CONDENSER. A form of electrical appa ratus used to accumulate a charge of electricity, or, in other words, to store up electrical energy. A condenser in its simplest form consists of two conductors which are separated from each other by an insulating medium or dielectric. and is illustrated by the Leyden jar or Franklin plate. The name dates from the time of the fluid theory of electricity, when it was believed that a certain amount of the electric fluid could be collected or condensed on a conducting surface. The princi ple of the apparatus is illustrated in the Frank lin plate, which consists of a plate of glass with pieces of tin-foil on each side. If a positively charged body or the positive conductor of an electric machine is connected with one of the tin foil coatings, it will communicate to it by con duction a positive charge of electricity. if now the opposite plate is connected with the ground, the negative electricity is held bound, while the positive is repelled and passes to the ground. Accordingly we have accumulated equal anionnts of positive and negative electricity on the tin foil, and if the two surfaces are connected a bright spark results and the equilibrium is restored.
Otherwise, the charge remains on the surface of the conductors until it is dissipated by leakage. The energy which is stored up in the condenser is expended in producing the spark. The amount of electricity that can be accumulated depends upon the capacity of the condenser and the po tential of the charge. The Leyden jar consists of a Franklin plate in a cylindrical form, and as it is more compact and has greater capacity, it is more often used. In practice, however, the usual form of condenser consists of sheets of tin foil separated from each other by paralfined paper, or in the case of standard condensers sheets of mica, with the alternate sheets of tin foil connected together to give considerable ca pacity. The apparatus may be arranged so as to afford various amounts of capacity, and is much used in cable-testing and other branches of elec trical work. The unit of capacity is the farad (q.v.), but condensers are generally constructed to give capacities in fractions or multiples of a micro-farad which is the unit or dinarily employed. See ELECTRICITY.