These uses are, in application, the most startling in the entire history of civilization. They have come about, and their applications have been made effective, within twenty years, and largely within ten. This subtlest and most elusive essence in nature, not even now entirely understood, is a part of common life. Some years ago we began to spell our thoughts to our fellow-men across land and sea with dots and dashes. Within the memory of the present high school boy we began to talk with each other across the miles. Now there is no reason why we shall not begin to write to each other letters of which the originals shall never leave our hands, yet which shall stand written in a distant place in our own characters, indisputably signed by us with our own names. We apparently produce out of nothing but the whirling of a huge bobbin of wire any power we may wish, and send it over a thin wire to where we wish to use it, though every adult can remember when the difficulty of distance, in the propelling of machinery, was thought to have been solved to the satisfaction of every reasonable man by the making of wire cables that would transmit power between grooved wheels a distance of some hundreds of feet. We turn night into day with the glow of lamps that burn without flame, and almost without heat, whose mysterious glow is fed from some distant place, that hang in clusters, banners, letters, in city streets, and that glow like new stars along the treeless prairie horizon where thirty years ago even the beginnings of civilization were unknown. Yet the mysterious agent has not changed. It is as it was when creation began to shape itself out of chaos and the abyss. Men have changed in their ability to reason, to deduce, to discover, and to construct. To know has become a part of the sum of life; to understand or to abandon is the rule. When the ages of tradition, of assertion without the necessity for proof, of content with all that was and was right or true because it was a standard fixed, went by, the age not necessarily of steam, or of steel, or of electricity, but the age of thought, came in. Some of the results of this thought, in one of the most prominent of its departments, I shall attempt to describe. A wire is the usual concomitant in all electrical phenomena. It is almost the universally used conductor of the current. In most cases it is of copper, as pure as it can be made in the ordinary course of manufacture. There are other metals that conduct an electrical current even better than copper does, but they happen to be expensive ones, such as silver. The usual telegraph-line is efficient with only iron wire.