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.”
FIG. 28.—PROF. HENRY’S ELECTRIC MOTOR.
““To the wires q r two other wires are soldered so as to project in an opposite direction, and dip into the thimbles l m. The wires of the galvanic magnet have thus, as it were, four projecting ends; and by inspecting the figure it will be seen that the extremity p, which dips into the cup m, attached to the copper of the battery in G, corresponds to the extremity r which dips into the cup t, connecting, with the zinc in battery F. When the batteries are in action, if the end B is depressed until q r dips into the cups s t, A B instantly becomes a powerful magnet, having its north pole at B; this, of course, is repelled by the north pole D, while at the same time it is attracted by C; the position is consequently changed, and o p comes in contact with the mercury in l m; as soon as the communication is formed, the poles are reversed, and the position again changed. If the tumblers be filled with strong diluted acid, the motion is at first very rapid and powerful, but it soon almost entirely ceases. By partially filling the tumblers with weak acid, and occasionally adding a small quantity of fresh acid, a uniform motion, at the rate of seventy-five vibrations in a minute, has been kept up for more than an hour; with a large battery and very weak acid the motion might be continued for an indefinite length of time.””