In the Thury system of power transmission high-voltage direct current is used throughout. Only one supply in this country is of this kind, but several are in operation on the Continent. Pressures up to 100,000 volts are used.
Electric Telegraph. See Telegraph.
Electric Traction and Electric Tramway. In electric traction the mechanical power required for the propulsion of the vehicle is obtained from electric motors. These motors are usually series direct-current motors, but for railway work single-phase and three-phase A.C. motors have also been successfully employed (see Electric Motors). Up to the present, electric traction on railways has only been employed for suburban traffic in this country. In one instance (L.B. & S.C.R. electrification) single-phase alternating current is used. In all the others the power supply is direct current (see Railways, Electrification of).
In electric tramways, except in some few instances where there are objections to the use of an overhead construction, the current is conveyed to the motors through a trolley pole carrying a wheel running on an overhead bare copper wire. A hand-operated drum controller, directly controlling the driving and electric braking of the motors, is used. A hand-brake, and commonly a separate electro-magnetic brake, are provided.
Except in very small tramway systems, the power is generated as high-tension alternating current, and transformed and converted at substations suitably placed in the area covered by the tramway (see Electric Power Transmission and Distribution). The low-tension D.C. power is distributed from the substations to the trolley wire. The car rails are earthed, and provide a return path for the current. In order to minimize the flow of current to other conductors in the vicinity of the car rails, copper cables returning directly to the substation are connected to the rails at suitable intervals. These earth-return cables are connected in series with special low-voltage dynamos (called negative boosters) at the substation. This arrangement automatically keeps the P.D. between the most distant point of the car rails and the substation within a prescribed maximum, and effectively prevents the corrosion of pipes laid near the car rails.
Elec´trode (Gr. hodos, a way), a term introduced by Faraday to denote the wires or other terminals by which electricity either enters or leaves a body which is undergoing electrolytic decomposition. He called the electrode at which the current enters the anode (ana, upwards), and the electrode at which the current leaves the electrolyte the cathode (kata, downwards). (See Electrolysis; Electro-metallurgy.) The word is now commonly used in a wider sense to denote the conductor by which contact is made with a medium. In this way electrodes are spoken of in connection with electric furnaces, electric welding appliances, vacuum tubes, and mercury vapour lamps, although the actions are not electrolytic.
Electrol´ysis (Gr. lysis, loosening) is the name give to the decomposition of fused salts or solutions of salts, &c., by means of the electric current, and is thus a branch of electro-chemistry. The substance through which the current is passed is termed the electrolyte, and must be either an acid, base, or salt in a fused state or in solution. The current enters the electrolyte by an electrode called the anode, or the positive terminal. The electrode by means of which the the current leaves the electrolyte is termed the cathode, or negative terminal.
During the passage of the current the electrolyte is decomposed, and the products of decomposition are released at the electrodes or terminals. According to the modern theory of electrolysis, all electrolytes contain a greater or smaller number of free ions. These ions are chemical radicles carrying a definite electric charge. The kind of charge, positive or negative, depends on the nature of the radicle. The ions exhibit none of the chemical properties of the uncharged radicle.
Thus, for example, in an aqueous solution of sulphuric acid, free ions of hydrogen H2 carrying a positive charge, and free ions of SO4 carrying a negative charge, exist. An uncharged SO4 radicle would react with the water present, and sulphuric acid would be formed and oxygen liberated. The ion SO4, however, is incapable of doing this. Owing to the nature of their charges, the hydrogen ions will move towards the negative electrode, and the SO4 ions towards the positive electrode. On reaching the electrodes the ions give up their charges, and immediately exhibit their ordinary chemical properties. Hydrogen is given off at the negative electrode, while at the positive electrode the uncharged SO4 radicle reacts with the water present, and oxygen is released.