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made, the arrangement is somewhat modified. The most simple arrangement is that in which the generators feed into a single distributing system. A switchboard adapted to meet this condition is shown in Fig. 19. Of the three bus bars, the lower one, marked 3, connects with the main switches S1 and S2, and through wires d d' which will be recognized at once as the equalizing wire, that is, wires. dd' together with bus 3, constitute the equalizing wire. These equalizing wires, as will be noticed, do not connect with the circuit breakers c b, and it is not necessary that they should; for if the connection with wire e and h is broken the circuit through the generator will be opened.

In this arrangement we have shown each generator connected with its own ammeter, but in some cases only one instrument is used, and this is arranged so as to measure the combined current of the two machines. When such an arrangement is adopted it is common to provide a switch by means of which the ammeter may be connected with each generator independently, when desired, so that the strength of each current may be determined. While we have shown two ammeters in Fig. 21 we have only provided one voltmeter, and this is arranged to indicate the voltage between the two busses 1 and 2. When a single voltmeter is provided, it is arranged so as to be connected with each generator independently so as to be used in adjusting the machines in the act of starting. Only two distributing circuits are shown in this diagram, but it will be noticed that, in one, a circuit breaker is provided as well as a switch. It is customary to provide circuit breakers in the distributing circuits when there are devices upon them that would be seriously injured by an excessive current.

For two generators.

Fig. 19 shows the simplest form of switchboard for two generators, as it is arranged for only one system of

distributing circuits. In Fig. 20 a more complicated arrangement is shown in which two independent distributing

More elaborate arrangement.

systems are provided for. It will be noticed that this diagram is simply an elaboration of Fig. 12, and that two double throw main switches, S1 and S2, are provided instead of one, but these are necessary on account of there being two generators. In all other respects the diagrams are identical. The main switches in Fig. 20 are shown in the position that connects the generators with the lower set of busses, marked A. If either switch is thrown to the opposite side it will connect its generator with the top busses B. Both generators can be connected with the top busses and either one can be connected with either set of busses, but the whole system cannot be tied together as in Fig. 12. To accomplish this result we would have to provide a switch to correspond with ss of that figure, that is, a switch to connect the two sets of busses. If each generator is connected with a separate set of busses, we can run them at different voltages, but if both feed into the same set as represented in the diagram, the voltage must be equalized. Therefore, unless the two voltmeters indicate the same pressure, the generators cannot be connected with the same set of busses.

In starting up two generators, connected in parallel, the course of procedure is to set the machines in motion, then close the circuit through one and adjust its voltage by means of the field regulator to the proper point. This much being done, the circuit breaker of the second machine is closed, and then the voltage developed by the armature is indicated notwithstanding that the main switch is opened, for the circuit between wires d and e will be closed through the voltmeter and the wires and i leading to the ammeter. By moving the field regulator of the second machine, its voltage is brought to the same

What might happen.

point as that of the machine already in operation, and when the two machines are at the same voltage the main switch of the second one is closed. If the switch of the second generator were closed when the voltage of the first machine was higher than the second, then the latter might be overpowered, and the current could be forced through it in the backward direction if the difference in the voltages were great enough. Under such conditions, the second generator would act as a motor and possibly drive the engine at a speed high enough to do damage. If the second. machine were developing the higher voltage when they were connected, it might reverse the current through the first one, and convert it into a motor. Unless the difference in the voltages is very great there is no danger of such an occurrence, for as soon as the overpowered machine begins to lose its current the speed of the engine will increase, and thereby increase the voltage and thus check the further decrease of current, but it is not wise to bank on such action taking place, although it would in most cases. The safest plan is to take it for granted that if the two generators are not adjusted to the same voltage within say one or two volts, they cannot be connected in parallel without doing damage. When only one voltmeter is provided, as in Fig. 19, it must be shifted from one machine to the other to adjust the voltages in starting up the second machine, and this takes time, and is not so reliable a method as when each generator has an independent instrument. Hence, if economy dictates the reduction in the number of instruments, it is better to provide one voltmeter for each generator and only one ammeter for the two. The best arrangement of all, is to provide a differential voltmeter, to be used for the purpose of connecting the generators.