Both Galvani and Volta passed into shadow, when, in 1820, Professor H. C. Oersted, of Copenhagen, discovered the law upon which were afterwards slowly built the electrical appliances of modern life. It was the great principle of INDUCTION. The student of electricity may begin here if he desires to study only results, and is not interested in effects, causes, and the pains and toils which led to those results. The term may seem obscure, and is, doubtless, as a name, the result of a sudden idea; but upon induction and its laws the simplest as well as the most complicated of our modern electrical appliances depend for a reason for action. Its discovery set Ampère to work. They had all imagined previously that there was some connection between electricity and magnetism, and it was this idea that instigated the investigations of Ampere. It was imagined that the phenomena of electricity were to be explained by magnetism. This was not untrue, but it was only a part of the truth. Ampere proved that _magnetism could also readily be produced by a current of electricity_. From this idea, practically carried out, grew the ELECTRO MAGNET, and to Ampère we are indebted for the actual discovery of the elementary principles of what we now call electrodynamics, or dynamic electricity, [Footnote: In all science there is a continual going back to the past for a means of expression for things whose application is most modern. _Dynamic_; DYNAMO, is the Greek word for power; to be able. Once established, these names are seldom abandoned. There is no more reason for calling our electrical power-producing machine a "Dynamo" than there would be in so designating a steam engine or a water-wheel. But, a term of general significance if used at all, it has come to be the special designation of that one machine. It is brief, easily said, and to the point, but is in no way necessarily connected with _electrical_ power distinctively.] in which are included the Dynamo, and its twin and indispensable, the Motor. Ampère is also the author of the _molecular theory_, by which alone, with our present knowledge, can the action of electricity be explained in connection with the iron core which is made a magnet by the current, and left again a mere piece of iron when the current is interrupted. Ten years later Faraday explained and applied the laws of Induction, basing them upon the demonstrations of Ampère. The use of a core of soft iron, magnetized by the passage of a current through a helix of wire wrapping it as the thread does a spool, is the indispensable feature, in some form meaning the same thing, with the same results, in all machines that are given movement to by an electric current. This is the electro-magnet. It is made a magnet not by actual contact, or by being made the conductor of a current, but by being placed in the "electrical field" and temporarily magnetized by induction.