Electricity.

Date:
[1829]
Catalogue details

Licence: Public Domain Mark

Credit: Electricity. Source: Wellcome Collection.

    10/150 (page 20)
    46. Of the first kind, viz. friction, the instances are most numerous, and, as Mr. Singer remarks, under certain limitations, universal; they may indeed be obtained by rubbing any of an exten¬ sive list of resinous and silicious substances; and of dry, vegetable, animal, and mineral productions. The electricity thus excited, is most readily rendered visible by its effects on the gold leaf electrometer. 47. Examples of the second kind are also very numerous. If a small quantity of sulphur be melted and poured into a conical wine glass, it will contract a little, and become electrical in cooling. A silk thread with a small hook at the end of it, or a rod of glass should be inserted in the sulphur while in a fluid state, to serve as a handle for separating it from the glass when cold. On being separated from the glass, the sulphur will exhibit other signs of electricity; if kept in the glass it will retain its electric virtue for years, and evmce it very perceptibly on every attempt to separate the two substances. 48. Mr. Henly discovered that chocolate, fresh from the mill, becomes strongly electrical, as it cools in the tin pans. It soon loses this property, but recovers it once or twice, by being melted in an iron ladle and poured into the tin pans. When the mass becomes dry, the electricity cannot be restored by melting, unless olive oil be mixed with it in the ladle; in which case it completely recovers its electric power. M. Chaptal observed the same circumstance during the congelation of glacial phosphoric acid. Calomel also, when it fixes by sublimation to the upper part of a glass vessel, has been found strongly electrical. The condensation of vapor, and the evaporation of fluids, though apparently opposite processes, are alike sources of electrical excitation. 49. Various crystallised gems, and a stone called the Tourmalin, become electrical by the mere application of heat; but no other sub¬ stances have yet unequivocally manifested the same property; though the effects of friction are generally increased, if it is preceded by a mode¬ rate elevation of temperature. 50. The contact of dissimilar bodies is pro¬ bably in all cases the real primary cause of elec¬ trical excitement, but it is rarely employed alone, for electricity is known to us only by its effects, which are constantly the result of an artificial arrangement, and consequently may not immedi¬ ately succeed the primary cause of electric powers, similar in their separate action on the electrometer, and other indifferent matter; but exerting a mutual influence on each other, destructive of their individual properties. 51. It was at first supposed that these pheno¬ mena were peculiar to the substances by which they were produced: hence the power excited by the frietion of glass was termed vitreous electri¬ city; and that by the friction of sealing-wax, resinous electricity. It has, however, long since been proved that both powers are produced in every case of electrical excitation; and, because their mutual counteraction of effect resembles that of an affirmative and negative power, they have been styled positive and negative electricity. 52. The determination of these two states of electricity in different excited bodies, continues Mr. Singer, is of importance to the practical electrician, and may be thus effected :—Sealing- wax when rubbed on woollen cloth is negatively electrified. Glass, when rubbed with silk is positively electrified. Let an electrometer be made to diverge by its being approached by an excited stick of sealing-wax: while in this state, approach it with any excited body, the electri¬ city of which is to be determined. If the diver¬ gence of the electrometer increase, the presented body is negative ; if it be diminished, the pre¬ sented body is positive. In other words, all those substances that lessen the divergence occa¬ sioned by excited wax, are positive; and such as increase it, negative: whilst those which lessen the divergence produced by excited glass, are negative; and such as increase it positive. Examining, by this test, the effects produced in some of the instances of excitation already con¬ sidered, we find the truth of the preceding state¬ ments, and the relation of the different electrical states to the processes by which they are pro¬ duced, become more intelligible. Care ought to be taken to destroy the divergence of the elec¬ trometer after every experiment of this nature; this is best effected by touching its cap with one end of a piece of brass wire. 53. As an illustration of the doctrine here advanced let the following simple and easily per¬ formed experiments be made. (1.) Roll up a warm and dry piece of flannel, so that it may be held by one extremity, while a stick of sealing-wax is rubbed with the other. After a slight friction present the flannel to an electrometer, which will instantly diverge; while this divergence continues, bring the stick ,of sealing-wax near the cap, and the leaves of the electrometer will quickly collapse. Both these substances, it is obvious, are electrified by mutual friction, but their electricities are opposite * that of the wax being negative, and that of the flannel positive. (2.) The electricities thus produced are equal to each other: for if the friction be repeated, and the two substances be both presented to the electrometer at the same time, no signs of elec¬ tricity appear: the opposite electricities, when applied together, producing a reciprocal'counter¬ action of effect. (3.) If a black and a white silk riband be ex¬ cited in contact, in the manner already described, the black riband will be found to be negatively, and the white one positively electrified. (4.) Take the sulphur cone described at 47, apply it and the glass separately to the elec¬ trometer ; the cone will be found to be negatively and the glass positively electrified. 54. From the above experiments it appears, that in all cases of excitation positive and nega¬ tive electricity are produced at the same time, and may be observed by the use of proper means. But it also appears that by friction with the same substance, different bodies are variously affected; for glass rubbed with silk evinces positive elec¬ tricity: but wax rubbed with silk is rendered negative. Again, polished glass, when rubbed with silk, skin-wool, or metal, becomes positive; but if it be excited by friction against the back of a living cat, it appears negative. Wool, silk,