PROVISIONAL TIIEOKY.] ELECTRICITY 17 Fis. 2. ducting support being called the insulator, a name which has on that account been given to non-conductors gene rally. lc . We have remarked above that a neutral pith ball attracts equally the positive and negative balls of the electroscopic needle ; this leads us to re mark, more explicitly than we have hitherto done, that an electrified body in general and in the first instance attracts a neutral or unelec- trified body. The explana tion of this action is that the originally neutral body in presence of the electri tied body becomes itself electrified for the time. It is said to be electrified by induction, and it is very easy to show, by using large bodies, not only that the originally neutral body is actually electrified, but that it is oppositely electrified in different parts. Thus (fig. 2) A and B are two bodies suitably insu lated and placed one above the other. If B be originally neutral, and A be positively electrified, then the lower end of B will be negatively, and the upper end positively electri fied ; as may be easily shown by exploring with a small posi tively electrified pith ball suspended by a dry white silk thread ; the little ball will be attracted towards the lower end of B, and repelled from the upper. If we remove the body A, or, which (as we have seen) amounts to the same thing, connect it with the earth, and so " discharge " its electrification, we shall find that all traces of electrical ac tion in B have disappeared i.e., the small positively elec trified pith ball will be attracted everywhere; and, if we discharge it too, it will neither be attracted nor repelled anywhere. Provisional Theory. Before going further into detail, it will be convenient to give a working theory of electrical phenomena, so far as we have considered them. The use of such a theory at the present stage is to enable us to co-ordinate and classify the results of experiment, and to furnish a few leading principles under which we may group results which appear to be due to a common cause. Such a theory is invaluable as a memoria technicct for experi mental results, and is useful in suggesting directions for experimental inquiry ; but in framing it we must be careful to make it contain as little as possible beyond the results of actual experiment, and in using it we must be on our guard against allowing it to prepossess our minds as to what may be the ultimate explanation of the phenomena we are considering. Following the caution of Coulomb and the example of Sir William Thomson, we shall avoid the use of the term electrical fluid, and substitute instead the more succinct and of less misleading word electricity. We suppose that a body s which exhibits electrical properties (as above defined) has
- n " associated with its mass a certain quantity of something
which, without attempting further definition, we shall call electricity. Of our right to use the word quantity here we shall give experimental justification by-and-by, and then the question of the appropriate unit will (vide infra, " elec tric quantity") be discussed. We may suppose that elec tricity is distributed throughout the whole mass of a body, and speak of electrical " volume density," meaning the quan tity of electricity in an element of volume divided by the element of volume. We shall also speak of an element of elec tricity, meaning the electricity in an element or very small portion of a body. In certain cases we shall find that electricity resides on the surface of a body ; electrical " surface density " then means quantity of electricity on au element of surface divided by the element of surface, and element of electricity the electricity on an element of surface. For shortness, we shall denote positive or vitreous electricity by the mathematical sign + , and resinous or negative electricity by the sign - , remarking that the choice of the signs is arbitrary, and reserving for the pre sent the question of how far we may associate with th6se signs the corresponding mathematical ideas. We shall assume that every element of electricity repels every other element of the same sign, and attracts every other element of opposite sign. The precise law of this force will be investigated further on This force considered as acting on any element of elec tricity we shall call an electric force. In perfectly con ducting substances electricity moves with perfect freedom under any electromotive force, however small. In perfect non-conducting substances electricity will not move under any electromotive force, however great. Any case in nature lies somewhere between these extremes, but into questions of gradation, &c., we do not enter for the present. When the forces due to other electrical elements acting on the electricity jn any element of a body have a resultant, that resultant acts on the element itself, and is called the ponderomotive force, to distinguish it from the electro motive 1 (or electric) force which tends to move + electri city in one direction and - electricity in the opposite direction. When a body is neutral, we shall assume that it contains equal and equally distributed quantities of + and - elec tricity, and we shall further suppose those to be practically unlimited in amount. A + electrified body is then to be conceived as a body which has excess of + electricity and a - electrified body as one which has excess of - electricity. Communication of + electricity to a body is in accordance with this to be regarded as equivalent to the abstraction of an equal amount of - electricity, and conversely. It is easy to see that the above assumptions will explain in a general way the phenomena already described. Thus the + electricity of the electrified pith ball C acting tm the + electricity of the ball A of the electroscopic needle repels it, and this force by our assumption is equally exerted on the matter of A, therefore A tends to move away from C, and will do so as long as it is free to move. The action on the - electrified ball B is similarly explained. Conduction and discharge to earth may be explained in a similar manner. The attraction of an electrified body ( -f- let us sup pose) A on a neutral insulated body B is thus explained. The + electricity on A (fig. 3) attracts the - electri city in B and repels the + electricity, so that, though there is still on the whole as much + elec tricity as - electricity, yet the distribution is no longer the same, for, the electricity being free to move, the - electricity under the attraction approaches A until the non-conducting air Fig. 3. 1 It might be well to use the term " electric force " here, for " elec tromotive force" is afterwards used to mean the line integral of a force (see below, p. 24).
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