88 



STATIC ELECTRICITY 



downwards through holes in the brass plate so large that the rods 

 do not touch the sides. The rods turn outwards, ending in binding 

 screws. A third vertical brass rod passes through an insulating 

 block in the brass plate, and can turn about its own axis so as to 

 bring a horizontal wire at its upper end in contact with a platinum 

 wire which is attached to the needle, and passes vertically down 



FIG. 66. 



into the acid in the jar. When the jar is to be charged the 

 contact is made and the jar may be charged by two or three sparks 

 from an electrophorus cover. The contact is then broken and the 

 horizontal wire turned well out of the way so that the needk 

 free to move. 



The instrument is contained in a glass case resting on the brass 

 plate, which is itself supported on levelling screws. The glass 

 should be lined with tinfoil except where the indicating beam of 

 light passes, and the case should be put to earth by a wire going to 

 the gas or water pipes. The instrument is thus completely secured 

 from external electrifications. 



The general principle of its action is simple. To begin with, 

 the needle should be adjusted in its zero position so that it- 

 median line is parallel to one of the lines of separation of the 

 quadrants. If now one opposite pair of quadrants is connected 

 by the outside terminal to earth, and the other pair to a body 

 of a potential differing from that of the earth, the needle, being 

 at the potential of the jar, which we may suppose positive, is 

 itself positively charged and tends to move so as to carry its 

 charge to the region of lowest potential. If the outside body 



