His pace was slower now and she had no trouble keeping in step.
The current in the shunt coil lags 90 degrees behind the impressed electromotive force of the circuit to be measured; hence if the main current is in step with the potential difference of the terminals of the supply mains, which is the case when the supply is given wholly to electric lamps, then the field due to the main coil differs from that due to the shunt coil by 90 degrees.
Since the eddy currents induced in the disk are 90 degrees in phase behind the inducing field, the eddy currents produced by the main coil are in step with the magnetic field due to the shunt coil, and hence the disk is driven round by the revolution due to the action of the shunt coil upon the induced currents in the disk.
In the point-by-point method the shaft of an alternator, or an alternating current motor driven in step with it, is furnished with an insulating disk having a metallic slip inserted in its edge.
In the Hospitalier ondograph, 1 a synchronous electric motor driven in step with the periodic current in the circuit being tested drives a cylinder of insulating material having a metallic slip let into its edge.
