The third element of the telegraph is the electro-magnet. This, and its arrangement as a relay in a local circuit, was a most important invention, and contributed quite as much to the success of the telegraph as did the inventions of Prof. Morse. It may be well to say that an electro-magnet is a magnet which attracts an iron armature when an electric current is sent through its coil of wire, and loses its attractive force when the circuit is cut off, thereby rendering it possible to produce mechanical effects at a distance through the agency of electrical impulses only. For the electro-magnet the world is chiefly indebted to Prof. Joseph Henry, formerly of Princeton, N. J., but later of the Smithsonian Institution. In 1828 he invented the energetic modern form of electro-magnet with silk covered wire wound in a series of crossed layers to form a helix of multiple layers around a central soft iron core, and in 1831 succeeded in making practical the production of mechanical effects at a distance, by the tapping of a bell by a rod deflected by one of his electro-magnets. This experiment may be considered the pioneer step of the telegraph. [Illustration: FIG. 4. HENRY. STURGEON. ] Great as was the work of Prof. Henry, he must share the honors with a number of prior inventors who made the electro-magnet possible. Electro-magnetism, the underlying principle of the electro-magnet, was first discovered in 1819 by Prof. Oersted, of Copenhagen. In 1820 Schweigger added the multiplier. Arago in the same year discovered that a steel rod was magnetized when placed across a wire carrying an electric current, and that iron filings adhered to a wire carrying a voltaic current and dropped off when the current was broken. M. Ampere substituted a helix for the straight wire, and Sturgeon, of England, in 1825 made the real prototype of the electro-magnet by winding a piece of bare copper wire in a single coil around a varnished and insulated iron core of a horse shoe form, but the powerful and effective electro-magnet of Prof. Henry is to-day an essential part of the telegraph, is in universal use, and is the foundation of the entire electrical art. It is unfortunate that Prof. Henry did not perpetuate the records of his inventions in patents, to which he was opposed, for there is good reason to believe that he was also the original inventor of the important arrangement of the electro-magnet as a relay in local circuit, and other features, which have been claimed by other parties upon more enduring evidence, but perhaps with less right of priority. [Illustration: FIG. 5.--MORSE’S FIRST MODEL PENDULUM INSTRUMENT.]