Diplex telegraphy is the simultaneous transmission of two separate messages in the same direction over one line. Without going into details it may be said that for this purpose two different transmitting keys are required, one of which alters the direction, and the other the strength of the current though the line wire. The receivers are arranged so that one responds only to a strong current, and the other only to a current in one particular direction. A line also may be quadruplexed, so that it is possible to transmit simultaneously two messages from each end, four operators being required at each station, two to transmit and two to receive. Systems of multiplex telegraphy have been devised by which very large numbers of messages can be sent at once over a single wire, and the Baudot multiplex telegraph has proved very successful. [Illustration: FIG. 30.--Diagram to illustrate principle of Duplex Telegraphy.] The wires for telegraphic purposes may be conveyed either above or below the ground. Overground wires are carried on poles by means of insulators of porcelain or other non-conducting material, protected by a sort of overhanging screen. The wires are left bare, and they are generally made of copper, but iron is used in some cases. In underground lines the wires formerly were insulated by a covering of gutta-percha, but now paper is generally used. Several wires, each covered loosely with thoroughly dry paper, are laid together in a bundle, the whole bundle or cable being enclosed in a strong lead pipe. The paper coverings are made to fit loosely so that the wires are surrounded by an insulating layer of dry air. As many as 1200 separate wires are sometimes enclosed in one pipe. In order to keep telegraph lines in working order frequent tests are necessary, and the most important British Postal Telegraph lines are tested once a week between 7.30 and 7.45 a.m. The earth is generally used for the return circuit in telegraphy, and the ends of the return wires are connected either to metal plates buried in the ground to a depth at which the earth is permanently moist, or to iron gas or water pipes. The current for telegraph working on a small scale is usually supplied by primary cells, the Daniell cell being a favourite for this purpose. In large offices the current is generally taken from a battery of storage cells.