ELECTRICITY, the name used in connection with an extensive and important class of phenomena, and usually denoting either the unknown cause of the phenomena or the science that treats of them. Most of the phenomena in question fall under the three chief heads of frictional electricity, galvanism, and magneto-electricity. The present. article is confined to the first.
Historical Sketc71.—Thales, about 600 B. c., refers in his writings to the fact that amber, when rubbed, attracts light and dry bodies. This was the only electric fact known to the ancients. The science of E. dates properly from the year 1600 A.D., when Gilbert of Colchester published a book, entitled De Arte Magnetica, in which he gives a list of substances which he found to possess the same property as amber, and speculates on magnetic and electric forces. He is the inventor of the word E., which he derived from the Greek word electron, amber. Otto von Guericke, burgomaster of Magdeburg, in his work Experimenta Nova Magdeburgica (1672), describes, among his other inventions, the first electric machine ever made, which consisted of a globe of sulphur turned by a handle, and rubbed by a cloth pressed against it by the hand. Hawksbee (1709) con structed a machine in which a glass cylinder, rubbed by the dry hand, replaced Guericke's sulphur globe. Grey and Wehler(1729) were the first to transmit E. from oue point to i another, and to distinguish bodies into conductors and non-conductors. Dufay (1733-45) showed the identity of electrics and non-conductors, and of non-electrics and conductors, and was the first to discover the two kinds of E., and the fundamental principle which regulates their action. Between the years 1733 and 1744, much attention was given in Germany to the construction of electric machines. Up to this time, notwithstanding the inventions of Guericke and Hawksbee, the glass tube rubbed by a piece of cloth which Gilbert first introduced, was used in all experiments. Boze, a professor at Wit tenberg, taking the hint from Hawksbee's machine, employed a globe of glass for his machine, and furnished it with a prime conductor. Winkler, a professor at Leipsic, was the first to use a fixed cushion in the machine. The Leyden jar was (1746) discovered accidentally at Leyden by Muschenbroek; but the honor of the discovery has been con tested also in favor of Cuneus, a rich burgess of that town, and Kleist, canon of the cathedral of Camin, irr Pomerania. Franklin (1747) showed the electric conditions of the Leyden jar, and (1752) proved the identity of lightning and E. by his famous kite experiment. The last was performed with the same object about the same time, and quite independently, by Romas of the town of Nerac, in France. In 1760, Franklin made the first lightning-conductor. Canton, Wilke, and YEpinus (1753-59) examined the nature of induction. Ramsden (1768) was the first to construct a plate-machine, and Nairn (1780) a two fluid cylinder-machine. The electrophorus was invented by Volta in 1775, and the condenser by the same electrician in 1872. In 1786, Galvani made the discovery which led to the addition of the new branch to the science which bears his name, and which now far exceeds the older branch in extent and practical value. See GALVANISM. In 1787, Coulomb, by means of his torsion-balance, investigated the laws of electric attraction and repulsion. In 1837, Faraday published the first of his researches on induction. Armstrong, in 1840, designed his hydro-electric machine.
Fundamental Facts.—Under the head Conductors (q.v.) it is stated that bodies which do not conduct E., or non-conductors, are capable of electrical excitation from friction, and are, in consequence, termed electrics, and that conductors not so affected are called non-electrics. The fundamental principles of electricity are illustrated by the electric pendulum. A glass tube bent at right angles, so as to project horizontally, is placed on a convenient stand. On the hook in which its upper end terminates, a cocoon thread is hung, to the end of which a pith-ball is attached. The ball is thus doubly insulated by the glass and the silk thread. If a tube of glass be rubbed by a dry silk handkerchief,
and brought near the ball, the ball is at first briskly attracted, and then as briskly repelled; and if the tube be then moved towards it, it moves off, keeping at the same distance from it. The ball being so affected, or charged, as it is called, a rod of shell lac or of sealing-wax, after being rubbed with flannel, attracts it, if possible, more briskly than before, and again sends it off exactly as the glass had done. If the rdass tube be now again taken up and rubbed a second time, if necessary, the ball will act towards it as it did towards the sealing-wax. The same series of attractions and repul sions would have taken place if we had begun with the sealing-wax instead of the glass tube. We interpret this experiment in the following way: When glass is rubbed with silk, and the silk removed, it is charged with what is called positive electricity. The ball is attracted by it, and becomes on contact also charged with positive E., and is then repelled. When sealing-wax is rubbed with flannel, and the flannel removed, it becomes charged with negative whieli is Alic counterpart of positive E., for it attracts the positively charged ball, and communicating its own E. to it, finally'repels it. From such an experiment as this, we conclude that bodies electrified either positively or negatively, attract neutral bodies and bodies with E. of an apposite name to their own, but repel those affected with E. of the same name; and that E. can be communicated from one body to another by contact. For positive and negative (written also + and — ), the terms vitreous and resinous are also employed, as glass and resin are the typical substances from which they may be obtained. Contact is not the only way in which E. is communicated. We find, when we deal with larger bodies than the pith-ball of the experiment, and some times even with it, that the passage of a spark between two bodies without contact com municates the E. of the one to the other. The part played by the rubbers in the above experiment must not be overlooked. The silk handkerchief employed to rub the glass assumes the resinous or—electrical state, and the flannel rubber of the sealing-wax the vitreous or -I-. This cannot, however, be clearly shown, as the experiment is performed, for the rubbers are in each case tightly embraced by the hand, which neutralizes their peculiar electricity. We can perform our fundamental experiment in a way clearly to show this. Let us take for our rubbing and rubbed surfaces two india-rubber balloons inflated with air (such as children play with), and hold them tightly one in each hand. They may be in all respects perfectly alike. Let us then rub them briskly on each other, and then hold the rubbed sides closely together. On bringing the two in contact near the pith-ball. it remains indifferent to them ; but if we pull them apart, and put one on each side of the pith-ball, the ball plays actively between them, being attracted and repelled by each in turn. The fact of no attraction occurring when the balloons arc together, shows that in the rubbing both electricities are generated in equal quantities, for they neutral ize each other when brought near; and the fact that the balloons must be separated proves that all electric phenomena take place in an electric field, with positive E. at its one termination, and negative E. at its other. The non-conducting nature of the india rubber prevents the electricities finally neutralizing in contact, and disappearing by the hands when apart. It is also instructive that as force is exerted and work is done in pulling them apart, we have the equivalent of that work in the form of an electric field capable of doing work. The motion of the pith-ball, and the heating caused by the tiny sparks which charge it, are evidences of the truth of the statement. It is again worthy of note that both balloons appear exactly alike, and yet they assume opposite electricities. That there must be some difference may be drawn from the next para graph.