the magnet is utilized. The current enters through the commutator, which reverses it at each half-revolution just as the poles of the armature are passing those of the field-magnet. The armature, therefore, rotates under these alternate attractions and repulsions. The rate of speed can be regulated with nicety by turning the brushes around the commutator cylinder to and from the neutral points. The speed is rendered uniform under a varying load by a very simple centrifugal governor, consisting of a small spring, one extremity of which is attached to an end of the armature coil, and the other rests against the commutator. When the speed increases beyond the normal rate, the free end of the spring is thrown out from contact with the commutator, and the current interrupted until this rate is regained. This governor has proved very sensitive in use, controlling the speed within variations of 1700 of its mean rate.
Excellent as this motor is, it has the defect common to all machines in which the armature is approaching or receding from the poles of the field-magnet during but a small part of each revolution. Currents are induced only during these periods, and hence much of the effective power of the field-magnet is lost. M. Trouvé has recently constructed a machine in which this defect is removed in a very simple manner. Instead of making the grooves in the Siemens armature parallel with its axis, they are cut in a spiral form, so that portions of the armature cores are approaching and receding from the poles of the field-magnet during the entire revolution. The impulse received by the armature is, therefore, a continuous one, and dead points are avoided.
The various electro-motors may of course be worked by currents furnished by ordinary batteries, and for running a sewing-machine a few hours a day, at a comparatively small cost. But, as a means of furnishing currents for power for any considerable time, such batteries are out of the question. As pointed out by Professor Ayrton, even if an electro-motor were a perfect machine—that is, if its efficiency were unity—it would be thirty-three times as expensive as a steam-engine, if operated by currents from such a source. The costliness of present batteries is, however, no necessary index of future possibilities. The most feasible way of obtaining power by electricity to-day seems to be through distribution of the current from a central point of supply; but it is not impossible that a battery may yet be produced which, furnishing electricity as cheaply as a machine, will remove the need of distribution, and at the same time greatly enlarge its field of usefulness.
