This principle or construction is obviously capable of many modified applications, most of which follow as a matter of course from the constructions described; for instance, the armature or induced coils or those in which the current are set up by induction, may be held stationary and the alternating currents from the generator conducted through the rotating inducing or field coils by means of suitable sliding contacts. It is also apparent that the induced coils may be movable and the magnetic parts of the motor stationary.
An advantage and a characteristic feature of motors constructed and operated in accordance with this plan, is their capability of almost instantaneous reversal, by the reversal of one of the energizing currents from the generator.
This will be understood from a consideration of the working conditions. Assuming the armature to be rotating in a certain direction following the movement of the shifting poles, then let the direction of the shifting be reversed which may be done by reversing the connections of one of the two energizing circuits. If it be borne in mind that in a dynamo-electric machine the energy developed is very nearly proportionate to the cube of the speed, it is evident that at such moment an extraordinary power is brought to play in reversing the motor. In addition to this the resistance of the motor is very greatly reduced at the moment of reversal so that a much greater
