414 



ELECTRICITY. 



I leci 1 



cnJ of a lever of 7 or ft lines, does not exceed the 



'x~. of a grain, and therefore when the whole os- 

 tK) 



dilation* are not greater than 40 or GO, the force of tor- 



quanlity """ lhat k 



Detcriptive 

 ricitjr. 



On thedis- 

 dpMfai !! 

 electricity. 



HOB 



ran h. . rccptible influence on the duration of 



' ii . and consequently on the results of the 



experiment^. 



The preceding method may be applied without any 

 > to tin- determination of the repulsive forces of si- 

 milarly electrified bodies; but the method already adopt- 

 ed by the use of the torsion balance is much more 

 simple and exact. 



SECT. X. OH the Dissipation of Electricity. 



WHEN any electrified body is insulated in the most 

 perfect manner, the quantity of electricity which it 

 possesses is found to suffer a gradual, and sometimes 

 a rapid diminution, till the whole of its electricity is 

 completely dissipated. It becomes a matter of the great- 

 est importance, therefore, to determine the law according 

 to which this dissipation takes place, and we are fortu- 

 nately able to lay before our readers a series of beautiful 

 experiments which have been made by Coulomb, by 

 means of the very delicate apparatus which we have al- 

 ready described. 



When any insulated electrified body dissipates its 

 electricity, the effect is produced by three causes; 1. By 

 the conducting power of the air which surrounds the 

 electrified body ; 2. By the deposition of humidity on 

 the surfaces of the electrics, which are employed to in- 

 sulate the electric body ; and, 3. By the imperfect in- 

 Kulation produced by the best non-conductors. The 

 last of these causes has probably the smallest share in 

 the dissipation of the electric fluid, and cannot easily be 

 :ted separately from the rest. 



Fn measuring the dissipation produced by the two 

 first causes, the reader must see, tnat there is some dif- 

 ficulty in separating the effects of the one cause from 

 those of the other ; for as the electrified body must al- 

 ways IK' insulated by imperfect conductors, the real 

 quantity of electricity dissipated during a given time 

 imi.-t he owing to both causes. Coulomb, whose inven- 

 tive genius overcame every difficulty, has succeeded 

 completely in separating the two effects. He saw, that 

 if the portion of the surface of the electrified body, 

 which communicated with the insulating support, was 

 made extremely small, the loss of electricity along this 

 support must be immeasurably minute, in comparison with 

 the In-, which was due to the humidity of the ambient 

 air. 1 le therefore put this idea to the test of experiment, 

 and fount!, that when the electrified body did not pos- 

 sess much electricity, such as a pith ball 5 or 6 lines in 

 diameter, he could insulate it completely by a cylinder 

 of sealing-wax, or gum lac. about half a line in diame- 

 ter, and 18 or 20 lines long; that a very fine silk 

 thread, |>enetrate'.l with malted wax, and covered with 

 wax, so as to form a cylinder -J of a line in diameter, had 



vily 



by the con- 

 tart nl air. 

 PLATE 



,.IV. 



J-'ig- li- 



the same insulating power when it was 5 or G inches 



,|Ual degne of iiiMil.ition could not _^__ 

 Hired by a line llir. ' <> inchi- 



.1 h.iir, or hv a fibre of silk, unless when the air 

 commonly dry, or the electric density ut the ball 

 .k. 

 Dissipation of Electricity by I lie Contact of Air. 



I Living suspended a pith ball similar to that upon On die dii. 

 the needle n g. to a very fine .-ilk fibre, covered with 

 I'.'-wax.'aiid terminated with a cylinder of gum I:- 1 . 1 

 18or^ii;iii,-in diamctcr.Coulomb introduced it through 

 the opening .- in his torsion balance, so as to touch the 

 pith ball ii, as in the former experiments l'\ m< 

 the ((inductor, Fig 1 I. he communicated the -:inie kind 

 of electricity to the two balls, which immediately re- 

 pelled one another to such a distance, that the repul- 

 sive force was equal to the force of torsion. Let us 

 ' suppose that the angular distance of the balls was I' 

 then twisting the thread of suspension through 111'. 

 Coulomb brought the ball back to W. Me next ob- 

 served the in -slant when this bell pointed exactly to 20". 

 As the electricity was dis.-ipating. the balls approached 

 one aiiotlwr some minutes after the operation ; so that, 

 in order to be able to observe always at the same dis- 

 tance of 20", lie tw isted the thread of suspension ,SO by 

 means of the index, and as the force of torsion was di- 

 minished by these 30, the balls repelled a little more 

 than 20'. He then waited the instant when the ball of 

 the needle arrived at 20", and counted, with great ex- 

 actness, the time which elapsed between the two opera- 

 tions. Suppose that this time was three minutes, then 

 it will follow, that at the first observation the distance 

 of the balls was 20, and the repulsive force 1 H + '~'i 

 160 ; that three minutes after, the repulsive force at the 

 same distance of 20, was only 1 10 +20= 130, or was 

 diminished 30, or 10 per minute. Hence as the mean 



160+1 

 force between the two observations was - 



145, and diminished 10 per minute, it follows that 

 the electric force of the two balls diminished j^j- per 



minute. . _ 



In this way Coulomb constructed the following la- 

 ble, which contains the observations made on the 2 

 May, the 29th May, the 22<1 June, and the 2d .Inly, 

 which he selected from a multitude of others, as on th 

 four days the variation of the humidity of the i 

 very great, while its temperature was very nearly t 

 same. 



In this Table, the first column represents the instant 

 of observation; the second, the distance of 

 the third, the degree of torsion given by the mtcromt 

 ter; the fourth, the time which elapsed bctv 

 consecutive observations ; \\wjiflh, the loss of the e 

 trie force during that time ; the tA, the mean tc 

 of repulsion between the two observations, as mcasurctl 

 by the mean torsion indicated by the micrometer addet 

 to the distance of the two balls ; and the .><;,//,, the rat 

 of the electric force lost in one minute to the total force. 



