Page:The American Cyclopædia (1879) Volume VI.djvu/510

502 ELECTRICITY either compound or simple ; bnt it may be that the luminiferous ether which most physicists now suppose to be the common medium for the transmission of light and heat will play an important part in a future theory of electricity. The fact that what is called the electric fluid cannot be discharged through a nearly perfect vacuum presents apparently a strong objection to the idea that the ether may be an electric medium ; but this consideration does not affect the possibility that it may exert a controlling influence, and it is not improbable that the re- cent investigations into the molecular nature of bodies in relation to heat and light will lead to others by which the problem may at last be solved. With regard to the kinds of electricity which may be developed upon a body, it is found that it not only depends upon the body itself, but upon the body with which it is rub- bed ; thus, although glass in almost every in- stance collects positive electricity by friction, if the fur of the cat be used for this purpose it will be found to be negatively electrified. The surface also has much to do with the nature of the fluid which a given body may acquire ; thus, if a piece of polished and a piece of rough glass be rubbed together, the former will be positively and the latter negatively electrified. If two silk ribbons are rubbed across each other, the one transversely rubbed will be neg- atively and the other positively electrified ; and again, if two bodies of the same substance and having the same kind of surface, but dif- fering in temperature, are rubbed together, negative electricity will be developed upon the warmer body. The kind of excitement a body may manifest also depends upon the color. When a piece of white is rubbed with a piece of black silk, the former acquires posi- tive, and the latter negative electricity. The general deductions of Coulomb from numerous experiments are, that when two bodies are rubbed together, the one whose particles are least disturbed is more disposed to collect positive electricity. Other forms of mechani- cal action besides friction will develop statical electricity. Several minerals when pressed between the fingers exhibit free electricity. Calc spar, topaz, and fluor spar are examples, the first retaining its excitation for several days. According to the observations of Bec- querel, the excitation is not perceptible while the pressure continues, but during the act of separation. Two similar bodies do not develop electricity when pressed together un- less one is colder than the other, the colder body always becoming negative. Cleavage or the tearing asunder of laminae will produce electrical disturbance in minerals, card board, or other laminated bodies. Vibration devel- ops electricity. A wire composed of iron and brass, connected with a galvanometer, on be- ing made to vibrate so as to produce a musical sound, will cause a deflection of the needle. Statical electricity is also developed by heat ; a familiar example is the attraction and repulsion of ashes when tourmaline is placed in hot em- bers. Electrical Induction. If a globe, C, fig. 2, supported upon an insulating pillar, and charged with positive electricity, be placed near one end of a cylinder, A B, also insulated, but not previously electrified, from which small gilt FIG. 2. Electrification by Induction. pith balls are suspended, the latter will be seen to separate, the separation being greatest at the two ends, while at a place midway no excite- ment will be manifested. It will be found that that portion of the cylinder nearest the globe will have negative and the further end positive electricity, the amount in either case diminish- ing from the ends to the place between where there is no excitement. The greatest diver- gence of the pith balls will be upon the end nearest the globe, and the point of no excite- ment will be nearer this than the other end. If the globe is removed to a considerable dis- tance from the cylinder, the latter will assume its natural state, the excitation having been caused simply by a separation by induction of the combined fluid upon the cylinder, and with- out any abstraction of either positive or nega- tive fluid. If, however, while the globe is near the cylinder, the latter be placed in communi- cation with the earth, the positive electricity which is repelled to the further end will escape, and a corresponding quantity of negative elec- tricity will flow in the contrary direction to the cylinder, and, being attracted by the posi- tive fluid of the globe, will increase the charge upon the end nearest it. Then, if communica- tion with the earth be interrupted and the globe removed, the cylinder will be charged with negative electricity, which will manifest itself in the separation of the pith balls, the divergence being now equal along the whole length of the cylinder. If the experiment is made with two cylinders, placed end to end, they will both become excited by induction, the induced electricity of one causing induction in the other. In this manner a number of cyl- inders may be electrified, but the excitation will become feebler in proportion to the de- parture from the globe. The ordinary expla- nation includes the idea that the action takea place at a distance, the non-conducting medium