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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 lamps 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 neutraliza

tion. The current flows out on 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 ampère, and 15 lamps between c and d, then dynamo B will tend to send a current of 5 ampères back along the neutral wire, and dynamo A will tend to send a current of 7 ampères out along the neutral wire. The result will be a current of 2 ampères out along the neutral wire, 5 ampères from dynamo A being neutralized by the 5 ampères 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

used as would be necessary in the two-wire system running at the same voltage, with the same loss. For, as has been noticed, without the neutral wire the system is working at a pressure of 220 volts, and the copper required is only one-fourth as much as would be required with a system running at a pressure of 110 volts. If the neutral wire is the same size as one of the outside wires, the weight of copper is increased fifty per cent and becomes three-eighths as much as would be required with the 110-volt circuit.

CHAPTER IV.

METHODS OF WIRING.

SINCE electricity first came into general use there has been a constant improvement in the methods of wiring. Partly from an imperfect realization of the needs, and partly from the desire for cheap construction, all the early wiring was little more than a stringing of paths for the electricity to follow. It was soon found, however, that unless special precautions were taken to confine the current to these paths, there was danger of fire, and the service was poor and uncertain.

The first wiring was generally done with a wire inadequately covered, and the conductors were fastened directly against any convenient support by means of wooden cleats or iron staples. While everything was perfectly dry, and as long as the current was not greater than the wire could carry without excessive heating, little trouble would be found. But of course it was not possible to be sure of this dryness. Natural dampness in the atmosphere, leaky roofs, the accidental

spilling of water, might at any time so affect the insulation that there would be considerable leakage. With this there would be, perhaps, a diminution in the available energy, and probably an arcing and heating that introduced a serious fire hazard.

There are two ways in which high temperatures are caused by electric currents. One is mentioned on page 13; it is the heating that takes place in any conductor when a current passes through it. A large current passing through a small conductor sometimes even causes fusion. But ordinarily much the higher temperature is produced by the formation of an electric arc, which is familiar from its practical use in the arc lamp. This arc is formed when a current passes from a conductor, through gases, to a conductor again, as explained on page 33. At ordinary potentials the current can "jump across" only a very short space, and these arcs have to be formed by the gradual separation of the conductors. Suppose that two wires when brought together complete an electric circuit; if the pressure forcing them together be now diminished, the electrical resistance at the contact is increased, and the current overcoming this resistance creates heat. If the wires are separated, there is at the instant of separation a great increase in the resistance and a proportionate increase in the heat generated; the ends of the conductors become volatilized and the

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