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General Discussion of the Electrical Arc.—The name of the electrical arc lamp is derived from the arch-like appearance of the vapors which give out the light when the carbons are placed horizontally. The horizontal arc was the earliest form, hence the name which it carries to this day.

The arc proper is due to the vapors of volatilized carbon or other materials forming the electrodes, which may be consumed by the passage of an electrical current from one electrode to another through the intervening medium. In order that an arc may be formed, it is necessary first to bring the electrodes together. This, if the circuit is properly arranged, starts the current and when the circuit is partly interrupted, as by slowly separating the points of the electrodes, the current passes through the intervening space, with the result that a high degree of heat (about 3,500 centigrade) is produced. This results in volatilizing the carbon or any other material of which one or both electrodes may consist.

As long as the distance between the electrode points is small, the current will be quite strong and a hissing or frying sound will be given out. In order to keep the current within bounds during the time that the electrodes are together or while they are separated only a very short distance, some resistance, or reactance in the case of alternating-current arcs, is always connected in series in the circuit. If this were not done, there would be a short circuit at the time of starting or striking the arc.

The arc formed with very short separation of electrodes is generally spoken of as a low tension arc and requires very hard carbons and about 25 volts. This type of arc is very little used for illuminating purposes.

If the distance between the electrodes is increased gradually, the light becomes very unsteady and flickers considerably until at a certain point it begins to improve and give the long quiet arc. This condition will occur when, with direct current, the electrodes are about one-eighth of an inch apart. It will then be found that the voltage across the arc is from 45 to 50 volts, which is the best voltage to use with open arcs. If the separation be carried still further, the arc will grow longer and become flaming until finally it breaks entirely.

The resistance of the arc is closely proportional to the cross section of the electrodes and increases with the distance of the arc gap. It acts, however, very much as though there were a small counter e. m. f. set up within it.

The color of the light given off varies with the length of the arc somewhat, but depends mainly upon the material of which the electrodes consist. In the so-called flaming arcs, the peculiar color is obtained by certain chemicals imbedded in the material composing the electrodes. Whenever an arc is allowed to burn down until it reaches the electrode holders, a greenish light is given off which is due to the volatilization of the metal—usually brass—in these holders.

The light of a strong arc is extremely injurious to the eyes and should only be viewed through colored glass. Many very painful experiences have resulted from persons gazing upon arcs of 200 or 300 amperes, such as are used sometimes in cutting away metals of old buildings, etc.

The most powerful arcs known at the present time are those used in some steel mills for refining steel. These use upward of 10,000 amperes.

FIGURE 3.FIGURE 4.

The length of the ordinary arc varies from one thirty-second of an inch to one inch. The light is not of much use and is rather unsteady until the electrodes have assumed a shape somewhat similar to that shown in Figure 3 for direct current, and Figure 4 for alternating current. With direct-current arcs, a crater is formed at the bottom of the positive electrode and, from this crater, about 80 per cent of the light is emitted. Where the light is wanted in a downward direction, the crater is always formed at the top and for this purpose the top electrode must be made positive; that is, the electricity must flow from the top electrode into the lower one. In some cases, where special illumination effects are desired, the bottom electrode is made the positive with the result that most of the light is thrown upward. In such cases strong shadows are thrown against the ceiling and the lamp is said to be burning “upside down.”

The positive electrode can always be distinguished from the negative (a) by the shadows cast; (b) by the form of the electrodes; and (c) by the fact that since it is heated to a greater degree, it will, when the lamp is turned off, remain hot for some time after the negative electrode has cooled off.

In case the arc is drawn out very long and operated in this way for a considerable time, the crater will almost wholly disappear and the electrodes will appear rounded off.

In an alternating-current circuit, the positive and negative poles reverse generally about 120 times per second and both electrodes in the alternating-current arc are positive and negative to the same degree. They are therefore very nearly alike, except that the heat rising from the lower one increases slightly the volatilization of the upper. The positive electrode in the direct-current arc is consumed approximately twice as fast as the negative electrode. The consumption of the two electrodes in an alternating-current arc is about equal and a crater much smaller than the kind formed in a direct-current arc is, therefore, formed on each electrode, instead of only on the positive electrode as in the case of the direct-current arc.

The general form of alternating-current arc carbons is given in Figure 4. The small elevations shown in the cuts are due to impurities and do not appear with first-class carbons.

When arc lamps are operated on alternating-current circuits, the best voltage for the arc is about 28; and consequently, for the same quantity of light, the current must be increased so that the amperage of alternating-current lamps is always much greater than that of direct-current lamps.

The alternating-current arc is much noisier than the direct-current arc, but with very high frequencies this noise ceases.

In general, arc lamps do not work very well on low frequencies. The time at which the current is practically zero is long enough to allow the vapor between the electrode points to cool off sufficiently to interfere with successful operation.

Any arc light is affected by draughts of air and can even be blown out. If this occurs often, there will be rapid feeding, a short arc, and great waste of electrode.

A magnet held close to an arc can be made to blow it out or force it to one side. This fact is made use of in some lightning arresters.

Generally speaking, arc lamps are of two kinds, open and enclosed. The enclosed arc operates at a much higher voltage and is but little used about theaters. The open arc is almost universally used for stage work and this is about the only place where it is still considered useful. This kind of arc lamp is, however, very hazardous in localities where inflammable material abounds and for this reason it is always enclosed with wire mesh when possible.

Lens lamps can be tightly enclosed since none of the light is wanted except that which passes through the lens in front.

FIGURE 5.FIGURE 6.

The so-called flood lamps are usually provided with wire gauze in front of the arc, which prevents the escape of pieces of the electrodes and also prevents parts of scenery, etc., from coming in contact with the arc.

The lamp houses should be of such dimensions that, with the highest amperage the lamp is capable of using, the outer walls will not become excessively hot.

Illustrations of standard lens and flood lamps, as made by the Chicago Stage Lighting Company, are shown in Figures 5 and 6.