DOMESTIC MOTORS. 495 



the magnet is utilized. The current enters through the commutator, 

 which reverses it at each half -revolution just as the poles of the arma- 

 ture 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 at- 

 tached 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 commuta- 

 tor, and the current interrupted until this rate is regained. This gov- 

 ernor has proved very sensitive in use, controlling the speed within va- 

 riations of yi- 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 effec- 

 tive power of the field-magnet is lost. M. Trouve has recently con- 

 structed 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 armatui'e 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, fur- 

 nishing electricity as cheaply as a machine, will remove the need of 

 distribution, and at the same time greatly enlarge its field of useful- 

 ness. 



