If we wind around an iron bar a number of turns of insulated wire, and an electric current is allowed to pass through the coil, the bar becomes a strong electromagnet. But it remains a magnet only as long as the current is passing. Now, the magnetic effects obtained with the electromagnet are identical with those obtained from a permanent magnet—such as the familiar horseshoe magnet, commonly seen on the flywheel of the Ford automobile, or in the ordinary telephone generator for calling up “Central”. In the case of a telegraph instrument, it is important that the iron is a temporary magnet. On the other hand, a permanent magnet is an essential part of every Bell telephone receiver. This permanency is secured by employing a bar of steel instead of a piece of iron—a temporary magnet. The power produced from a dynamo—or electric generator—depends upon the fact that when a magnet is put into a coil of wire, only a momentary current of electricity passes through the wire, in one direction. If the magnet is withdrawn, a current starts in the opposite direction. Copper wire coiled about an iron core forms the “armature” of the dynamo. The rotating coils are said to “cut the magnetic field.” On this principle of electricity, intense electric currents are produced, furnishing the “power” for the electric motors in electric cars, elevators, musical instruments, etc., and for electric lights—incandescent and arc. Dynamos may contain either permanent magnets or electromagnets. They produce the magnetic field in which the “armature” or conductor—the coils of wire wound around the iron core—rotates. A machine with permanent magnets is usually termed a _magneto_, and is never made in large sizes. The current for the electromagnets may be derived wholly from an outside source, or part of the current which it generates may be used for that purpose. The current generated in the armature winding is alternating, but may be rectified to a direct current by a _commuter_ if desired; otherwise it is conveyed to the line circuit by _collector_ or slip rings and brushes. We owe much of our knowledge of magnetism and electricity to Michael Faraday (1791-1867), who brilliantly covered the whole field of these sciences. Faraday was distinguished alike as a chemist and as an experimenter in electricity and magnetism. Örsted had shown that magnetism could be produced by a current of electricity, but it remained for Faraday to produce current electricity by a magnetic “field of force”, thus laying the foundation for those modern industries which derived motive force for their machinery from the gigantic dynamos of our “power houses”.