RECENT ADVANCES IN ELECTRIC LIGHTING. 379 



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 em- 

 ployed 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 de- 

 monstrable that there is economy in the use of small machines. Trials 

 made for the Trinity House have shown that more efficiency is ob- 

 tained 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 Avith 

 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 pro- 

 ducing 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 fila- 

 ment, which offers obstruction to the flow and produces light by in- 

 candescence, 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 temj^erature 

 of 3,150^ C. (5,702 Fahr.), and is broken up and fired in a fierce bom- 

 bardment of white-hot molecules across the air against the uj^jDer pole 

 the positive which is beaten up by incessant imjDacts into a higher 

 temperature of 3,900 C. (7,052 Fahr.), the arc itself being 4,800 C. 



