4S8 ELECTRICITY. 



already described in the Tract on Gravitation. By 

 means of this, very minute forces could be accurately 

 measured. Coulomb cut out a small disc or circle of 

 gilt paper, which he attached to a silk thread. This he 

 called the proof plane, and by applying this successive- 

 ly to the various parts of an electrified body, and bring- 

 ing it to his balance, the degree to which the various 

 parts were charged, were very accurately ascertained. 

 The results of these experiments were found to corres- 

 pond very exactly with the theory. Some of the general 

 principles in regard to the manner in which electricity 

 diffuses itself over the surfaces of bodies, thus doubly 

 proved, are as follows. 



(a) Tf a spherical body be charged with electricity, 

 vitreous or resinous, the whole fluid is, in consequence 

 of the repulsion of its own particles, accumulated in a 

 thin, uniform stratum at the surface. 



(b) If the body, instead of being spherical is elonga- 

 ted, then there will be a greater charge toward the ci- 

 trcmities, than near the middle. 



(c) If the elongation of the body is carried as far as 

 possible, i. e. if it assumes a pointed form, the accumu- 

 lation of the fluid at the extremities, will be increased 

 to a very high degree, and the electricity will pass off 

 in a rapid stream. 



(d) The electricity of a charged body, resides at the 

 surface. Coulomb ascertained this by experiment in 

 this way. He caused little holes or pits to be bored in a 

 brass ball, and then after having charged it, he let down 

 his proof plane to the bottom. On withdrawing it, and 

 applying it to his balance it gave no signs of electricity. 



2. Attraction and Repulsion. 



It has been before stated, in the brief notice of the The- 

 ory of Electricity, that a repulsion always exists between 

 bodies similarly electrified, and an attraction between those 

 which are in opposite states. It will also be remembered 

 that, by the process called induction, which has been de- 

 scribed on a previous page, whenever a body electrified in 

 either way, is brought into the vicinity of other bodies Us 

 mere presence electrifies them. An electrified body will 



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