usual pressure adopted is that of one hundred volts. (The volt is the unit-measure of pressure.) Suppose, therefore, a circuit contained one hundred such lamps, ten thousand volts would be required in order that they might give their normal light. Such a pressure is fatal to life. A pressure exceeding five hundred volts with continuous current, and two hundred volts with alternating current, may prove fatal under certain conditions.
In the parallel system, which is free from danger, since high pressure is rarely employed with it, the flow and return mains may be supposed to be of equal length and laid parallel with each other. At various points along these mains, branch wires start, a lamp or motor being placed in the course of each branch, so that the current, in passing from one main to the other, traverses the lamp or motor. If this system is drawn on paper, it will have the appearance of a ladder, the lower ends of the sides, which may be taken to represent the mains, being connected to the dynamo and the top ends free. The rounds of the ladder will then represent the connecting cross-wires, each having in its course a lamp or any other piece of apparatus. Evidently, if the large mains, represented by the sides of the ladder, are of very low resistance, the current traversing the branches will simply be pro- portional to the resistance of each branch; and for lamps made to give equal light, placed in these branches, the resistances of the latter must be made approximately equal.
The resistance of the one-hundred-volt sixteen-candle-power glow- lamp most commonly in use at the present time is about one hundred and seventy times greater than that of the mains and branches leading to it. Consequently, if the flow- and the return-wire of such a lamp were to come in contact before reaching it, what is termed a short circuit would result, since the electric current, like steam and water, flows in the direction of least resistance. In this case the wires leading to the lamp will pass one hundred and seventy times more current than was intended, which would raise these wires to a white heat, or even fuse them, if no safety-junction is inserted in the circuit. If it exists, the fusible wire melts, and no mischief will be done.
In practice, one horse-power will produce one thousand candle-power in an arc-light; but an increase in the horse-power gives a far larger corresponding increase of light in the case of the arc lamp. For instance, seven horse-power will produce as great a light as fifteen thousand candles, or more. But, on the other hand, the light given by the incandescent lamp is directly proportional to the power of production; and, in practice, one horse-power will incandesce about eight sixteen-candle lamps of this type.
Although electricity was known to the ancient Greeks, the uses to which it might be applied have remained unknown for thousands of years. It was reserved to Ampere, Volta, Faraday, and a few others, to discover the laws which govern this force. The development of the science in its application to the practical needs of human life may be considered to date from the time of Faraday, whose career ended about the middle of the present century. In the past forty years electrical science has advanced in a degree probably unequalled by the progress
