The shunt circuits are represented as being connected to the positive and negative terminals of the respective machines, but in many cases where the load is subjected to sudden variations, and when a large number of machines is connected to the bus bars, the shunt coils are frequently connected direct to these. In such circumstances this method is preferable, as by means of it the fields of the idle dynamos can be excited almost at once direct from the bus bars by the current from the working dynamos; hence, if a heavy load come on suddenly, no time need be lost in building up a new machine previous to switching it into parallel. The pressure of the lamp circuit is given by a voltmeter whose terminals are placed across the bus bars; and the pressure at the terminals of each of the machines is indicated by separate voltmeters or pilot lamps, the terminals of which are connected to those of the respective machines. [Illustration: Fig. 687.--Diagram showing method of coupling shunt dynamos in series. The ends of the shunt coils may be connected to the terminals of their respective machine, or they may be connected in series as shown.] Ques. Describe a better method of parallel connection. Ans. Better results are obtained by connecting both the shunt coils in series with one another, so that they form one long shunt between the two main conductors, the same as in fig. 687. When arranged in this way, the regulation of both machines may be effected simultaneously by inserting a hand regulator (R) in series with the shunt circuit as represented. [Illustration: Fig. 688.--Diagram showing method of coupling shunt dynamos in parallel.] Switching Dynamo Into and Out of Parallel.--In order to put an additional dynamo in parallel with those already working, it is necessary to run the new dynamo up to full speed, and, when it excites, regulate the pressure by means of a hand regulator until the voltmeter connected to the terminals of the machines registers one or two volts more than the voltmeter connected to the lamp circuit, and then close the switch. The load upon the machine can then be adjusted to correspond with that upon the other machines by means of the hand regulator. Ques. In connecting a shunt dynamo to the bus bars, must the voltage be carefully adjusted? Ans. There is little danger in overloading the armature in making the connection hence the pressure need not be accurately adjusted. It is, in fact, common practice in central stations to judge the voltage of the new dynamo merely by the appearance of its pilot lamp. Ques. How is a machine cut out of the circuit? Ans. When shutting down a machine, the load or current must first be reduced, by gradually closing the stop valve of the engine, or inserting resistance into the shunt circuit by means of the hand regulator; then when the ammeter indicates nine or ten amperes, the main switch is opened, and the engine stopped.