That was the beginning of electric lighting; and perhaps it will be well to bridge the long and comparatively uninteresting interval which elapsed between this discovery and the equally important one which alone gave it commercial value—­I refer to the production of suitable currents by mechanical means.  That is to say, the substitution of energy obtained from coal in the form of steam power reduced the cost to a fraction of what it necessarily was when the galvanic elements were used.  Here is the point; the cost of zinc today is something over fifty times that of coal, while its energy as a vitalizing agent is only about five times greater, leaving a very large margin in favor of the “black diamonds.”  This is not the only advantage, for the resulting impulse in the case of mechanical production is much more uniform in action, and therefore better suited to the end in view, while the amount of adjustment and attention required is beyond comparison in favor of the latter means.

The machines adopted were of the magneto variety, and many ingenious machines of this class were operated with more or less success, being, however, quickly abandoned upon the introduction of the dynamo-machine, which gave currents of much greater electromotive force from the same amount of material, the advantage being chiefly due to the large increase of magnetic intensity in the field magnets.  At this period lights of enormous power were produced with ease and by the use of costly lamps.  With complicated mechanism a new era in artificial illumination seemed close at hand, but a grave difficulty stood in the way—­namely, the proper distribution or subdivision of the light.  It was quickly found that the electric difficulty of subdividing the light, added to the great cost of the lamps then made, was an apparently insurmountable obstacle to its general adoption, and the electric light was gradually taking its place as a brilliant scientific toy, when the world was startled by the introduction of the Jablochkoff candle, which may fairly claim to have given a greater impetus to the new light than any previous invention, a stimulus without which it is even probable that electric lighting might have slumbered for another decade.

The Jablochkoff candle embodies a very beautiful philosophical principle, and though its promises have not been fulfilled in general practice, we must not forget that we owe it much for arousing scientific men from a dangerous lethargy.

Up to this time the light had always been produced by approximation of carbon rods with their axes in the same plane; but the Jablochkoff candle consisted of like rods arranged parallel to each other and about one-eighth of an inch apart, the intervening space being filled with plaster of Paris, and the interval at the top bridged by a conducting medium.  The object of the plaster, which is a fairly good insulating material at ordinary temperatures, is to prevent the passage of the current except at the top, where the