Dynamo is from the Greek _dunamis_, meaning power. Motor is from the Latin _motus_, or _moveo_, to move. Dynamo is the every-day term applied to the dynamo-electric machine. Motor is the every-day term applied to the electric motor. The dynamo and motor are quite alike in principle of construction, yet direct opposites in object and effect. Perhaps it might be well to designate both as dynamo-electric machines, and to say that, when such machine is used for the conversion of mechanical energy or power of any kind into electrical energy or power, it is a dynamo. When a reverse result is sought, that is, when electrical energy or power is to be converted into mechanical energy or power, the machine that is used is a motor. In practical use for most purposes they are brought into coöperation, the dynamo being at one end of an electric system, making and sending forth electricity, the motor being at the other end, taking up such electricity and running machinery with it. Both machines were epoch-making in the midst of a wondrous century, and both were results of those marvelous evolutions in electrical science which characterized the earlier years of the century. We have seen how the simple glass cylinder or sulphur roll became, when rubbed, a generator of electricity. In a later chapter of electrical history, we saw a new and more powerful generator of electricity in the voltaic cell, by means of opposing metals acted upon chemically by acids. The greatest, grandest, most powerful, and most economic of all generators of electricity was yet to come in the shape of the dynamo. We see its beginnings in those investigations of Faraday which led to the discovery of the induction coil and the principles of magneto-electric induction. In 1831, he invented a simple yet, for that date, wonderful machine, which was none the less the first dynamo in principle, because he modestly called it “A New Electrical Machine.” He mounted a thin disk of copper, about twelve inches in diameter, upon a central axis, so that it would revolve between the opposite poles of a permanent magnet. As the disk revolved, its lower half cut the field of force of the magnet, and a current was induced which was carried away by means of two collecting brushes, fastened respectively to the axis and circumference of the disk. This was the first electric current ever produced by a permanent magnet. The Faraday machine and others that derived the mechanical energy which was converted into electric current from a permanent magnet were classed as magneto-generators. Soon the electro-magnet took the place of the permanent magnet, because it produced a much stronger field of force. But then the electro-magnet had to have a current to excite it. This current was supplied by a magneto-generator, placed somewhere on the dynamo. Now came the thought, suggested by Brett in 1848, that the induced currents of the dynamo could themselves be turned to account for increasing