ELECTRICITY. 



21 



circle : the 360th part of this force, 

 therefore, or less than the 100,000th of 

 a grain, might be estimated by each 

 degree of its angular motion. 



In order to apply to the instrument 

 only such forces as it is capable of 

 measuring, and of collecting at the 

 same time from the different parts of 

 bodies such minute quantities of elec- 

 tricity as are exactly proportional to 

 those with which they are themselves 

 charged, Coulomb employed what he 

 calls a proof plane, which is simply a 

 small circular disc of gilt paper, d, (fig. 

 15,) fixed to the extremity of a very 

 slender cylinder of gum-lac, and thus 

 completely insulated. If we wish, then, 

 to ascertain the proportions in which 

 electricity is distributed on the surfaces 

 or interior of any particular body, we 

 first insulate that body as completely 

 as possible, and impart to it a small 

 quantity of electricity by a spark from 

 the prime conductor. We next touch 

 any of the points on its surface, the 

 electricity of which we may wish to 

 measure, with the little gilt disc, holding 

 it by the other end of its insulating 

 handle ; then carrying the plane to the 

 torsion balance, of which the moveable 

 ball has been previously charged with 

 an electricity of the same kind, we bring 

 it for an instant in contact with the 

 fixed ball. We then withdraw it, and 

 the fixed ball being now electrified in 

 the same manner as the moveable one, 

 repels the latter with a force measured 

 by the angle of torsion, at which the 

 moveable ball stops. While the little 

 plane and the balls of the balance remain 

 the same, the division of the electricity 

 between the little plane and the move- 

 able ball preserves the same uniform 

 proportion ; and thus the repulsive force 

 which results, and which drives off the 

 moveable ball, is proportional to the 

 quantity of electricity with which the 

 little plane is charged. It has been 

 proved, by a series of well-contrived 

 experiments, that this quantity is ex- 

 actly proportional to the quantity of 

 electricity which really exists at the 

 point of the body with which it has 

 been placed in contact. By applying 

 the same test and method of admeasure- 

 ment to various other points of the body 

 we are studying, we may determine the 

 manner in which the electricity is dis- 

 tributed in all its parts ; for the method 

 is applicable even to the interior of the 

 body, if we pierce it with a small hole 

 terminating at the part whose electricity 



we wish to examine, and pass the proof 

 plane into it till it is applied to the 

 bottom of the aperture. Care must be 

 taken, however, in conducting these last 

 experiments, that the proof plane be not 

 suffered to touch any other part of the 

 body except that of which the electricity 

 is to be determined, and not even the 

 sides of the aperture through which it 

 is introduced, as such contact would 

 entirely falsify the result. 



The following are among the princi- 

 pal results of these investigations : 



(77.) In a solid body having the form 

 of a perfect sphere, and charged with 

 positive electricity, the whole of the 

 fluid is, in consequence of the repulsion 

 of its own particles, which is every- 

 where directed from the centre out- 

 wards, accumulated in a thin stratum 

 at the very surface of the sphere. If the 

 body be charged with negative elec- 

 tricity, the deficiency of fluid will take 

 place only in the superficial stratum of 

 matter. 



(78.) If, instead of being spherical, 

 the body have any other form, the elec- 

 tricity will still be chiefly confined to 

 the surface ; and if it have an elongated 

 form, there will be a greater charge in 

 the remoter parts than in those nearer 

 to the middle. 



(79.) This result of theory, respecting 

 the limitation of electricity to the mere 

 surface, is confirmed in the most deci- 

 sive manner by the experiments of 

 Coulomb. A conducting body of the 

 form represented by the section, /g. 16, 



Fig. 16. 



/ ' . I3||\ 



^ tll^ 



had small pits made in various parts of 

 its surface. They were half an inch in 

 diameter, and some of the most shallow 

 were not depressed more than one- tenth 

 of an inch below the surface. When 

 the body was electrified, and the small 

 proof plane applied in accurate contact 

 to the bottom of these pits and depres- 

 sions, care being taken that it should 

 not touch their margin, and then applied 

 to the electrometer, no indication of 

 its having received any electricity could 

 be perceived ; whereas the contact of 

 the same proof plane with any part of 

 the even surface showed the latter to be 

 strongly electrified. . 



