Sir Humphry Davy was the first to discover that when the terminal wires of a powerful electric battery were furnished with carbon-points and brought into such a position that they almost touched, the space between them became bridged over with a dazzling arc of light. The excessive cost of producing this light (owing to the rapid consumption of the metal-plates and acids which together form the battery-power) rendered it for a long time almost inapplicable to any other purpose than that of lecture-room demonstration. But it was evident to all that a means of illumination so nearly approaching in its intensity the light of the sun, would, if practicable, be of immense value to society at large. Apart from its cost, there were many other hindrances to its ready adoption. The incandescent carbon-points--which we may here remark are cut from a hard form of gas-coke--were found to waste away unequally. Some plan had therefore to be hit upon of not only replacing them at certain intervals, but also, in view of this inequality of consumption, of preserving their relative distance the one from the other; otherwise the light they gave became intermittent and irregular. These difficulties were met by employing clock-work as a regulator, and more recently by a train of wheelwork and magnets set in motion by the current itself. These arrangements naturally led to complications, which required the constant supervision of skilled operators, and the coveted light was necessarily confined to uses of a special nature where the question of cost and trouble was unimportant.
The use of the battery for the electric light has for some years been almost entirely superseded by the magneto-electric machine. The construction of this machine is based upon Faraday's discovery, that when a piece of soft iron inclosed in a coil of metal wire is caused to pass by the poles of a magnet, an electric current is produced in the wire. The common form of this machine consists of a number of such iron cores so arranged upon a revolving cylinder that in continual succession they fly past a number of stationary horse-shoe magnets placed in a frame round its circumference. By a piece of mechanism called a commutator, the various small streams of electricity thus induced are collected together into one powerful current. This invention forms one of the most advanced steps in the history of the electric light. But although it produces electricity without the consumption of metal involved in the battery system, another element of cost comes into view in the expense of the steam-power necessary to work it; besides which the original outlay is considerable.