[Illustration: Fig. 673.--Connections for two shunt wound dynamos to run in parallel. The positive lead of each machine is connected to the same bus bar. In starting, if but one machine is to be used, the dynamo is first brought up to speed and the voltage regulated by means of the rheostat R and the voltmeter V. The main switch is then thrown in. The connections for the field are taken off the dynamo leads so that the opening of the main switch will not open the field circuit and for this reason the field will begin to build up as soon as the machine is started. When but one of the machines is running, the idle machine is brought up to speed with the main switch open, and the voltage regulated by means of the rheostat and voltmeter until the voltages of the machines are the same. Then the main switch is thrown in and the load on the machines (which is ascertained by the ammeters) is equalized by means of the rheostats. Should there be any great difference in voltages, the higher one will run the other as a motor without changing the direction of rotation. The field current will remain unchanged, and the armature current of the low dynamo will be reversed, which will cause it to run as a motor in the same direction as it ran as a dynamo. When dynamos feeding current to motors are to be shut down, the switches on the motors should first be opened. Otherwise some of the motor fuses will blow. As the voltage goes down the motors will draw more current to do the work. If a plant be shut down with the motor switches "in" it will generally be found impossible to start a shunt dynamo, the low resistance in the mains not allowing enough current to flow around the shunt fields to energize them.] Ques. Should the brushes be raised out of contact in starting? Ans. The brushes should not be in contact in starting if there be any danger of reverse rotation, as might happen when the dynamo is driven by a gas engine. Aside from this, it is desirable that the brushes be in contact, because they are more easily and better adjusted, and the voltage will come up slowly, so that any fault or difficulty will develop gradually and can be corrected, or the machine stopped before any injury is done. [Illustration: Fig. 674.--Connections for two shunt dynamos to run on the three wire system. The two machines are connected in series, three wires being carried from them, one from the outside pole of each machine and one from the junction of the two machines. The voltage between the outside wires is equal to the combined voltage of the two machines and the voltage between the outside and the central or neutral wire is equal to the voltage of the corresponding machine. If the load on each side of the system be equal, there will be no current in the neutral wire, while if the loads be unequal, the neutral wire will have to carry the difference in current between the two outside wires.] Ques. How should a series machine be started?