Ans. The external circuit should be closed, otherwise a closed circuit will not be formed through the field magnet winding and the machine will not build up. Ques. What is understood by the term "build up"? Ans. In starting, the gradual voltage increase to maximum. [Illustration: Fig. 675.--Connections for two compound wound dynamos to run in parallel. The series fields of the machines are connected together in parallel by means of wire leads or bus bars, which connect together the brushes from which the series fields are taken. This is known as the equalizer and is shown by the line running to the middle pole of the dynamo switch. By tracing out the series circuits it will be seen that current from the upper brush of either dynamo has two connections to its bus bar. One of these leads through its own field, and the other, by means of the equalizer bar, through the fields of the other dynamo. As long as both machines are generating equally there is no difference of pressure between the brushes of either, but should the voltage of one be lowered, current from the other would flow through its fields and thereby raise the voltage, and at the same time reduce its own until both are equal. The equalizer may then be called upon to carry much current, but to have the machines regulate closely it should be of low resistance. It may also be run as shown by the dotted lines, but this will leave all the machines alive when any one is generating. The ammeters should be connected as shown. If they were on the other side they would come under the influence of the equalizing current and would indicate wrong, either too high or too low. The equalizer switch should be closed a little before the main switches are closed.] Ques. How should a shunt or compound machine be started? Ans. All switches controlling the external circuits should be opened, as the machine excites best when this is the case. If the machine be provided with a rheostat or hand regulator and resistance coils, these latter should all be cut out of circuit, or short circuited, until the machine excites, when they can be gradually cut in as the voltage rises. When the machine is giving the correct voltage, as indicated by the voltmeter or pilot lamp, the machine may be switched into connection with the external or working circuits. Ques. In starting a shunt dynamo, should the main line switch be closed before the machine is up to voltage or after?