ELECTRICITY. 377 



little farther up, terminates in another brass ball B, 

 which should be so high as to touch the prime con- 

 ductor A, which is supposed to stand before the 

 electrical machine. The electrometer consists in 

 a glass rod, F D, cemented to two brass caps, F 

 and D; from the latter of which a strong perpen- 

 dicular brass wire proceeds, the extremity of which 

 comes as high as the centre of the ball B, and is 

 furnished with an horizontal spring socket, through 

 which the wire C E, having the brass ball C at 

 one end, and the open ringE at the other, maybe 

 slided backwards and forwards, so as to set the 

 brass ball C at any required distance from the ball 

 B. This distance, at most, need not be greater 

 than half an inch ; hence the electrometer may be 

 very small. Sometimes small divisions are marked 

 upon the wire C E, which serve to set the balls 

 B and C at a given distance from one another with 

 more readiness and precision. Now, suppose that 

 the jar is set contiguous to the prime conductor, 

 that is, with the ball B touching the conductor; 

 that the ball C be set at one-tenth of an inch dis- 

 tant from the ball B, and that, by means of a chain, 

 a conducting communication be formed from E to 

 the outside coating of the jar, by a chain a\- in 

 this case, if the electrical machine be put in motion, 

 the jar will be charged; and when the charge is 

 so high as that the electric fluid accumulated within 

 the jar, can leap from the ball B to C, which we 

 have supposed to be one-tenth of an inch asunder, 

 the discharge will take place, a spark appear be- 

 tween the said balls, and the shock pass through 

 the chain x : for the part F D of the electrometer, 

 being of glass generally covered with sealing-wax, 

 is impervious to electricity, consequently the elec- 

 tric fluid has no other way through which it can 



