The operating coil of each DC line switch is in the same circuit as the operating coil of its associated motor line switch, so that the generator line switch closes when the motor is started.

If power is applied to the motor of one of the sets and both of the generator line switches could be closed at the same time, the generator of the other machine would run as a motor, which is not desired. To avoid this, the "Start" button for one motor-generator set opens the line switch for the other generator and vice-versa.

As an additional safeguard, a mechanical "inter-lock" is also added to the panel. This "inter-lock" consists of a bar pivoted between the two contactors (see Figure 25) . When one contactor closes, it pushes one end of this bar down, and the other end is pushed up against the other contactor so that it cannot close. On account of these features, in case of emergency it is not necessary to push the "Stop" button of a machine which has been operating. Simply push the "Start" button for the other machine. This will automatically turn off the power to the machine which was being used and start up the stand-by machine.

To adjust the voltage of the DC generators push the "Start" button of one of the push-button stations. Adjust the voltage at the generator panel, by means of the generator field rheostat which is directly below the line switch which is closed, so that the generator panel meter reads 110 volts when two projector motors are running. After the voltage of one generator is adjusted, stop the machine, start the other, and adjust it in the same manner. The generator voltages should seldom require readjustment after they are once set properly.

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