It is evident that with two or more magnetic fields, secured by increasing the number of poles, the armature inductors revolving between them cut more magnetic lines in one revolution than with a single field, hence, a given voltage is obtained with less speed of the armature than in the bipolar machine. For instance, if a bipolar dynamo be required to run at say 900 revolutions per minute to generate 125 volts, a four pole machine of equal output will require only 450 revolutions, and one of eight poles only 225 revolutions per minute. =Ques. What is a self-exciting dynamo?= Ans. A machine in which the initial excitation of the field is due to the residual magnetism retained by the cores. =Ques. What may be said of the field due to this residual magnetism?= Ans. It presents a very weak field, and the voltage that could be generated by the armature revolving in such a field would be only about two to ten volts. [Illustration: FIG. 190.--Series wound dynamo, used for series arc lighting, and as a booster for increasing the pressure on a feeder carrying current furnished by some other generator. The coils of the field magnet are in series with those of the armature and external circuit, and consists of a few turns of heavy wire. The characteristic of the series dynamo is to furnish current with increasing voltage as the load increases. If overloaded, the voltage will drop.] =Ques. How then can commercial voltages such as 100 or more volts be obtained with a self-exciting dynamo?= Ans. Part or all of the current induced in the armature is passed through the windings of the field magnets, thus strengthening the field. The voltage, therefore, will “build up,” increasing until the maximum has been reached. The maximum voltage will depend upon the capacity of the field magnets as determined by the construction, and upon the strength of current used to excite them. =Ques. How long does the process of “building up” require?= Ans. The time required to fully excite the field magnets is from ten to twenty seconds, the rise in field strength being indicated on the voltmeter or by the gradual increase in the brilliancy of the _pilot lamp_. =Ques. Name three important classes of dynamo.= Ans. Series wound, shunt wound, and compound wound. =Ques. Describe the winding of a series dynamo.= Ans. In this machine, the field magnets are wound with a few turns of thick wire joined in series with the armature brushes as shown in fig. 190. =Ques. What is the effect of this arrangement?= Ans. All of the current generated by the machine passes through the coils of the field magnets to the external circuit. The current in passing through the field magnets, energizes them and strengthens the weak field due to the residual magnetism of the magnet cores, resulting in the gradual building up of the field. =Ques. For what service is the series dynamo adapted?=