As early as 1802 an Italian savant, Gian D. Romagnosi of Trent, learning of Volta’s discovery, observed and announced in a public print the deflection of the magnetic needle when placed near a parallel conductor of the galvanic current. In the years 1819 and 1820 so many brilliant discoveries and inventions were made by eminent men, independently and together, and at such near and distant places, that it is hard telling who and which was first. It was in 1819 that the celebrated Danish physicist, Oersted of Copenhagen, rediscovered the phenomena that the voltaic current would deflect a magnetic needle, and that the needle would turn at right angles to the wire. In 1820 Prof. S. C. Schweigger of Halle discovered that this deflecting force was increased when the wire was wound several times round the needle, and thus he invented the magnetising helix. He also then invented a galvano-magnetic indicator (a single-wire circuit) by giving the insulated wire a number of turns around an elongated frame longitudinally enclosing the compass needle, thus multiplying the effect of the current upon the sensitive needle, and converting it into a practical _measuring_ instrument--known as the galvanometer, and used to observe the strength of currents. In the same year Arago found that iron filings were attracted by a voltaic charged wire; and Arago and Davy that a piece of soft iron surrounded spirally by a wire through which such a current was passed would become magnetic, attract to it other metals while in that condition, immediately drop them the instant the current ceased, and that such current would permanently magnetise a steel bar. The elements of the _electro-magnet_ had thus been produced. It was in that year that Ampère discovered that magnetism is the circulation of currents of electricity at right angles to the axis of the needle or bar joining the two poles of the magnet. He then laid down the laws of interaction between magnets and electrical currents, and in this same year he proposed an electric-magneto telegraph consisting of the combination of a voltaic battery, conducting wires, and magnetic needles, one needle for each letter of the alphabet. The discoveries of Ampère as to the laws of electricity have been likened to the discovery of Newton of the law of gravitation. Still no practical result, that is, no useful machine, had been produced by the electro-magnet.