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Page:Popular Science Monthly Volume 19.djvu/393

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Here is a sphere in which gas companies may maintain their dividends. Water furnishes a convenient source of energy wherever it can be found available. Sir William Armstrong makes his brook light his house, producing from it, by the aid of a turbine, a force giving six horse-power. The caloric-engine at the Lizard Lighthouse has been found to be economical, useful, and very suitable for an isolated place where it is hard to provide water.

The difference in the value of the several excellent machines employed for generating the current is not great. Each is especially adapted for its own particular work, either by a variation in velocity or by a variation in the manner in which the wire is wound, so as to produce a variation in the current produced to suit the particular light required. In both the Siemens and the Gramme machines ninety per cent, of the power is converted into useful current. It is easily demonstrable that there is economy in the use of small machines. Trials made for the Trinity House have shown that more efficiency is obtained by joining small machines in multiple arc than by using a larger machine, or joining the same small machines in series.

For conducting-wires the preference is given to copper, the purest that can be got, and wire of the largest dimensions consistent with economy, so as to keep the resistance as low as possible and avoid waste of energy. When it can be carried overhead, facility is given for the radiation of the heat into the air, and the wire is kept cool and conveys more electricity. Since the currents to be carried over these wires are three thousand times larger than those used in telegraphy, the difficulties to be encountered in their safe transmission are greatly magnified. The disturbing effects produced by the inductive influence of such currents are so serious that apprehensions are entertained that it will be impossible to maintain electric light and telegraph currents close together.

The electric light is coincident with electric heat; the art of producing a brilliant light is the art of producing a high temperature. No greater illusion is extant than the idea that the electric light is a cold light, for the electric arc is the greatest source of heat known. This heat can be produced either by causing the electricity to fly across an air-space, in which case we have light by the arc, or an arc light, or by causing it to flow through a small wire or a carbon filament, which offers obstruction to the flow and produces light by incandescence, or the incandescent light. The forms of arc-lamps are very numerous. In every case carbon rods are opposed to each other, and are disintegrated and consumed in the fierce blast to which they are subjected. The lower pole—the negative—acquires a temperature of 3,150° C. (5,702° Fahr.), and is broken up and fired in a fierce bombardment of white-hot molecules across the air against the upper pole—the positive—which is beaten up by incessant impacts into a higher temperature of 3,900° C. (7,052° Fahr.), the arc itself being 4,800° C.