Tel′egraph, an instrument for transmitting signals to a distance. Unless otherwise specified the word is generally used to mean the electric telegraph, which is the only one we shall here discuss.

Electric signals appear to have been first transmitted on a commercial scale by Wheatstone and Cooke in England in 1837. At each end of the “line,” which consisted of five insulated metallic wires, they placed five galvanometers. Different combinations of deflections to the right and left indicated different letters. (See Galvanometer.) Later a single galvanometer was used, different combinations of deflections indicating as before different letters. About the same time S. F. B. Morse (q. v.) in America succeeded in perfecting a receiver in which the signals could not only be seen and heard, but were automatically recorded. This is the instrument which was in universal use in the United States until operators learned to read the message by the click of the instrument. Accordingly, for ordinary work a telegraph line is provided at each end with a battery for producing an electric current, a key for opening and closing the circuit and a small electromagnet, with pivoted armature, the so-called sounder or receiver. More recently the Wheatstone bridge has been applied to the telegraph line in such a way that two messages may be sent over a line in opposite directions at the same time. This is called the duplex telegraph. Edison has invented a method for sending two messages over the same line in the same direction at the same time. This is known as the diplex telegraph. By combining these two, four simultaneous messages may be sent. This system is called the quadruplex telegraph. Various plans have been suggested for sending even more than four messages at once. This is called the multiplex telegraph. Rowland (q. v.) perfected a system which will not only transmit six or eight simultaneous messages, but will deliver them printed on a typewriter; and not only so, but the sending instrument of the Rowland telegraph is merely the ordinary keyboard of a typewriter. For details concerning these various systems and concerning the important subject of submarine telegraph, first worked out by Lord Kelvin, the student is referred to the larger treatises.