Coupling Series Dynamos in Series.--Series wound dynamos will run satisfactorily together without special precautions when coupled in series, if the connections be arranged as in fig. 685. The positive terminal of one dynamo is connected to the negative terminal of the other, and the two outer terminals are connected directly to the two main conductors or bus bars through the ammeter A, fuse F, and switch S. If it be desired to regulate the pressure and output of the machines, variable resistances, or hand regulators R, R^1, may be arranged as shunts to the series coils as shown, so as to divert a portion or the whole of the current therefrom. Series Dynamos in Parallel.--Simple series wound dynamos not being well adapted for the purpose of maintaining a constant pressure, are in practice seldom coupled in parallel; the conditions or working, however, derive importance from the fact that compound dynamos, being provided with series coils, are subject to similar conditions when working in parallel, which is frequently the case. Ques. What may be said with respect to coupling two or more plain series dynamos in parallel? Ans. The same procedure cannot be followed as in the case of plain shunt dynamos, for the reason that if the voltage of the dynamo to be coupled be exactly equal to that of the bus bars when connected in parallel, the combination will be unstable. [Illustration: Fig. 685.--Diagram showing method of coupling series dynamos in series. R and R' are two hand regulators which are placed in shunt across the coil terminals to regulate the pressure and output of the machine.] Ques. Why is this? Ans. If, from any cause, the pressure at the terminals of one of the dynamos fall below that of the others, it immediately takes a smaller proportion of the load; as a consequence, the current in its field coils is reduced, and a further fall of pressure immediately takes place. This again causes the dynamo to relinquish a portion of its load, and again occurs a further fall of pressure. Thus the process goes on, until finally the dynamo ceases to supply current, and the current from the other dynamos flowing in its field coils in the reverse direction reverses its magnetism, and causes it to run as a motor against the driving power in the opposite direction to that in which it previously ran as a dynamo. Under such circumstances the armature is liable to be destroyed if the fuse be not immediately blown, and in any case it is subjected to a very detrimental shock. This tendency to reverse in series dynamos can be effectually prevented by connecting the field coils of all the dynamos in parallel. [Illustration: Fig. 686.--Diagram showing method of coupling series dynamos in parallel. In the diagram A, A', are ammeters; F, F', fuses; S, S', switches.] Ques. How are the field coils of all the dynamos connected in parallel?