Page:The American Cyclopædia (1879) Volume VI.djvu/509

 ELECTRICITY 501 French academy was perhaps the first to ap- proach the subject in a theoretical way. He found that all bodies were susceptible of elec- trical excitement by friction if they were in- sulated, and was the first to propose the theory of two fluids, which he termed vitreous and resinous. The results of the labors of other investigators will be mentioned in the descrip- tions of apparatus or experiments. The dis- coveries of Franklin and some further notice of statical electricity will be found in the article LIGHTNING. The ordinary phenomena of fric- tional electricity may be observed by taking a dry glass rod, rubbing it on a piece of dry silk, and presenting it to small scraps of paper or pieces of pith, when they will fly against the rod, and after remaining a short time, the length of which depends on the intimacy of contact and the nature of the light bodies, they will fly off with a sudden motion, as if repelled. Similar effects may be produced by using a stick of sealing wax, a material mainly com- posed of gum shellac, in place of the glass rod. It was supposed for a long time that only certain bodies were capable of becom- ing electrically excited, and they were called electrics, while those which did not distinctly manifest excitement under similar circum- stances were called non-electrics. When the conduction of electricity was made the subject of investigation by Stephen Gray, it was found that all the so-called electrics were non-con- ductors, and that those which were termed non- electrics were conductors. This discovery led to the insulating of conductors and subject- ing them to friction, when it was found that they also were readily excited. The division into electrics and non-electrics was thus found to be incorrect, and was abandoned, the fact being recognized that under proper circum- stances all bodies are electrics. Bodies differ so greatly in the power of ' conducting elec- tricity that they may be classed into conduc- tors and non-conductors ; but there are none which are perfect non-conductors except pos- sibly dry gases. The following list of solids and liquids exhibits the conductors in the order of their conductivity, and the non-conductors in the inverse order, the best non-conductors being placed first : CONDUCTOKS. All metals, Snow, Well burned charcoal, Living vegetables, Plumbago, Flax, Concentrated acids, Hemp, Dilute acids, Saline solutions, Living animals, Flame, Spring water, Moist earth and stones. Rain water, NON-CONDTTCTOKS. Shellac, Diamond, Baked wood, Amber, Ebonite, Porcelain, Resins, Sulphur, Gutta percha, Silk, Marble, Camphor, Wax, Wool, Caoutchouc, Jet, Feathers, Chalk, Glass, Dry paper Oils, Mica, Leather, Metallic oxides. If an electrically excited insulated conductor is placed in contact with another conductor which is also insulated, a portion of its electri- city disappears, and the two become equally excited. If the first body is large in propor- tion, it loses but little of its electricity ; but if it be smaller than the other body, it parts with a proportional quantity ; and if the difference is vast all sensible excitement disappears, as when communication is made with the earth, which is often called the common reservoir. Non- conductors may be used as insulators. Dry air is an excellent insulator ; if it were not so, it would be impossible under ordinary circum- stances to collect electricity upon any body, because it would be conveyed away as fast as generated. Damp air is a partial conductor, and therefore experiments in frictional electri- city cannot be well performed during moist weather. It was for a long time observed that different bodies when electrically excited seemed to possess opposite states of electricity. FIG. 1. Attraction and Eepulsion. Thus, if a gilt pith ball be suspended by a silk thread, as in fig. 1, and approached by a glass rod which has been excited by friction with a piece of silk, it will be attracted to it ; but as soon as contact is made it will be repelled, and the repulsion will continue as long as the ball retains its charge. If, however, a stick of ex- cited sealing wax be brought toward it before its charge is lost, it will be attracted, and more powerfully than it was at first, by the glass rod. To explain this phenomenon different theories have been proposed, which are dis- cussed in the article ELECTBO-MAGNETISM. In this article, for convenience of explanation, the existence of two fluids will be assumed, termed positive and negative, which in the natural state of bodies are supposed to be combined, but which are separated or decomposed by mechanical or some other force, and are more- over mutually attractive, but self-repulsive. There is no proof of a special electric fluid,