CHAMBERS'S INFORMATION FOR THE PEOPLE. 



^3.) The electrical excitement is only found on 

 the surface of a conductor. This may be shewn 

 by a hollow brass ball with an opening of an inch 

 diameter or so, and fixed on a glass stand. How- 

 ever strongly we electrify the ball, not the slightest 

 trace of electricity will be found inside. Faraday 

 took a cylinder of wire-gauze, and, setting it on an 

 insulating stand, electrified it powerfully. Yet, 

 strange as it may seem, none could pass by the 

 meshes to the inside. 



Electrical Theories. Many theories as to the 

 nature of electricity have been proposed, but its 

 real character is yet beyond our reach. The two 

 most historically important theories are the fluid 

 theories of Franklin and of Symmers. 



Franklin's theory is, that all bodies, when in the 

 neutral state, contain a definite quantity of an 

 extremely elastic, imponderable fluid which repels 

 itself, but attracts matter. Bodies are positively 

 electrified when they have more than their natural 

 share of it, and negatively when they have less. 

 Symmers' theory is, that bodies in the neutral 

 state contain equal amounts of two electrical 

 fluids, of opposite characters. By friction and 

 other means, these can be separated, one going to 

 each body rubbed. Each repels itself, but attracts 

 the other, and one is peculiar to rubbed glass, and 

 the other to rubbed sealing-wax. 



These fluid theories were long exclusively 

 adopted, and on them are based all the leading 

 terms of the science. 



The modern tendency, however, is to depart 

 from all material theories of electricity, and to 

 adopt some one more or less analogous to those 

 of heat and light. They are based on the fact, 

 that heat may be transformed into light and 

 electricity, and also that electricity may be con- 

 verted into light and heat Light and heat are 

 now ascribed to vibrations of an extremely elastic 

 ether, which is so rare or fine as to pervade the 

 densest matter as easily as air does the branches 

 of a tree. So electricity is ascribed to some other 

 modification of this ether, such as condensation or 

 rarefaction of it at the surface of bodies, there 

 being evidently some mutual influence between 

 matter and the ethereal medium. 



One thing is certain electricity is not known 

 to exist apart from matter, and it is very probable 

 that it is but some particular state of the particles 

 of matter. Faraday and Grove ascribed all elec- 

 trical phenomena to a polarisation of the mole- 

 cules of bodies, acting by attraction or repulsion, 

 according to very definite laws. Faraday, by a 

 series of testing experiments, went far to establish 

 this theory, which would overturn all the fluid 

 theories. 



Induction. An electrified body has the power 

 of inducing, or of affecting a neutral one in much 

 the same way as a magnet does a piece of soft 

 iron. This may be shewn 

 very simply in the 

 following way. A brass 

 cylinder with rounded 

 ends, and fixed on a glass 

 support, has pairs of piths 

 hung at each end (fig. 7). 

 If, now, an insulated con- 

 ductor, such as a brass ball, 

 be charged positively, and 

 brought near one end of the 



Fig. 7- 



cylinder, both pairs of piths will instantly diverge, 



262 



and as quickly collapse when it is taken away. It is 

 easily shewn that the two ends of the cylinder are 

 in opposite electrical states, and separated bv a 

 neutral line of no electricity. Next the positive 

 ball there is negative electricity induced, and so it 

 is always. A charged body induces an opposite 

 kind of electricity on the side of another next it, 

 and the same kind on the farther side. If we 

 had a series of cylinders like that in the figure, 

 placed end to end in a line, without contact, 

 we should find that the positive end of the first 

 cylinder would act on the next just as the ball 

 did on itself. Thus the induction might be trans- 

 mitted through the whole of them ; and, on our 

 removing the inducing ball, they would all return 

 to the neutral state instantly. 



Opposite forces are here, as in a magnet, resi- 

 dent in opposite sides of the conductors. They 

 are therefore said to be polarised, and the facts of 

 electric induction are all in accordance with the 

 laws of polarity. Indeed, there seems every reason 

 to infer that all electric action is of a polar nature. 

 When a rubbed glass attracts a pith, it is an 

 attraction between the positive of the glass and 

 the negative it has induced on the face of the pith 

 next it This is quite analogous to the action of a 

 magnet on a piece of soft iron. There is, how- 

 ever, an apparent contradiction of this polar doc- 

 trine, for we may give to a body by induction only 

 one kind of electricity, and this it retains perma- 

 nently. If, while the cylinder is near the positively 

 charged ball, we touch it for a moment with the 

 finger, we uninsulate it, and allow its electricity to- 

 spread over the earth and become lost. Only the 

 positive, however, will escape ; the negative will 

 be held bound to the positive inducing charge. If 

 after thus touching the cylinder, we remove the 

 ball, we shall have remaining on the cylinder only 

 negative electricity. This is very different from 

 what we can do with magnetism. There seems 

 here no second electric polarity. 



The explanation, however, is not far to seek. 

 All matter can be excited electrically, but only one 

 kind magnetically. In the case of electric polarity, 

 then, it is possible to transfer one of the poles- 

 by induction to surrounding objects, whereas, in 

 magnetism, this transference cannot take place. 

 Not only is it possible, but it is what must take 

 place before we can have any electricity at all. 

 Faraday has shewn that a body cannot be electrified 

 at all, either positively or negatively, unless there 

 be surrounding objects to which it can transfer 

 the opposite electricity by induction, and that the 

 degree of charge which it can take is just in pro- 

 portion to this facility for induction. 



When a body is charged by induction, then, 

 we see its electricity is opposite to that of the 

 charging body. But when we charge it by con- 

 tact or by spark, the electricity is of the same 

 name as that of the charging body. Yet the latter 

 is due to induction as well as the former. For 

 when we bring the ball (fig. 7) near the cylinder, 

 the positive electricity of the ball tends to unite 

 with the negative it induces next it. The ten- 

 dency increases as they approach, till at last it is 

 sufficient to burst through the interval of air ; and 

 the two flash together with a spark, and neutralise 

 each other. Thus, the positive electricity at the 

 other end of the cylinder is left alone without its 

 negative, just as if the positive of the ball had 

 passed or flowed over to it 



