32 



ELECTRICITY. 



redundant matter in N will, by its at- 

 traction for the fluid, draw more of it 

 into the upper plate, which will be sup- 

 plied from the conductor of the ma- 

 chine through the wire M ; and such 

 an additional quantity will be accumu- 

 lated in P, as will balance the increased 

 attraction of the matter inN, and main- 

 tain it at the same intensity as the fluid 

 in the prime conductor. That this is 

 what really happens will be rendered 

 evident by placing an electroscope upon 

 the prime conductor ; for the moment 

 the plate N communicates with the 

 ground, the balls of the electroscope 

 collapse, showing that the intensity of 

 the fluid in the prime conductor is sud- 

 denly reduced by the great quantity 

 that has been absorbed by the plate P. 

 The machine must now again be set in 

 motion, in order to supply the electri- 

 city which has been thus abstracted 

 from the conductor. The operation of 

 each plate on the other may be con- 

 sidered as that of increasing its electri- 

 cal capacity, or of rendering a large 

 proportion of its electricity latent or 

 disguised. 



(115.) It is evident that the quantity 

 of electric fluid driven out of the lower 

 plate by the action of the fluid in the 

 upper one, can never be quite equal to 

 that of the fluid with which the upper 

 one is itself charged, and the difference 

 will be greater in proportion to the dis- 

 tance of the plates. When they are 

 very close to each other, these two quan- 

 tities approach very near to an equality ; 

 and this circumstance it was that mis- 

 led Franklin into the belief that they 

 were actually equal. 



(116.) The capacity for accumula- 

 ting electricity corresponding to a given 

 intensity in the upper plate depends 

 upon the distance between the plates, 

 provided always that the intervening 

 electric opposes a sufficient obstacle to 

 the direct transfer of the electricity 

 from the one to the other; and is in 

 some inverse ratio to that distance. 

 The lower plate, N, which communi- 

 cates with the ground by the wire W, 

 although strongly negative, is rendered, 

 by the vicinity of the fluid in the upper 

 plate P, neutral with respect to fluid in 

 the wire W : that is, the attraction of 

 its unsaturated matter, although nearer, 

 is exactly balanced by the repulsion of 

 the redundant fluid in the upper plate, 

 which, although really stronger, is, from 

 the greater distance at which it acts, 

 only equal to the former. With refer- 



ence to fluid in the wire M, however, 

 the action of the redundant fluid in P, 

 is not balanced by that of the unsatu- 

 rated matter in N, which latter is both 

 weaker in itself and more distant. Thus, 

 while N is neutral with respect to the 

 conductors which touch it, P is in a 

 slight degree active, in consequence of 

 this small preponderance of force, and 

 a portion of its fluid tends to escape. 

 Hence, if N be again insulated, by 

 removing the wire W, and the wire M 

 be now made to communicate with the 

 ground, this portion of the fluid in P 

 will pass off by it ; but not any larger 

 quantity, for the remaining portion is 

 retained by the attraction of the unsa- 

 turated matter in N. P is, by this loss, 

 rendered neutral, as N had before been, 

 and it now no longer acts on the fluid 

 beyond it in M. The influence of P on 

 that fluid is greater than that of N in 

 respect to its greater vicinity, but less 

 in as far as regards the intensity of 

 action, and the compensation is exact, 

 feut under these circumstances, N, 

 which was before neutral, becomes in 

 its turn active, and now that the repul- 

 sion of the fluid in P is diminished, will 

 absorb a certain quantity of the fluid as 

 soon as it is touched by W, after P has 

 been again insulated. By this contact, 

 N is again restored to the neutral state, 

 a fresh portion of fluid in P is released 

 from the attraction of N, and P is again 

 active. By repeating these alternate 

 contacts a sufficient number of times, 

 we gradually deprive the plates of their 

 whole charge of electricity ; alternately 

 imparting small portions to the negative 

 plate, and taking away the like portions 

 from the positive one, until they are 

 both brought to their natural unelectri- 

 fied state. The quantities of fluid wh.ch 

 are thus successively added and ab- 

 stracted were found, by the calculations 

 of Laplace, to be in geometrical pro- 

 gression. 



(117.) The most convenient mode of 

 obtaining the accumulated electricity 

 arising from induction is by the employ- 

 ment of coated glass, that is, of a plate of 

 glass, on each side of which is pasted a 

 sheet or coating of tin- toil. Care must 

 be taken to leave a sufficient margin of 

 glass uncovered by the metal, for pre- 

 venting the transfer of electricity from 

 the one coating to the other round the 

 edge of the glass ; and all sharp angles, 

 or ragged edges in the coatings, should 

 be avoided, as they have a great tendency 

 to dissipate the charge. 



