ELECTRICITY. 487 



winch is made of glass, in order the more effectually to 

 prevent the escape of the electric fluid from the cylin- 

 der. This is not, however necessary, nor is it even ne- 

 cessary to have the pillars of glass. / R, is the rubber, 

 and IRK, is the silken floss. This cushion or rubber 

 is fastened to a spring, which proceeds from a socket ce- 

 mented on the top of the glass pillars S. The lower part 

 of this pillar is fixed into a small board which slides upon 

 the bottom board of the machine, by means of a 

 screw-nut, and may be fixed more or less forward in order 

 that the rubber may press more or less upon the cylinder. 

 N F is a glass pillar which is fixed in the bottom board, 

 and supports the prime conductor M L, of hollow brass 

 or tin plates, which has the collector or pointed wires at 

 L, and knobbed wire at M. From this brass knob O, a 

 larger spark may be drawn than from any other part of 

 the conductor. But this knobbed wire is only screwed 

 into the conductor, and may be easily removed from it. 



These three articles of electrical apparatus, the Elec- 

 trical Machine, the Prime Conductor, and the Leyden 

 Jar, are the most important instruments used for excit- 

 ing and accumulating the electrical power. We come 

 now to consider our third division. 



HI. EFFECTS PRODUCED BY ELECTRICITY WHEN ACCU- 

 MULATED. 



1. Distribution of electricity over the surfaces of bodies. 



It was early observed that the electric fluid resides on 

 the surface, not within the substance of the electrified 

 body, but the manner in which it distributes itself over 

 the surface was left, strange as it may appear, to be dis- 

 covered by the application of purely mathematical rea- 

 soning. The existence of the fluid, and the repulsion 

 between its particles, were the data in this investigation, 

 and from these, the manner in which it must distribute 

 itself over bodies of every possible variety of form, may 

 be precisely ascertained. In order to determine whether 

 these results of theory should correspond with observed 

 fact, M. Coulomb made use of his celebrated torsion 

 balance, the principles of whose construction have been 



