ELECTRICITY

(Gr. »jXe»T|«v, amber; the substance in which the property of attracting light substances after friction was first observed.) This truly extraordinary power of matter, independent of the interest that always belonged to it, has of late years acquired much importance from its influence over chemical phenomena, and its connection with those of magnetism. When a clean gla tube is rubbed with the dry hand, or with a piece of silk, it attracts and repels any light substances, — such as feathers, bran, or little pieces of paper, — which are brought near it; a stick of sealing wax rubbed upon dry flannel exhibits the same appearances, and, to a superficial observer, seems to be exactly in the same state as the gla ; and they are said to be electrically excited. But, on more close examination, it is found that when light bodies are attracted by excited gla , they are repelled by excited sealing wax, and vice vers a, so that the gla and wax are said to be in opposite electric states; and hence the terms vitreous and resinous or positive and negative electricity. But these two states are always coexistent: thus when gla is rubbed by silk the gla becomes positive, but the silk becomes negative; and in the case of sealing wax rubbed by flannel the wax is negative, but the flannel is positive. A similar excitation of electricity is seen in an infinity of other cases; as when we rub a cat's back with the hand, or when a piece of silk riband is drawn briskly between the fingers, or a sheet of paper rubbed with India-rubber, or a metal rod with a silk handkerchief. These, and other extraordinary phenomena connected with them, are hypothetically referred to the presence of a peculiar form of matter called the electric fluid; it is supposed to appertain to all matter, but to become evident only when in redundance or deficiency. When gla is rubbed with silk the equilibrium of the electric fluid is disturbed, the silk imparts it to the gla ; and hence the former, losing electricity, becomes minus or negative, and the latter, acquiring electricitv', becomes plus or positive. This is commonly called " Franklin's Theory," having been proposed and defended by that celebrated electrician. Others have a umed the existence of two fluids as e ential to the explanation of electrical phenomena; both equally subtile, elastic, and universally diffused, and each highly repulsive as to its own particles, and attractive of those of the opposite kind. Electrical quiescence is referred to the combination of these fluids, and their consequent mutual neutralization; and electrical excitation is the consequence of either being free or in exce . It is supposed that they are separated by friction, and by all those other causes which give rise to the appearance of free electricity. Either of these hypotheses may be adopted as facilitating the explanation of electrical phenomena, and as conferring meaning on terms which would otherwise be unintelligible: of the two the simplest, or that which refers the phenomena to one fluid, is perhaps the most generally applicable. Both are apparently equally consistent with facts; but the existence of any fluid, or form of matter, as the cause of electrical phenomena, is at best extremely problematical. There are two series of distinct phenomena presented by electrified bodies: the one seems to result from the accumulation of electricity upon the surfaces of bodies; they are commonly included under the term electricity cj tension, and are well exhibited by the common electrical machine and its prime conductor. It affects all neighbouring bodies, and they are thrown by it into a polar electrical state by what is termed induction: it has a tendency to pa off in sparks through the air, or gradually to escape from points. The thunder storm furnishes a magnificent specimen of this state of electricity. The other state of sensible electricity is that exhibited by electricity in motion; as when a current of electricity is pa ing through a wire or other conducting medium: in this case a vast quantity of electricity may be concerned in the phenomena without any apparent intensity; but, whilst the current is continuous, it produces magnetic phenomena of a most extraordinary character; and, when the perfect conductor is broken by the intervention of certain other media, they suffer in some cases chemical decomposition, and in others become heated, and even ignited. The phenomena of electricity in motion are best exhibited by the Voltaic apparatus. In all electrical experiments remarkable differences are observed in respect to the transfer of the electric fluid through different bodies: some, such as the metals, allow its free and nearly unimpeded pa age through their substance; while others receive and retain it more superficially, such as gla , resin, and other substances which exhibit attractive and repulsive powers when rubbed. Hence the division of bodies into conductors and nonconductors. Many most important electrical phenomena depend apparently upon inducli07i, a subject which has beeu ably Cc 2 [s. 400]