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there will be 109.95 volts at the lamp. Thus, if the resistance of the wires leading to the lamps be kept as low as .1 ohm, either one lamp or twenty may be turned on and there will not be a difference in the pressure of more than .95 volt, an amount so small that no change in the strength of the light could be detected by the eye. Even at the dynamo or at any intermediate point, a lamp would have practically the same pressure.
It can be seen from the examination of this case how necessary it is for satisfactory service, that there be equality of pressure at all lamps at all times. The calculations are often complicated by the number of branches and the positions of the lamps, and experience assists one to see quickly what will probably be the best arrangement; but the loss may always be found by an application of Ohm's law, and even with experience one must in the end rely upon this to ascertain the exact sizes of wire.
The first cost of a plant and the cost of maintenance may in several ways be affected by the proportions of the wires and the method of laying out the system. The wires may be so small that a large part of the energy delivered by the dynamo is wasted in worse than useless heat, as has been shown; but the conductors may be of sufficient size to prevent excessive loss and to keep the pressure constant at the lamps, and yet be arranged so that a much larger amount of copper is used than is necessary. Since the length of the wire, as well as the sectional area, makes a difference in the resistance, there is need of judgment in placing the main wires, in choosing "centres of distribution," and in dividing the circuit into the different branches. In a large building there is opportunity for a good grade of engineering skill and to have this work done by any one except those who have studied this branch of engineering and who are familiar with the requirements, is sure to result in loss.
There should always be equality of pressure at the lamps for another reason that is not commonly considered. All lamps are not made of the same " efficiency." That is, for a light of sixteen candle-power, some will require more energy than others — more coal will be burned under the boilers. The most efficient lamps require a practically constant pressure, of the amount for which they are designed. If the pressure rises more than a very little above this, the lamps have a "short life," and it is only in plants where the regulation at the dynamo is of the best, and where the wiring is calculated for a very small loss, that they can be used. If the wires are so small that turning off a number of the lamps causes the pressure at those remaining to increase by several volts, the more efficient lamps cannot be used on account of the expense of renewals, and it is necessary to use those that will not be too much affected by the variation. Such lamps can be had and are widely used, because there has been much poor engineering and the breakage must be kept small. Their use is, however, made necessary only by the variation of the pressure, and they are a constant expense because of the greater cost to operate them.
SYSTEMS OF DISTRIBUTION. — THE SERIES SYSTEM. — THE MULTIPLE SYSTEM. — HYDRAULIC ANALOGUES.—THE THREE-WIRE SYSTEM.
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.
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