its movements to be seen. 



Fig. 56 is an enlarged view of the needle and quad- 

 rants. The needle is a thin, flat piece of aluminium, 

 shaped like a figure 8 ; represented by the dotted lines 

 in Fig. 56 ; and seen edgewise in its place at w, in Fig. 

 55. Through its center passes a piece of stout platinum 

 wire to which it is attached, and which terminates 



