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allows it to drive the engine but automatically disengages when the engine starts so that the engine will not drive the motor. If such a device were not fitted the generator might be injured by the motor driving it at too high a speed.
As already explained, the function of the automatic cutout is to disconnect the generator from the battery when the engine is stopped or turning so slowly that its voltage is below that of the battery. If this cutout were not provided the storage battery would discharge back into the generator.
The cutout consists of an electro-magnet with two windings. One is a shunt winding of many turns of fine wire and the other a series winding of a few turns of heavy wire, both windings being over a soft iron core. The shunt winding is permanently connected across the positive and negative terminals of the generator, so that when the generator comes up to charging speed, this winding energizes the magnet core and the magnet core attracts a steel arm that closes the circuit between the generator and the battery.
So long as the cutout points are closed the current must pass through the series winding of the cutout. This current adds its magnetizing influence to that of the shunt winding and holds the points together. The cutout is designed so that it closes at a carspeed of 12 miles per hour and opens at 10.
If, now, the speed of the generator drops below charging speed, the current begins to flow through the cutout series winding in the reverse direction. This weakens the pull and allows the points to fly apart, through the agency of a spring.
Now that a general idea of the different parts of the Gray & Davis system has been obtained, the path of the current in the different wires will be explained. The illustration shows this system with a very complete equipment. Besides the usual head, side and tail lights, there are pillar lights, dome lights, a speedometer light and an electric horn connection. It will be noticed that the return circuits are through the frame, with the exception of the connections between the storage battery and the starting motor.
First we will trace out the flow of current when the starting switch is closed, this circuit being shown by the heavy black lines. Current flows from the plus terminal of the storage battery out
Fig. 197.-Technical Wiring Diagram Showing Circuits in Gray & Davis
Two Unit Starting and One Wire Lighting System.
through wire A to the motor, where it passes through the series field and the armature and from thence through the wire T to the starting switch and from there through the wire C to the negative pole of the battery.
Below 9 or 10 miles an hour or when the motor is at rest the cutout is open and therefore current for the lights must be furnished by the battery, and its path is as follows: It runs out through wire A to one terminal of the starting motor, where it goes to the frame through the ground wire Z. From thence it runs to the lamps. From the lamps the current passes to the junction switch, where all the lamp terminals are connected to the terminal P, and from here the current flows through the series field of the generator and on out through wire F to a terminal on the cutout, and from thence to the ammeter over the sh vire E. From the am
it goes via wire D to a binding post on the starting switch, from which it connects with the other pole of the battery by wire C. At or over 12 miles per hour the cutout contact points are closed as previously described. Current is then supplied to the storage battery if it needs charging and also to any of the lamps that are in circuit.
If the battery needs recharging it is of course below the voltage of the generator and therefore current will flow to it until its voltage becomes equal to that of the generator, when the flow will automatically stop because the electrical pressure at the two points is the same. The current passes from the positive terminal of the generator through wire G to the series coil of the cutout and from thence through wire Y to the frame. It flows through the frame up through wire Z to one terminal on the motor and from thence through wire A to the plus pole of the battery. The return circuit is through wires C and D to the ammeter and from thence through wires E and F back to the generator. The flow of current from the generator to the lamps is as follows: Through wire G and the series coil of the cutout and wire Y to the frame. This part of the circuit is identical with that for charging the storage battery. Then the current goes through the frame and up through the ground wires to the lamps, from whence it passes to the terminals on the junction switch and on through wire P to the generator. It will