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Figure 6 shows an extended electrical multiple system. The mains, AA, will have to carry all the current passing through the lamps, or, if there are twenty lamps each taking J ampere, the mains will have to carry 10 amperes. The branches, BB, carry only the current required by the lamps they feed, or, as
the system is laid out in the figure, they will carry five amperes each. The taps, CO, will carry amperes each. In this way the current may be distributed throughout a building, branches being led to various points and the taps taken off as lamps are required. With what is called the "closet" system the taps are all taken off at a few centrally located places, and in recesses in the wall may be set all the safety-fuses and the switches, so that the controlling may all be done from these few points. Figure 7 shows a system arranged in this way. This is the system used almost exclusively where the wiring is concealed. It requires
more wire, but it does away with the safety-fuse blocks scattered about in unsightly and inconvenient places, and provides a special place for them in a receptacle that is unobtrusive.
The three-wire system is a special form of the multiple system and is largely used because it effects a great saving of copper. Figure 8 will serve to illus
voltage are used, and are connected as shown in the diagram. Suppose the voltage of each to be 110 volts. Then A is creating a difference of pressure between c and d of 110 volts, and B is creating a difference of pressure between d and « of 110 volts, so that there is
a resulting difference of pressure between c and « of 220 volts. The "positive side" (or the side the current starts from) of A is the negative side (or the side the current returns to) of B just as it might be with two pumps, one creating a certain pressure and passing the water to the next, which adds a like pressure. If now the lamps were connected directly between o and e, the
trate the principle. Two dynamos having the same
system would be working at a pressure of 220 volts, and, as indicated on page 34, the wire necessary to carry a current with a given per cent loss would be only one-fourth as much as would be necessary with onehalf the voltage, or 110 volts. But with 220 volts it would be necessary to run two 110-volt lampi in series or to have 220-volt lamps. There are disadvantages in both these arrangements, however, that would counterbalance the advantage in the saving of copper, and 110-volt lamps are used with the two dynamos in the following way: —
The wire d, called the "neutral wire," is run with the other wires, c and e. Suppose that all the lamps are turned off, excepting one lamp between e and d. Dynamo A will have no work to do and will be running idle, while dyamo B will be supplying current to the one lamp through the wires e and d as if the circuit were a simple multiple system. But suppose now that a lamp is turned on between c and d. In the conventional way of speaking, dynamo B will be tending to send the current required by one lamp between e and d, from its positive side through the wire e to the lamp, and back through the wire d to the negative side of the dynamo; while dynamo A, on the other hand, will be tending to send the same strength of current for the lamp between c and d, from its positive side out through the wire d. The result of the tendencies is a neutralization. The current flows out ou e through the two lamps back to c, just as it would if the wire d were not there at all and the two lamps were in series on a 220volt circuit; but, since the wire is there, if anything happens to cause a break or a short circuit at either lamp, the other lamp will be run by its dynamo through the neutral wire, as in the ordinary two-wire system. The result is similar with any number of lamps on the two sides of the neutral wire. If there are 10 lamps between d and e, each requiring % ampere, and 15 lamps between c and d, then dynamo B will tend to send a current of 5 amperes back along the neutral wire, and dynamo A will tend to send a current of 7 J amperes out along the neutral wire. The result will be a current of 2£ amperes out along the neutral wire, 5 amperes from dynamo A being neutralized by the 5 amperes from dynamo B.
In wiring a building for the three-wire system, the three wires are run together in the mains and branches, and the taps are taken off from the neutral wire and either of the other two in such a way that the loads on the two sides of the system will balance as nearly as possible. It is not possible in practice to get an absolute balance, but the neutral wire may frequently be only one-half or two-thirds the size of one of the outside wires. Even though the three wires are all the same size, there is only three-eighths as much copper