Unless the line be automatically opened under such a condition either by a fuse or circuit breaker, the machine and its driving engine may be damaged. To avoid this danger fuses or automatic circuit breakers are employed. =Ques. Mention another service for which the compound dynamo is used.= Ans. In some isolated plants, as small country residences where it is frequently necessary to have a dynamo capable of charging a storage battery during the day, and of furnishing current for lighting during a certain portion of the evening. Under such conditions the compound machine with slight modification is used, the ordinary shunt dynamo not being capable of maintaining the necessary consistency of voltage, without attention to the shunt regulator in driving the lamps direct, the ordinary compound dynamo on the other hand, being unsatisfactory for charging storage batteries. =Ques. How is the compound dynamo modified to adapt it to the dual service of lighting and battery charging?= Ans. It is furnished with _alternative compound winding_, in which the series winding is provided with a switch, which may be fixed either upon the machine itself or upon the switchboard. This switch permits the series coils to be either short circuited in part or cut out of the circuit entirely while the machine is charging the storage battery, being again cut into circuit when the machine is required to furnish current for the lamps. [Illustration: FIG. 198.--Separately excited dynamo. Current for field excitation is supplied by a second and smaller generator.] =Separately Excited Dynamos=.--In this class of machine the current required to excite the field magnets is obtained from some independent external source. Though used by Faraday, the separately excited dynamo did not come into favor until, in 1866, Wilde employed a small auxiliary magneto machine to furnish currents to excite the field magnets of a larger dynamo. A separately excited dynamo is shown in fig. 198. This method of field excitation is seldom used except for alternators; it is, however, to be found occasionally in street railway power houses, the shunt fields of all the dynamos being separately excited by one dynamo. In common with the magneto, the separately excited machine possesses the property that, with the exception of armature reactions, the magnetism in its field and therefore the total voltage of the machine is independent of variations in the load. [Illustration: FIG. 199.--Diagram showing principle of Dobrowolski three wire dynamo. This type of machine is shown in more detail in fig. 795 on page 708.]