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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 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.

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 pulley for strap-belting, by means of which the water or steam power used is made to turn the dynamo machine.

But we must not forget the motor as a companion of the dynamo, as its indispensable brother, in turning to practical account the electricity sent to it. As we have seen, the motor is the reverse of the dynamo, at least in its effects. It is fed by the dynamo, and it imparts its power to the machinery which it is to set in motion. It is to the dynamo what the water-wheel is to the water. In one sense it is an even older invention than the dynamo, but its extended commercial application was not possible until the dynamo had reached certain stages of perfection. It is generally agreed that the first motor of importance was that constructed by Professor Jacobi, through the liberality of the Czar Nicholas, of Russia. Jacobi used two sets of electro-magnets, by means of whose mutual attraction and repulsion he rotated a wheel on a boat with a power equal to that of eight oarsmen. But as Jacobi’s electro-magnets required an electric current to magnetize them, and as there were then no means of producing such current except by the costly use of the voltaic battery, his invention was unripe as to time.

In 1850, Professor Page, of the Smithsonian Institution, constructed a motor which worked ingeniously, but was still open to the objection of cost in supplying the necessary electric current for the electro-magnets. Though various inventions came about having for their object a commercially successful motor, such a thing was impossible till Gramme produced his improved and effective dynamo in 1871. This dynamo was found to work equally well as a motor, and hence it became necessary for electricians to greatly enlarge their understanding of the nature of electro-magnetic induction. They soon discovered many curious things respecting the behavior of induced currents, with the result that rapid and simultaneous improvements were made in both dynamos and motors. One of the most curious of these discoveries was that a motor automatically regulates the amount of current that passes through its circuit in proportion to the work it is called upon to do; that is, if the work the machine has to do is decreased, the motor attains a higher speed, which higher speed induces a counter electro-motive force sufficient to check up the amount of current passing through the motor. So when the motor is required to do increased work, the machine slows up; but with this slowing up, the counter electro-motive force decreases, and consequently the current passing through the motor increases.

As with the dynamo, one of the marvels of the motor is its efficiency. In perfect machines, ninety to ninety-five per cent of the electrical energy supplied can be converted into mechanical energy. For this reason it has become a competitor with, and even successor of, steam in countless cases, and especially where water-power can be commanded. A prime motor, in the shape of a water-wheel, may be made to drive scores of secondary motors in places hundreds of miles away. The power developed by the waterfall at Lauffen, Germany, is transmitted one hundred miles to Frankfort, with a loss of only twenty-five per cent of the original horse-power.

THE GOLDEN CANDLESTICK.

In its adaptation for practical use, the motor, like the dynamo, assumes all sizes and embraces a host of ingenious devices, yet its power and usefulness always centre around, or are contained in, its two efficient parts, its armatures and fields of force. We have seen how in the dynamo the armatures became the source of induced currents by being made to cut the fields of force of electro-magnets. Now, a dynamo can be made to work in an opposite way; that is, by making the magnetic fields of force rotate in front of the coils or armatures. In the motor, the field of force is mostly established by the current directly from the dynamo. This current passes also through the armature, which begins to rotate, owing to the force of the field upon it. This rotation of the armature through the field of force produces in the armature conductors an electro-motive force, which is the measure of the power of the motor, be the same great or small.