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The Arc-light.—If two pointed pieces of carbon are joined by wires to the terminals of a generator of electric currents, and are brought into contact for a moment and then drawn apart to a short distance, a kind of electric flame called the arc or the voltaic arc is produced between the points of carbon, and a brilliant light is emitted by the white hot points of the carbon electrodes.

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This phenomenon was first noticed by Humphrey Davy in 1800, and its explanation appears to be the following: Before contact the difference of potential between the points is insufficient to permit a spark to leap across even 10 of an inch of airspace, but when the carbons are made to touch, a current is established. On separat. ing the carbons the momentary extra-current due to self-induction of the circuit, which possesses a high electromotive-force, can leap the short distance, and in doing so volatilizes a small quantity of carbon between the points. Carbon vapor being a partial conductor allows the current to continue to flow across the gap, provided it be not too wide; but as the carbon vapor has a very high resistance, it becomes intensely heated by the passage of the current, and the carbon points also grow hot. Since, however, solid matter is a better radiator than gaseous matter, the carbon points emit far more light than the arc itself, though they are not so hot. In the arc the most infusible substances, such as flint and diamond, melt;

FIG. 153.

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and metals such as gold and platinum are even vaporized readily in its intense heat.

When the arc is produced in the air the carbons slowly burn away by oxidization. It is observed, also, that particles of carbon are torn away from the + electrode, which becomes hollowed out to a cup-shape, and some of these are deposited on the electrode, which assumes a pointed form.

In the alternating current arc lamp, since the current flows alternately in each direction, the carbons burn away with uniform rapidity and the ends assume about the same form. The light will be thrown up and then down as the direction of the current changes, and for this reason reflectors are placed above the arc to catch the upward rays.

The familiar prism and spectrum experiment shows that all ordinary "white" light is made up of colors, shades of red, yellow, green and blue. The quality of any light will depend upon both the presence and intensity of these component color-groups. When the incandescent lamp burns low the red and yellow rays are mainly present; as it grows hotter the greens and blues are added, until, when all are present in correct proportion, the light is pure white. Analysis of the components of ordinary lights shows that the candle and oil flames are made up mainly of red and yellow, with some green and faint blue rays.

The two pencils in the modern arc lamp are separated by a distance of from one-sixteenth to three-sixteenths of an

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an arc or part of a circle. The other carbon will be found slightly rounded with a little pyramid just beneath the crater of the upper carbons. Under these conditions, it will be noticed that, the positive carbon, or one nearest the positive brush of the dynamo, will burn away much faster than the other, and because the greater amount of light is desired below the lamp, the positive carbon is placed above the other.

Long before the incandescent electric light had been invented, the arc light made by an electric arc formed between

* NOTE.-It was about this time that popular explanations begun to be made of the nature and source of electricity; on the installation of a 4,000 candle light arc lamp, one man who had gathered a crowd around him thus proclaimed, calling attention to the solenoid at the top of the lamp: "That is the can that holds the oil," and, speaking of the side rod of the lamp, "That is the tube which conducts the oil from the can to the burner." This season also witnessed the introduction of the arc light into "Wannamaker's" at Philadelphia. One gentleman on that occasion looked the whole apparatus over very carefully, perhaps a half hour, sized it up, and then, pointing to the line wire, he said, "How large is the hole in that wire that the electricity flows through?" Another gentleman, one connected with a great manufacturing company, observed in complete silence the machine running for perhaps five minutes. Then he fully digested the whole thing, and was ready to tell all about it. He said, "The electricity in that thing is generated by that revolving business there, rubbing the air up against these iron blades (meaning the magnets), just as you get sparks when you rub a cat's back."

In the following year, 1879, the Cincinnati Exposition Buildings were lighted, and an expert from New York City was sent out to make matters go smooth, but one distressful night he failed to appear, and a self-examined engineer was put in charge, but the lamps would not "illumine"so the next morning Mike was sent for. "Well, Mike, why didn't your light burn last night?" "Sure, sir, I don't know. I do be thrying to do me part; it's more than the full of a box of matches I spint a-thrying to light thim two black sticks, and dev'l a light could I get." The great pile of burnt matches under the lamp bore evidence of the truth of Mike's assertion.