CHAP, iv.] ELECTRICITY AND MAGNETISM. 



217 



sists of a thin flat piece of metal hung horizontally by a 

 fibre or thin wire, thus charged with, say, + electricity, 

 will be attracted by a - charge, but repelled by a -f 

 charge ; and such attraction or repulsion will be stronger 

 in proportion to these charges, and in proportion to the 

 charge on the needle. Four quadrant -pieces of brass 

 are fixed horizontally below the needle without touching 

 it or one another. Opposite quadrants are joined with 

 fine wires. 



Fig. 1 01 shows a very simple form of the Quadrant 

 Electrometer, as arranged for qualitative experiments. 



Fig. i oi. 



The four quadrants are enclosed within a glass case, and 

 the needle, which carries a light mirror, M, below it, is 

 suspended from a torsion head, C, by a very thin metallic 

 wire, F. It is electrified to a certain potential by being 

 connected, through a wire attached to C, with a charged 



