ELECTROMETERS. 173 



charging electrode, used to connect the replenisher with 

 the inner coating of the Ley den jar. One pair of quad- 

 rants, A A f , Fig. 56, is connected with the electrode ?, 

 and the other pair, B B', with the electrode m. 



MODE OF USING THE QUADRANT ELECTROMETER. 

 The Leyden jar is connected with the replenisher by 

 the electrode />, and charged to a certain constant po- 

 tential, F", as indicated by the gauge ; and its constancy 

 maintained during the experiment: and the needle, 

 being connected with its inner coating, has therefore 

 the same constant potential V. 



By means of the electrodes I and m, a connection is 

 then made between the opposite pairs of quadrants, and 

 any two bodies \vhose difference of potential is required ; 

 one of which is usually the earth. Suppose the earth 

 connection to be made with the electrode m; then, if 

 the potential of the other body is higher than that of 

 the earth, the needle will move round from the higher 

 to the lower potential ; that is, from A A' to B B f : but 

 if it is lower, the movement will be from. B B f to A A f : 

 and the difference of potential will be indicated on the 

 scale by the movement of the spot of light, to the right 

 or left from zero ; and may be considered practically 

 correct, within certain limits. In this way the re- 

 quired potentials are compared with the constant poten- 

 tial V; and the results determined in absolute measure. 



In the Helmholtz quadrant electrometer the quad- 

 rants are maintained at the constant potential ; and the 

 bodies whose potential is required are connected with 

 the needle. 



There are various styles of Thomson's electrometers : 

 both of the attracted-disc and quadrant instruments. 

 Some of them are portable, and much simpler than 



