Ans. It is only used in special cases, as for constant load; if the voltage be not just right to give the required current, it may be adjusted by changing the speed of the engine. =Ques. What may be said of the second method?= Ans. In both the “ring” and “drum” types of armature, rotating in a bipolar field, there are two points situated at opposite extremities of a diameter of the commutator, at one of which the potential is a maximum and at the other a minimum, and it is at these points that the brushes must be placed in order to obtain the greatest difference of pressure, the difference being less at other points. Hence, by rocking the brushes around the commutator the pressure at the terminals of the machine may be varied and regulated as required. =Ques. What difficulty is experienced in rocking the brushes to regulate the voltage?= Ans. Sparking takes place at the brushes when they are moved any considerable distance from the neutral position. Special dynamos have been designed to overcome this objectionable feature, still this method of regulation is not extensively used. =Ques. What may be said of the third method of regulation?= Ans. The third method, that of variation of field strength, is the one in general use. =Ques. How is the field strength varied?= Ans. This may be done by the _two path method_, or by the _variable field coil method_. =Ques. Describe the two path method of field regulation.= Ans. An adjustable resistance or _rheostat_ is connected in parallel with the field winding as shown in fig. 191. This shunts more or less of the current from the field winding according to the amount of resistance made active by the lever, _L_. Thus, if the current in the armature and main circuit be 10 amperes and the resistance of the field winding 10 ohms, a resistance of 40 ohms in parallel with the winding would cause the current to split in the ratio of 40 to 10, or 4 to 1; 2 amperes would pass through the resistance and 8 amperes through the field. [Illustration: FIG. 191.--The two path method of regulating a series dynamo. The ends of the series winding are connected by a shunt containing a rheostat. The current induced in the armature, divides and flows through the two paths thus offered, the amount flowing through the shunt being regulated by the rheostat. In this way the field strength is easily regulated.] [Illustration: FIG. 192.--Regulation of series dynamo by variable field. A multipoint switch is provided with connections to the field winding at various sections, thus permitting more or less of the field winding to be cut out to regulate its strength.] =Ques. Describe the variable field coil method of field regulation.= Ans. This consists in dividing the field winding into a number of sections and throwing the sections in and out of circuit as shown in fig. 192.