ELECTRICITY. 



a body manifests this attractive power, it is said to 

 be in an electrical state, or to be electrified ' j and 

 the name electricity is given to the strange physical 

 cause of the power, which the rubbing has called 

 into existence. 



For experiments in this subject, it is useful to 

 have a number of small pith-balls, formed from the 

 pith of the elder-tree, or of the common sunflower. 

 One of these, hung by a silk thread from a con- 

 venient support, is called an electric pendulum, 

 and shews very readily the attraction of an electri- 

 fied body. 



But every substance which we rub as we did 

 the glass or sealing-wax, will not behave in this 

 manner. A rod of metal, held in the hand, will 

 shew no trace of electricity, though it be rubbed 

 ever so long. It is clear, therefore, that all bodies 

 are not alike with regard to the electrical state. 



The difference used to be explained by saying 

 that amber, glass, and such substances are electrics, 

 while the metals and others are non-electrics that 

 is to say, that the former class can be electrified 

 by friction, while the latter cannot. This idea is 

 now known to be erroneous. For, if the metal be 

 held by a glass handle, and rubbed, it will at once 

 shew its attractive power. 



The true explanation lies here. In some sub- 

 stances, the electrical sta/.e is no sooner produced 

 at any part than it instantly diffuses itself over the 

 whole. In others, it is confined to the part where 

 it is produced, or, at least, spreads over the rest 

 of the body very slowly, and with great difficulty. 

 According as bodies admit this instant diffusion 

 or transmission of the electrical state or not, they 

 are now termed conductors or non-conductors. 

 Non-conductors, such as dry air, glass, shellac, &c. 

 are also called insulators, because, if an electrified 

 body is surrounded by such, its electricity is 

 inclosed as in an island, and prevented from 

 escaping over other conductors. The earth is a 

 huge conductor, and any electricity, which we can 

 artificially excite, must be carefully insulated from 

 it, otherwise, if allowed to spread over the earth, 

 it will be lost in its immensity, like a wave on a 

 boundless sea. 



We can now easily understand the difference 

 between rubbing a rod of metal and one of glass 

 held in the hand. In the former case, the elec- 

 tricity is dissipated as fast as it is produced, while 

 the glass acts as the insulator of its own electricity. 



There is no such thing as a perfect conductor or 

 a perfect insulator. In other words, the very best 

 conductors offer some resistance to the diffusion of 

 electricity, and the very worst do not wholly pre- 

 vent it Nor is there any sharp line between the 

 two ; the difference is merely one of degree, and 

 depends on a variety of causes. Temperature, for 

 example, has a marked effect on conducting power. 

 Thus, water is a moderate conductor, yet ice and 

 dry steam are both insulators. 



The following is a list of bodies in order, from 

 the best conductors to the best insulators : Silver, 

 fopper, gold, zinc, iron, lead, mercury, charcoal, 

 acids, salt solutions, rarefied air, pure water, stone, 

 dry ice, dry wood, porcelain, dry paper, wool, silk, 

 glass, sealing-wax, sulphur, resin, gutta-percha, 

 india-rubber, shellac, paraffin, ebonite, dry air, and 

 gases. 



Looking more closely at the action of the rubbed 

 glass rod on the electric pendulum, we see that 

 the attraction is merely momentary, and is followed 



by as brisk repulsion. Any attempt to bring the 

 rod near to the pith, only serves to drive it farther 

 away. But if an excited stick of sealing-wax be 

 brought near, the pith instantly flies to its embrace, 

 only, however, to be in a moment cast off, as it 

 had been by the glass before. Banished from the 

 wax, it will now find favour with the glass for an 

 instant again ; and thus a continual exchange of 

 sympathy for the one or the other electrified body 

 may be kept up as long as the excitement con- 

 tinues. 



Now, this curious behaviour of the pith can be 

 accounted for only on the supposition that there 

 are two opposite states of the electrical excite- 

 ment, one peculiar to the glass, and the other to 

 the sealing-wax. What the one repels, the other 

 attracts, and the characters of the electricities 

 must therefore be as opposite as are attraction and 

 repulsion. 



In any case, the excitement is either like that of 

 glass or that of sealing-wax ; thus, any electricity 

 used to be defined as either vitreous or resinous. 

 But these terms are not quite correct, as either 

 kind may be got from the glass or from the wax, 

 by varying the nature of the rubber. For vitreous 

 and resinous, the terms positive and negative are 

 now used positive electricity being like that 

 evoked on glass by rubbing with silk ; and nega- 

 tive, that evoked on sealing-wax by rubbing with 

 flannel. 



Laws of Electricity. The laws of frictional 

 electricity are strikingly like those of magnetism. 

 Often both may be expressed in the same lan- 

 guage. 



(i.) As with magnetism, so with electricity. 

 Like kinds repel : unlike, attract each other. If 

 we take two piths hanging together by two silk 

 strings, and bring near a rod of glass or of sealing- 

 wax, they will repel each other, and remain apart. 

 But if we electrify one with the glass, and the other 

 with the wax, they will attract. 



We can thus at once tell the nature of any 

 electricity by its repelling a pith electrified by 

 glass or by sealing-wax. Mere attraction is no 

 test, as a neutral body is attracted. 



(2.) But as we never have one pole of a magnet 

 without its fellow, so both kinds of electricity are 

 always produced at the same time, and in equal 

 amount. One kind goes to the body rubbed, and 

 the other to the rubber. Faraday, whose name is, 

 so closely associated with this subject, shewed this 

 fact by a very simple experiment. He made a 

 small flannel cap to fit the end of a stout stick of 

 sealing-wax. After rubbing the cap round the 

 end of the wax, and presenting it to a pendulum, 

 not the slightest effect was seen before removing 

 the cap. But on drawing it off, by a silk string 

 attached to it, he found the end of the rod nega- 

 tively, and the cap positively, electrical. 



The kind of electricity that goes to each depends 

 on the nature of the rubbing bodies. Thus, if we 

 rub glass with silk, the glass becomes positive, but 

 if we rub it with cat-skin, it becomes negative. It 

 is a curious fact that no body has yet been found 

 which will give positive electricity when rubbed 

 with a cat-skin. 



If any two substances in the following list be 

 rubbed together, that one which comes first in 

 the list will be positive : Cat-skin, flannel, glass, 

 cotton, silk, the hand, shellac, metals, sulphur, 

 india-rubber, gutta-percha. 



