=260. The Telegraph.=--The invention of an effective electromagnet by Henry made possible the _electric telegraph_. In its simplest form it consists of a battery, _C_, a key, _K_, and a sounder, _S_, with connecting wires. (See Fig. 237.) The _sounder_ (Fig. 238) contains a _horseshoe electromagnet_ and a bar of soft iron across its poles called an armature, _A_, attached to a lever _L_. When the key is closed, the electromagnet draws down the armature and lever until the latter hits a stop _O_, making a click. When the key is raised, the magnet releases the armature which is raised by the action of a spring at _S_ until the lever hits a stop at _T_ making another click. Closing and opening the circuit at _K_ will start and stop the current which operates _S_ which may be 100 miles or more from _K_. One voltaic cell will work a sounder in the same room. But if many miles of wire are in the circuit, the E.M.F. of a single cell will not force sufficient current through the long wire to operate the sounder. [Illustration: FIG. 238.--A telegraph sounder.] [Illustration: FIG. 239.--A telegraph relay.] [Illustration: FIG. 240.--How the relay is used.] [Illustration: Samuel F. B. Morse (1791-1872). Inventor of the electromagnetic recording telegraph and of the dot and dash alphabet. SAMUEL F. B. MORSE "From Appleton's Cyclopedia of American Biography, Copyright 1888 by D. Appleton & Co."] [Illustration: Thomas A. Edison, Orange, New Jersey. Invented the incandescent lamp; phonograph; moving picture; most noted inventor of electrical appliances of the present day. THOMAS A. EDISON "Copyright, Photographische Gessellschaft," and "By Permission of the Berlin Photographic Co., New York."] A battery of several cells is then required. Even a large battery is insufficient to operate a long line containing many sounders in circuit. Recourse is therefore usually made to a more sensitive device called a _relay_. (See Fig. 239.) In the relay a very small current will magnetize its electromagnet enough to draw toward it the delicately hung armature thereby closing a second circuit which contains a sounder and a battery. (See Fig. 240.) when the current in the main circuit is stopped, the armature of the relay is drawn back by a light spring. This opens the _local_ circuit. Thus the local circuit is closed and opened by the relay just in time with the starting and stopping of the current in the main line. It is thus possible for a small current in the main line by the use of a relay, to close and open a second local circuit containing a local battery and sounder. Modern telegraph lines are operated in this manner. [Illustration: FIG. 241.--An electric bell and its circuit.]