Although the electric motor of to-day depends for practical value entirely upon the dynamo which supplies it with electric power, nevertheless the motor considerably antedated the dynamo. The genesis of the electric motor began in 1821 with Faraday’s observation of the phenomenon of the conversion of an electric current into mechanical motion. In his experiment a copper wire was supported in a vertical position so as to dip into a cup of mercury, while a small bar magnet was anchored at one end by a thread to the bottom of the cup and floated in the mercury in upright position. The mass of mercury being connected to one pole of a battery, and the vertical wire to the other, it was found that when the circuit was completed by clipping the wire into the mercury, the floating bar magnet would revolve around the wire as a center. [Illustration: FIG. 26.--BARLOW’S WHEEL.] In 1826 Barlow, of Woolwich, made his electrical spur wheel, Fig. 26, and in 1830 the Abbe Dal Negro, in Padua, is said to have constructed a sort of vibrating electrical pendulum, both of which devices were crude forms of magnetic engines. Dal Negro’s machine, see Fig. 27, consisted of a magnet A, movable about an axis situated about one-third of its length, and the upper extremity of which was capable of oscillating between the two branches of an electro-magnet E. A current being sent into the electro-magnet, passed through an eight-cupped mercurial commutator C, which the oscillating magnet controlled by means of a rod _t_ and a fork F. When the magnet had been attracted toward one of the poles of the electro-magnet this very motion of attraction acting upon the commutator changed the direction of the current, and the magnet was repelled toward the other branch of the electro-magnet, and so on. [Illustration: FIG. 27.--DAL NEGRO’S ELECTRIC MOTOR.] In 1828 Prof. Joseph Henry produced his energetic electro-magnets sustaining weights of some thousands of pounds, and gave prophetic suggestion of the possibilities of electricity as a motive power. In 1831 he devised the electric motor shown in Fig. 28, which is described in Prof. Henry’s own words as follows: “A B is the horizontal magnet, about seven inches long, and movable on an axis at the center; its two extremities when placed in a horizontal line are about one inch from the north poles of the upright magnets C and D. G and F are two large tumblers containing diluted acid, in each of which is immersed a plate of zinc surrounded with copper; _l m s t_ are four brass thimbles soldered to the zinc and copper of the batteries and filled with mercury. “The galvanic magnet A B is wound with three strands of copper bell wire, each about twenty-five feet long; the similar ends of these are twisted together so as to form two stiff wires _q r_, which project beyond the extremity B, and dip into the thimbles _s t_. [Illustration: FIG. 28.--PROF. HENRY’S ELECTRIC MOTOR.]