POLYPHASE CURRENTS.

IS)

field magnet, and the same is magnetized by induction, exhibitinga distinct pole on each of the pole-pieces. If a piece of soft ironis approached to the field magnet, it will at the start be attractedwith a rapid vibrating motion produced by the reversals of polar-ity of the magnet, but as the speed of the armature increases, thevibrations become less and less frequent and finally entirely cease.Then the iron is weakly but permanently attracted, showing thatsynchronism is reached and the field magnet energized by in-duction.

The disc may also be used for the experiment. If held quiteclose to the armature it will turn as long as the speed of rotationof the poles exceeds that of the armature; but when the normal

Q

Fig. 13.

speed is reached, or very nearly so, it ceases to rotate and is per-manently attracted.

A crude but illustrative experiment is made with an incandes-cent lamp. Placing the lamp in circuit with the continuous cur-rent generator and in series with the magnet coil, rapid fluctua-tions are observed in the light in consequence of the induced cur-rents set up in the coil at the startthe speed increasing, thefluctuations occur at longer intervals, until they entirely disap-pear, showing that the motor has attained its normal speed. Atelephone receiver affords a most sensitive instrument; whenconnected to any circuit in the motor the synchronism may beeasily detected on the disappearance of the induced currents.

In motors of the synchronous type it is desirable to maintain