By following this plan, the heavy sparking at the switch contacts is avoided, and the tendency for the engine to race, reduced. Ques. What precaution must be taken in reducing the current? Ans. Care must be taken not to reduce the current too much. Ques. Why is this necessary? Ans. There is danger that the machine may receive a reverse current from the other dynamos, resulting in heavy sparking at the commutator, and in the machine being driven as a motor. Ques. What provision is made to obviate this danger? Ans. Dynamos that are to be run in parallel are frequently provided with automatic cutouts, set so as to automatically switch out the machine when the current falls below a certain minimum value. Dividing the Load.--If a plant, composed of shunt dynamos running in parallel, be subjected to variations of load, gradual or instantaneous, the dynamos will, if they all have similar characteristics, each take up an equal share of the load. If, however, as is sometimes the case, the characteristics of the dynamos be dissimilar, the load will not be shared equally, the dynamos with the most drooping characteristics taking less than their share with an increase of load, and more than their share with a decrease of load. If the difference be slight, it may be readily compensated by means of the hand regulator increasing or decreasing the pressures of the machines, as the load varies. If, however, the difference be considerable, and the fluctuations of load rapid, it becomes practically impossible to evenly divide the load by this means. Under such circumstances, the pressure at the bus bars is liable to great variations, and there is also liability of blowing the fuses of the overloaded dynamos, thus precipitating a general breakdown. To cause an equal division of the load among all the dynamos, under such circumstances, it is needful to insert a small resistance in the armature circuits of such dynamos as possess the straightest characteristics, or of such dynamos as take more than their share of an increase of load. By suitably adjusting or proportioning the resistances, the pressures at the terminals of all the machines may be made to vary equally under all variations of load, and each of the machines will then take up its proper share of the load. Coupling Compound Dynamos in Series.--Since compound dynamos may be regarded as a combination of the shunt and series wound machines, and as no special difficulties are encountered in running these latter in series, analogy at once leads to the conclusion that compound dynamos under similar circumstances may be coupled together with equal facility. Ques. How are compound dynamos connected to operate in series?