372. Vitreous and Resinous Electricity.—Suppose now that you rub a rod of sealing-wax with woolen or silk and hold it near a pith ball which has been electrified from glass. It will attract the ball. The reason is that an electricity is excited on the sealing-wax of a different kind from that which is excited on the glass. The former is called resinous, and the latter vitreous electricity. They are supposed to be two fluids, which have a strong attraction for each other, while, on the other hand, the particles of either fluid are repellent to each other. It is this attraction between the two fluids which causes in the case just stated the sealing-wax to attract the ball to itself. We can illustrate this attraction in another way. Take two pith balls and electrify them, the one from glass and the other from sealing-wax. Brought near together they will attract each other, because they have two unlike electricities. This, you see, is just the reverse of the effect produced in an experiment cited at the conclusion of § 371, in which the electricities were alike in the two pith balls. Again, if you bring the rubbed sealing-wax near to the ball electrified from glass, the ball will be attracted, and the same effect will follow if you bring the rubbed glass near to the ball electrified from sealing-wax.
373. Franklin's Theory.—In § 372 is developed the theory now commonly received in regard to electricity. The theory of Franklin was different. He supposed that there is but one electric fluid, and that all bodies are in their usual state charged with a certain portion of it, some having more than others, according to their capacity for electricity. While a body is in its usual state there is no manifestation of electricity. The fluid is in a quiescent condition, because its particles are prevented from repelling each other by the attraction which exists between them and the particles of the substance. But this quiescence can be disturbed by friction and other causes. Thus if a glass rod be rubbed with a piece of silk, the natural equilibrium is disturbed, the glass having an excess and the cloth a deficiency of electricity. The glass is therefore said to be positively and the cloth negatively electrified. The equilibrium can be restored in the case of a positively electrified body by having its excess drawn off, and in the case of a negatively electrified body by having its deficiency made up by receiving electricity from other bodies. Though this theory is discarded, the terms positive and negative derived from it are retained, being applied to the two fluids[6] or electricities, and they are often designated by the two signs + and -.
374. Upon What the Kind of Electricity Excited Depends.—It depends on what a substance is rubbed with whether vitreous or resinous electricity is excited in it. Thus smooth glass rubbed with woolen cloth or silk will be positively electrified; while if it be rubbed upon the back of a cat it will exhibit negative or resinous electricity. So, also, if a resin, as gumlac or sealing-wax, be rubbed with silk or woolen cloth, it will be charged with resinous electricity, but it will be charged with vitreous or positive if it be rubbed with sulphur. The terms vitreous and resinous are therefore incorrect, for they are based upon the idea that one kind of electricity is always excited on glass, whatever the friction may be made with, and that the other kind is always excited on resins. The most decided illustration of the incorrectness of these terms we have in the fact, that while smooth glass rubbed with silk or woolen cloth becomes charged with positive (vitreous) electricity, roughened glass rubbed with the same gives us negative (resinous) electricity. Below I give a table of substances, any one of which has positive electricity developed on it when it is rubbed with any substance below it on the list, and negative when rubbed with any substance above it:
1. Cat-skin.
2. Polished glass.
3. Woolen cloth.
4. Feathers.
5. Wood.
6. Paper.
7. Silk.
8. Sealing-wax.
9. Amber.
10. Roughened glass.
11. Sulphur.
375. Conductors and Non-Conductors.—Electricity passes over the surface of some substances very readily; while over others it moves with very great difficulty, and therefore very slowly and sparingly. The former are termed conductors, and the latter non-conductors. As in the case of heat, so with electricity there are no substances which are wholly non-conducting. The best of all the conductors are the metals, those least liable to oxydation being the most perfect. Next come charcoal, water, living substances, flame, smoke, steam. The best non-conductors are gumlac and gutta-percha. Then come amber, resins, sulphur, glass, silk, wool, hair, feathers, cotton, paper. Non-conductors are sometimes called insulators, from the Latin word insula, as they serve to confine electricity within certain bounds, and prevent its escaping. Thus in the experiments with pith balls, already cited, the silk threads by which they are suspended prevent the electricity from escaping from them. So the glass knobs on which the wires of the telegraph rest are insulators, preventing the electric fluid from escaping down the poles into the ground.
Fig. 258.
376. Electricity Always on the Surface.—There is a marked difference between heat and electricity in the manner in which they are disposed of. Heat pervades all the particles of substances, and in its conduction spreads through them, while electricity in its ordinary movements operates altogether on the surface. A hollow ball, therefore, can contain as much electricity as a solid, and a hollow conductor of electricity is just as effectual as a solid one. The following experiment exhibits in a very striking manner this disposition of electricity to occupy the surface alone: Let a, Fig. 258, be a metallic ball supported by a glass stand, b; and let c c be metallic caps which will just cover the ball, having non-conducting handles, either glass or gumlac. Now, after having charged the ball with electricity, let the caps held by the insulating handles be carefully placed over the ball. On withdrawing them it will be found that the electricity of the ball has all passed to the outer surface of these caps.
377. Electrics and Non-Electrics.—It will be observed, on looking over the list of conductors and non-conductors, that among the non-conductors are those substances in which electricity is easily excited by friction, such as glass, amber, silk, etc. These were therefore called electrics. The conductors, on the other hand, were called non-electrics, it being supposed that electricity could not be excited with them. But this has been found not to be true. For example, if a metal be insulated by being placed on a pillar of glass or of gumlac, so that the electricity, when excited, can not pass off readily, its generation can be made manifest. It is probably true that every substance is more or less an electric, it being difficult to make this manifest in the case of conductors, because the electricity passes off as fast as it is generated.