[Illustration: Fig. 691.--Diagram showing another and better method of coupling compound dynamos in parallel. With this arrangement the idle machines are completely disconnected from those at work. The same reference letters are common in both diagrams. S, S' are switches; F, F' fuses; A, A' ammeters, which indicate the total amount of current generated by each of the machines; AC, AC', automatic switches, arranged for automatically switching out a machine in the event of the pressure at its terminals being reduced through any cause; R, R,' are hand regulators, inserted in the shunt circuits of each of the machines, by means of which the pressures of the individual machines may be varied and the load upon each adjusted. The pressure at the bus bars is given by the voltmeter V, one terminal of which is connected to each of the bars; a second voltmeter may be used, to give the pressure of any individual machine, by connecting "voltmeter keys" to the terminals of each of the machines, or a separate voltmeter may be used for each individual machine. The only essential difference between figs. 690 and 691 is, that in fig. 690 the equalizer is connected direct to the positive brushes of all the dynamos, while in fig. 691 the equalizer is brought up to the switchboard and arranged between the two bus bars, a switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working.] Ques. How is a compound dynamo, running in parallel, cut out of circuit? Ans. The load is first reduced to a few amperes, as in the case of shunt dynamos, either by easing down the engine, or by cutting resistance into the shunt circuit by means of the hand regulator, and then opening the switch. Previous to this, however, it is advisable to increase the voltage at the bus bars to a slight extent, as while slowing down the engine the load upon the outgoing dynamo is transferred to the other dynamo armatures, and the current in their series coils not being increased in proportion, the voltage at the bus bars is consequently reduced somewhat. Equalizing the Load.--When a number of compound dynamos of various output, size, or make, are running together in parallel, it frequently happens that all their characteristics are not exactly similar, and therefore the load is unequally distributed, some being overloaded, while others do not take up their proper share of the work.