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.
VI. “AND THERE WAS LIGHT.”
ANCIENT LAMP.
Mention of the “candlestick of pure gold” (Ex. xxv. 31) may lead to the inference that the primitive artificial light was that of the candle. But “candlestick” in connection with the lighting of the temple is clearly a misnomer. The lamp was the original artificial light-giver, unless we choose to except the torch; and if less indispensable than in patriarchal times, it is still a favorite dispenser of nightly cheer. Prior to the middle of the eighteenth century, the lamp had practically no evolution. It was the same in principle at that date as when it illuminated the desert tabernacle. Even the splendid enameled glass or decorated Persian pottery lamps of Damascus and Cairo, and the magnificent brass or bronze lamps of Greece, Rome, and the European cathedrals, gave forth their dull, unsteady flame and noisome smoke by means of a crude wick lying in a saucer or similar receptacle of melted lard, tallow, oil, or some such combustible liquid. A prime improvement was made in lamp-lighting in 1783, by Leger, of Paris, who devised the flat, metallic burner, through which he passed a neatly prepared wick. A further improvement was made in 1784 by Argand, of Paris, who introduced a burner consisting of two circular tubes, between which passed a circular wick. The inner tube was perforated so as to admit of a draught of air to feed the flame on the inside of the wick. In order to similarly feed the flame on the outside of the wick, he invented the lamp chimney, which was at first a crude thing of metal. It, however, soon gave way to the glass chimney, which has up to the present taken on many improved forms, designed to secure more perfect combustion and a brighter, steadier glow.
TALLOW DIP.
MODERN LAMP.
Improvement in lamp-lighting during the nineteenth century has consisted of an indefinite number of inventions, all aiming at economy, brilliancy, steadiness, convenience, beauty, and so on. But in no respect has this improvement been more rapid and radical than in the adaptation of lamps to the various combustible fluids that have bid for favor. While the various oils, animal and vegetable, were almost solely in vogue as illuminants at the beginning of the century, they were largely superseded at a later period by the burning-fluid known as camphene. This was a purified oil of turpentine, which found great favor on account of its economy, convenience, cleanliness, and brilliancy of light. But it was very volatile, and its vapors formed with air a dangerously explosive mixture. Yet with all this it might have held its own for a long time, had not Gesner, in 1846, discovered that a superior mineral oil, which he called “kerosene,” could be readily and profitably distilled from the coal found on Prince Edward Island. This kerosene or hydrocarbon oil speedily displaced camphene as an illuminant. Its manufacture rapidly developed into an important industry in the United States, and large distilling establishments arose, both on the Atlantic coast, where foreign coal was used, and throughout the country, wherever cannel or other convertible coal was found. With the discovery of petroleum in paying quantities on Oil Creek, Pa., in 1859, there came about a great change in kerosene lamp-lighting. It was found, upon analysis, that crude petroleum contained about fifty-five per cent of kerosene, which constituted its most important product. The manufactories of kerosene from cannel or other coal, therefore, went out of existence, and new ones, larger in size and greater in number, sprung up for the manufacture of kerosene or, popularly speaking, coal oil, from petroleum. This illuminant came into almost universal favor for lamp use, owing to its cheapness and brilliancy. It is not free from danger when improperly distilled, but under the operation of stringent laws governing its preparation and testing, danger from its use has been reduced to a minimum. In rural districts, in smaller towns and villages, wherever economy and convenience are essentials, and when beauty in lamp effects is desirable, the kerosene illuminant has become indispensable.