Chap. VIII Of Electric Attraction and Repulsion
No satisfactory theory of electric attraction and repulsion has, so far as our knowledge extends, ever yet been given. The phenomena have been differently accounted for, as the writers have embraced different opinions in regard to positive and negative electricity. One mode of explanation has been adopted by those who believe, with Franklin, that positive electricity is only an accumulation of the electric fluid in a body beyond its natural state; and that negative electricity is nothing more than a deficiency of this fluid in a body. Another mode of explanation is given by those who maintain, in opposition to Franklin, that positive and negative electricity are either two distinct fluids, or else vibrations of the same fluid—the positive electricity always rushing out of a body, and the negative always rushing in. Those who maintain this hypothesis endeavour to support it by the easy solution which they affirm it gives to the phenomena of electric attraction and repulsion. But after a careful examination of this theory, we think that, so far from being satisfactory, it is scarcely intelligible. We therefore do not choose to introduce it into our epitome, as affording any solution of the difficulties that occur on this part of our subject. We are besides of opinion that the evidence in favour of a single fluid is conclusive, as we shall show when we come to discuss the theory of electricity. Yet we confess that we cannot, on this theory, offer a rationale of electric attraction and repulsion, that satisfies ourselves. It is therefore the demand of candour, and in the spirit of the Newtonian philosophy, to avow explicitly that this part of our subject is yet involved in much obscurity. In the mean time we are acquainted with certain facts, and with the clear explanation which they give of certain phenomena.
1. That bodies positively electrified, repel each other.
2. That bodies negatively electrified, also, repel each other.
3. That bodies positively electrified, attract those which are negatively electrified.
4. That bodies either positively or negatively electrified, induce a contrary electricity in bodies in their natural state, brought within the sphere of their action.
This statement is easily verified by experiment, in the following manner.—By flaxen or hempen threads, suspend, from the prime conductor, two balls made of cork or elder-pith, so that they touch each other. On charging the conductor, these balls, being both electrified positively, will immediately repel each other, and be separated to a considerable distance.—Remove one of the balls, take it in your fingers, and bring it near to the one which remains positively electrified, and the two will immediately rush together; because there are now two substances of which one is electrified positively, and the other negatively.—Again. Suspend two balls, of the kind just mentioned, from an insulated cushion of an electric machine, and let them touch each other. Put the machine in motion and the balls, which are now both electrified negatively, will repel each other and separate, as in the case first described.
In attempting to explain the first of these phenomena Dr. Franklin once supposed that there was an electric atmosphere round each of the balls positively electrified, the particles of which atmosphere, by mutually repelling each other, separated the balls. He also supposed that as bodies negatively electrified, or not having their proportional quantity of the electric fluid, are always strongly disposed to receive it, this would account for the fact that when one of these bodies was brought near to one that had more than its proportional quantity, the two would naturally rush together; the one to impart, and the other to receive the fluid. But at this time he was not acquainted with the fact, that two bodies negatively electrified would repel each other. When this was discovered he candidly acknowledged the utter deficiency of his theory, in regard to electric attraction and repulsion. Some of his friends and followers, however, have endeavoured still to maintain it. But we think that though their zeal has been greater, their success has not exceeded that of the Doctor himself: and we have already stated that other theories are equally, if not more defective, than that of Franklin. Let us then leave the explanation of electric attraction and repulsion to be made when future and fortunate discoveries shall have furnished the means of making it, and let us proceed with the application of known facts and principles.
A pleasing exhibition of the phenomena of electric attraction and repulsion, may be made in the following manner.
Take a glass tube, and after having rubbed it, let a small light feather fall from your fingers, at the distance of eight or nine inches from it.—The feather will be immediately attracted by the tube and stick very close to its surface for some seconds, after which it will be repelled, and if the tube be kept under it, the feather will continue floating in the air, at a considerable distance from the tube, without coming near it again, except it touch some conducting substance; and if you manage the tube dexterously, you may drive the feather through the air of the room at pleasure.
The cause of this phenomenon is obvious. The feather, at first, not being electrified, rushes to the excited tube. There it becomes electrified and is then repelled, and cannot approach the tube again, unless it first touch some conducting substance; because it cannot part with its electricity while floating in the air, and therefore cannot acquire a contrary electricity; consequently it must remain in a state incapable of being again attracted by the excited tube.
There is a remarkable circumstance attending this experiment, which is, that if the feather be kept at a distance from the tube by the force of electric repulsion it always presents the same part towards the tube. The reason of this phenomenon is, that the equilibrium of the fluid in the different parts of the feather being once disturbed cannot easily be restored; the feather being an electric, or at least a very bad conductor. When the feather has acquired a quantity of electricity from the tube it is plain that, by the action of the excited tube, that superinduced electricity will, for the most part be forced to that side of the feather which, at first, happened to be farthest from the tube; hence that part will always afterwards be repelled the farthest.
This experiment may be agreeably varied in the following manner.—A person may hold an excited tube of glass, within a foot and a half of a stick of sealing-wax, or any other electric negatively electrified, held by another person; a feather let fall between these differently excited electrics will leap from one to the other alternately, and the two persons will seem to drive a shuttlecock by the force of electricity.
Another experiment calculated to shew the phenomena of electric attraction and repulsion is the electric spider.
Cut a piece of cork in the shape of a spider, and run a few short threads through it, to represent the legs; this done, suspend it by a silk thread from the ceiling of the room, or any other support, so that the spider may hang mid-way between the knob of a jar and the knob of a wire fastened to the table, or to the outside coating of the jar when not charged; let the place where the jar stands be marked; then charge and replace it. The spider will now begin to move from knob to knob, and continue this motion for a considerable time.
In this case, the knob of the jar is charged positively, and the spider, being in its natural state, is attracted by it; the knob then communicates to it some of its electricity, and the spider becoming possessed of the same electricity with the knob, is repelled by it, and immediately runs to the other knob, which communicates with the negative coating, or with the table, where it discharges its electricity and is again attracted by the knob of the jar. This attraction and repulsion continue till the jar is discharged, when the spider finishes its motion and seemingly expires.