192 



ELECTROMETRY. 



produced when an electrified sphere of large diameter is made 

 to act on a small disc charged with the opposite electricity, and 

 attached to a needle suspended "by a silk fibre. 



As the distribution of electricity on the sphere is not appreciably 

 altered by that of the disc, the action of the sphere is inversely as 

 square of its distance from the disc ; the time of oscil- 



lation, being inversely as the square root of the force, is then 

 proportional to the distance. 



Fig. i55- 



Fig. 156. 



This method would also furnish absolute measurements if the 

 dimensions of the apparatus were so chosen that the action could 

 be calculated as a function of the electrical masses or of the poten- 

 tials for instance, in the case in which balls of the same diameter 

 are used (802). 



If the experiment itself does not involve great accuracy, as when 

 sparks are produced, very approximate results are obtained by an 

 arrangement resembling that of Coulomb.* The insulated needle 

 is replaced by a conducting needle terminating in a small ball a, and 

 connected with the earth by the wire of suspension ; in the direction 



* MASCART. Traitt d'Elcctricite Statique, Vol. I., p. 52. 



