Electric Machine

(Phy.) The first E. M. was invented by Otto von Guericke. Fig. 298 represents the machine now most generally used. Between two wooden supports a circular gla plate, P, is suspended by an axis pa ing through the centre, and which is turned by means of a gla handle, M. The plate revolves between two sets of cushions or rubbers, F, of leather or of silk, one set above the axis and one below, which, by means of screws, can be pre ed as tightly against the gla as may be desired. The plate also pa es between two bra rods shaped like a horse-shoe, and provided with a series of points in the sides opposite the gla : these rods are fixed to larger metallic cylinders, C, which are called the prime conductors. The latter are insulated by being supported on gla feet, and are connected with each other by a smaller rod, r. The action of the machine is founded on the excitation of electricity by friction, and on the action of induction. By friction with the rubbers, the gla becomes positively, and the rubbers negatively electrified; but the rubbers communicate with the ground by means of a chain, and, consequently, as fast as the negative electricity is generated, its tension is reduced to zero by contact with the ground. The positive electricity of the gla acts then by induction on the conductor, attracting the negative electricity. This negative electricity collects in the points opposite to the P M D Fig. 298.-ELECTRIC MACHINE. gla . Here its tension or tendency to discharge becomes so high that it pa es acro the intervening space of air, and neutralizes the positive electricity on the gla . The conductors thus lose their negative electricity, and remain charged with positive electricity. Before use, the rubbers are coated either with powdered aurum musivum (sulphur et of tin), or graphite, or amalgam. If the hand be brought near the conductor when charged, a spark follows, which is renewed as the machine is turned. In this case, the positive electricity decomposes the neutral fluid of the body, attracting its negative electricity, and combining with it when the two have a sufficient tension. Thus, with each spark, the conductor rove rts to the neutral state, but becomes again electrified as the plate is turned. Electro-chemistry. That science which treats of the chemical changes which take place under the influence of electricity. It is generally divided into two parts: Electrolysis, which discu the decomposition, or separation into its constituent parts, of a compound body by the pa age of the electric current; and Electrometallurgy, which is the application of electrolysis to the arts. [s. 322]