ELECTRICITY. 



29 



They take place in an equal degree 

 whatever substance be interposed be- 

 tween the bodies which are exerting this 

 action on one another, provided the 

 interposed substance undergoes no 

 change in its own electrical state; a 

 condition which is fulfilled in electrics 

 only. Thus, induction will take place 

 just as effectually through a plate of 

 glass, as if no such substance had inter- 

 vened. 



(104.) Let us now suppose that the 

 acting body, E, is, instead of an electric, 

 a conducting body, a globe of metal, for 

 example, charged with positive electri- 

 city. The primary effects of this globe 

 on the cylinder will be the same as in 

 the former case ; but the electrical state 

 which the globe has induced on the 

 cylinder will re- act upon its own electri- 

 city. The negative electricity, that is, 

 the under-saturated matter at the nearer 

 end of the cylinder N, exerts a tendency 

 to induce positive electricity in the 

 globe, and more especially upon the 

 adjacent side, F : that is, it will tend, by 

 its attraction for the fluid, to draw it io 

 that side, and thus render it still more 

 highly positive than it was before. This 

 can only be done at the expense of the 

 other side, O, from which the fluid must 

 betaken, and which is, therefore.rendered 

 less charged with fluid, that is, less po- 

 sitive than before. But this new distri- 

 butiorj of the electric fluid in the globe, 

 by increasing the positive state of the 

 side, F, next to the cylinder, tends to 

 augment its inductive influence on the 

 fluid in the cylinder ; that is, to drive 

 an additional quantity of fluid from the 

 negative to the positive end. This is 

 followed, in its turn, by a corresponding 

 reaction on the globe, and so on, con- 

 stituting a series of smaller adjustments, 

 until a perfect equilibrium is established 

 in every part. When this has been 

 attained, the electrical states will, it is 

 evident, be of the same kind as those 

 consequent upon the immediate actions, 

 though somewhat increased in intensity 

 by the series of reactions. 



The following experiment is a practi- 

 cal illustration of the preceding reason- 

 ing. Furnish the metallic globe with 

 electroscopes on its opposite surfaces ; 

 when the globe is insulated and alone, 

 any electricity communicated to it will 

 diffuse itself equally over the surface, 

 and both the electroscopes will diverge 

 equally. But no sooner do we bring 

 near to it a conducting body, than the 

 balls of the electroscope at the side most 



distant from that body begin to col- 

 lapse, while those at the nearer side 

 diverge to a greater degree than before ; 

 thus showing the nature of the reflex 

 operation of the induced electricity of 

 the conductor upon the body from 

 which the induction originated. 



(105.) It should be recollected that in 

 all the changes we have thus traced as 

 the effects of induction, there has been 

 no transfer of electricity from either of 

 the bodies to the other ; as was suffi- 

 ciently proved, indeed, by their taking 

 place equally if a plate of glass be inter- 

 posed. Another proof is afforded by 

 the circumstance that the mere removal 

 of the bodies to a distance from one 

 another, is sufficient to restore each of 

 them to their original state. The globe 

 remains as positively electrified as be- 

 fore ; the cylinder returns to its condi- 

 tion of perfect neutrality ; nothing has 

 been lost, and nothing gained on either 

 side. The experiment may be repeated 

 as often as we please, without any varia- 

 tion in the phenomena. But this would 

 not be the case if the cylinder were 

 divided in the middle, and one or both 

 of the parts were removed separately, 

 while they still remained under the in- 

 fluence of the globe. The return of the 

 electric fluid from the positive to the 

 negative end being thus prevented, each 

 part will retain, after its separation, the 

 electricity which had been induced upon 

 it. The nearer portion will remain ne- 



fative ; the remoter portion, positive, 

 f the division had been in three parts, 

 the middle part only would have been 

 neutral. The experiment may be made 

 by joining two or more conductors end- 

 wise, as shown in Jig. 2G, so that they 



Fig. 26. 



-NT: 



may act as a single conductor when 

 placed near to the electrified globe, and 

 after induction has thus been produced, 

 removing them separately, and examin- 

 ing their electrical states. If E be po- 

 sitive, N will be found negative, P po- 

 sitive, and M neutral. 



(106.) Another modification of effect 

 will take place when an insulated con- 

 ductor, rendered electrical at both ends 

 by induction, is made to communicate 

 with another conductor. Let us first 

 suppose that a long metallic conductor 



