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SYSTEMS OF

TEM.

CHAPTER III.

DISTRIBUTION. THE SERIES SYS

- THE MULTIPLE SYSTEM.— HYDRAULIC ANALOGUES. — THE THREE-WIRE SYSTEM.

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LAMPS, or other electrical devices, may be connected in a number of different ways to form a distributing system, but considerations of safety, convenience, or practicability have narrowed the methods down until there are but two in general use. These are the plain "series" and " multiple" systems. Arc lamps, and incandescent lamps for street-lighting, are usually operated "in series," while incandescent lamps for general use, motors, and other devices, are almost always operated "in multiple."

When lamps are operated in series, the same current passes through each, and all lamps in the circuit are interdependent. If the circuit be broken in any place, all the lamps go out. With this system the current is kept constant, and if more lamps are added, the pressure at the dynamo must be increased to overcome the added resistance. To cut out any of the lamps, it is necessary first to make a by-path

round the lamp to preserve the continuity of the circuit, and then the connection to the lamp may be broken. But it is also necessary either to replace the lamp by an equivalent resistance or else to lower the pressure at the dynamo, for otherwise the strength of the current would be increased in proportion to the decrease in the resistance of the circuit. Figure 2 represents an electric circuit in which the lamps are connected in series.

FIG. 2.-Arc Lamps in Series.

The principle of this series system can perhaps be brought out more plainly by showing its analogy to a system of water-wheels operated by one stream of water. In Figure 3 are shown three water-wheels in series, or, as it is sometimes expressed in electrical parlance, "in tandem" or "in cascade." The pump, which is analogous to the dynamo, creates a pressure, raises the water to a height, and gives it a definite "head." This pressure or head corresponds to the voltage or pressure in the electrical

circuit. The water passes through a pipe to the wheels, in a stream corresponding to the electric current, and operates the water-wheels which here take the place of lamps. At each wheel there is a definite resistance to the flow of the water, and a part of the energy is transformed into the work done by the wheel. The water falls to the next wheel where it has a diminished head, but the same in proportion to the number of wheels it has yet to

FIG. 3.-Water-wheels in Series.

operate. If another wheel were added, the pump would have to be run so that it would give proportionately greater pressure or head, but the current or flow of water would remain the same, for the resistance would be proportionately increased.

It will be noticed that the working of the wheels is influenced only by the pressure or head that exists between the point where it leaves one wheel and the point where it leaves the next. It makes no difference how many or how few are in the series,

provided only that the total pressure is sufficient to keep this pressure for each wheel the same. It is the same in the electrical series; the current is kept constant, and the pressure necessary is the pressure needed for each device, multiplied by the number of devices. Thus, a "2000 candle-power" arc lamp requires, approximately, 50 volts pressure. If there are 50 lamps in series, the total pressure at the dynamo is 2500 volts; if there are 10 lamps in series, the total pressure is 500 volts. The current in the circuit would be the same in both cases, about 10 ampères.

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It will be seen, too, in the water-wheel illustration, that if one wheel be shut off by closing a valve in the pipe above it, all the wheels will stop, for the continuity of the circuit will then be broken. If one wished to stop one of the water-wheels, it would be necessary first to "short-circuit one of them by opening a passage round it. The water running through the regular passage to the wheel could then be shut off entirely, and the current or flow would continue through the by-passage. This passage round the wheel could be made to offer exactly the same resistance that the wheel did, and in that case the pump could keep on developing the same pressure as before, and the other wheels would all continue to work uninterruptedly. If, however,

the by-passage offered little or no resistance, the pump would, of course, have to be run so that it would not develop so much pressure, for otherwise the current would be greater would be greater through all the wheels, and might be more than they were made to withstand.

Water-wheels operated as shown in Figure 3, would all be transforming the energy due to the current of water into other forms. They would be doing work and "the rate of work" could be measured by multiplying the pressure, or head, by the flow of water, or gallons per minute. From this, the foot-pounds per minute, or horse-power, could be readily determined. And it is so with the electric circuit. There is work done at the lamps, where the electrical energy is transformed into the energy of heat and light, and this "rate of work" can be measured by multiplying the pressure, or volts, by the current, or ampères. One volt multiplied by one ampère gives one watt, the unit of electrical "rate of work." hundred and forty-six watts make one horse-power, and there is here the direct connection between electrical energy and the more familiar mechanical energy.

Seven

When an electric circuit is broken, energy is no longer being expended, for although the electrical pressure may be kept constant, the current, the other factor determining the amount of energy, has become

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