the strength of the electro-magnets used in inducing them. This was a most progressive step in the history of the dynamo. It led to rapid inventions, whose principle was based on the fact that every dynamo carried within the cores of its magnets enough of unused or residual magnetism to render the magnets self-exciting the moment the machine started. So the outside means of magnetizing the fields of force of the dynamo passed away. The dynamo speedily grew in size and importance. The electro-magnets or fields of force were greatly increased in number, size, and power. There were great improvements in the construction and efficiency of the wire coils or armatures which cut the fields of force, and a corresponding increase in their number. Commutators and brushes underwent like improvement. So, at last, the well-nigh perfect and all-powerful dynamo of the end of the century was evolved, with a capacity for delivering, in the form of electricity, ninety per cent of the mechanical energy which set it in motion. In the application of steam to machinery, eighty per cent, and sometimes more, of the energy supplied by a ton of coal is lost. [Illustration: A DYNAMO.] With the perfection of the dynamo, its uses multiplied. It became a prime factor in electric lighting. Trolley systems sprang up in city, town, and village, taking the place of horse and traction cars. In certain places, as in the Baltimore tunnel, the dynamo superseded the engine for hauling freight and passenger cars. The mighty dynamos which convert the inexhaustible energy of Niagara Falls into electricity send it many miles away to Buffalo, to be applied to lighting and to every form of machinery. The end of the century sees a power plant in operation in New York city capable of furnishing one hundred thousand horse-power, or enough to supply the lighting, rapid transit, and thousand and one mechanical needs of the entire municipality. The essential parts of an ordinary dynamo are: (1.) The electro-magnets, which, however numerous, are arranged in circular form upon part of the framework of the machine. (2.) The iron coils or armatures, mounted in a circle upon a wheel. When the wheel revolves, the armatures pass close in front of the electro-magnets, cutting through their fields of force, and thereby inducing electric current. (3.) The commutator, which consists usually of a series of copper blocks arranged around the axle of the armatures, and insulated from the axle and from each other. The current passes from the armatures to the commutator. If the current be an alternating one, the commutator changes it into a continuous one, and the reverse may also be accomplished. (4.) The brushes, which are thin strips of copper or carbon, are brought to bear at proper points upon the commutator, making connection with each coil or sets of coils. They carry the corrected current to the outside line or lines. (5.) The outside line or lines, to carry the current away to the motor. (6.) The