MOTORS, ELECTRIC. The term " electric motor " includes all apparatus by which electric energy is converted into mechanical energy. This can be accomplished (1) by the attraction that an electromagnet exerts upon an iron or steel armature ; (2) the mutual attraction between two electromagnets : (8) by analogous principles based upon the attract ive force exhibited between masses of magnetic metal, and (4) the action of a magnet upon a field of force created by the passage of currents in neighboring conductors. To these must be added that small experimental class which depend for their action on the attraction and repulsion of statically charged surfaces.
IlisroarcAL.—The discovery by Oersted, that the magnetic needle could be deflected by the passage of a current in proximity to it, was closely followed by that of Arago and Davy, who showed, independently of each other, that iron and steel could be magnetized by the passage of a current through a wire wound around them. Sturgeon utilized this in the con struction of the first powerful horseshoe magnets. These principles were soon applied to the construction of elementary electric motors, among them that of Barlow, known as Bar low's wheel. 1820. This consisted of a disk of copper between the poles of a magnet. The current was sent perpendicularly through the disk from axis to circumference, where it passed into a cup of mercury.
Prof. Joseph Henry may be said to have constructed the first electric motor acting upon the attraction and repulsion of electromagnets (1831). IL consisted of an oscillating electro magnet provided with a simple attachment for breaking and reversing the battery current, and thus reversing the polarity of the electro-magnet, which was alternately attracted and repelled by the poles of a permanent magnet.
A large number of inventors bad also constructed experimental motors, among them, Abbil Salvatore dal Negro, Dr. Shulthess. Davenport, Elias, Froment, Du Moncel, Wheat stone, Gaiffe, ITj8rth, ltoux, Larmenjeat, Bourbouze, Moses G. Farmer, and Thomas Hall.
Probably the most interesting of the early motors was that of Jacobi, which drove a boat on the Neva, at St. Petersburg, in 1838. This motor consisted of two sets of electro magnets. One set was fastened to a square frame, disposed in a circle, and with the poles projecting parallel with the axis. The other set was similarly fastened to a disk attached to the shaft, and revolving with it. Each set comprised four magnets, and there were conse quently eight magnetic poles. The current from a powerful battery passed through the commutator to the coils of the electromagnets, and as the magnets attracted each other, the disk rotated. By means of the commutator on the shaft. the current was reversed eight times during each revolution, just as the poles of two sets of magnets arrived opposite each other. Attraction ceasing, repulsion took place, and the motion was thus accelerated. As the poles were alternately of different polarity, the reversals had the effect of causing at traction between each pole of one set, and the next pole of the other. In his historic experi
ments of 11:38, Jacobi used a modified form of this motor, so as to obtain greater power.
The most celebrated early motor, next to that of Jacobi, was undoubtedly that of Prof. C. G. Page, of the Smithsonian Institution. This depended upon a different principle from that of the others. When the end of a bar of iron was held near a hollow electro magnetic coil or solenoid, the iron bar was attracted into the coil by a kind of a sucking action, until the bar had passed half way through the coil, after which no further motion took place. Professor Page constructed an electric engine on this principle about 1850. The solenoid was placed vertically, like the cylinder of an upright engine. A rod of iron, by way of armature, was fastened to a piston rod connected to the crank of a shaft carrying a ily-wheel. The core moved downward by its weight, until its upper end was tist leaving the solenoid, and thus one movement of the piston was accomplished. • On passing the cur rent, the core or piston was attracted upward, and thus the second movement was completed. A commutating device was attached to the shaft which automatically admitted the current into the coil and cut it off at the right moment. Professor Page soon improved on this single-acting electric engine by adding another solenoid, which could pull the piston in the other direction without the assistance of gravity.
A large motor of this description was constructed by Professor Page, in 1850, which developed over 10 horse-power. Professor Page sought to apply his motor to locomotion, and he actually constructed an electric locomotive to demonstrate the practicability of his scheme.
The most important of the early motors from a scientific standpoint, however, was the motor built, in 1861, by Professor Pacinotti, of the University of Pisa, and exhibited at Vienna in 1873, and in Paris in 1881. This motor, described in the Arum Ciinento for 1864, had an armature consisting of a toothed iron ring. and was wound and connected practically in the same manner as the Gramme armature. As to reversibility, he remarked with keen foresight : " This model further shows how the electro-magnetic machine is the complement of the magneto-electric machine, for in the first, the current obtained from any source of electricity, circulating in the bobbins, produces movement of the wheel with its consequent mechanical work; whilst in the second, mechanical work is employed to turn the wheel, and obtain, by the action of the permanent magnet, a current which may be transmitted by con ductors to any required point." Although the reversibility of the electric motor and the magneto-electric generator haul already been noticed, it was not until 1873, after the substitution of electro-magnets for per manent ones in electric generators, that the reversibility of the dynamo was fully realized, and pointed out by N. Fontaine, in the action of the Gramme machines exhibited at the Vienna exhibition of that year. Modern practice dates from this period.