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Operation of Arc Lamps.—From the standpoint of operation, arc lamps may be divided into two classes, viz.: hand-feed and automatic-feed. The hand-feed lamp is generally used in theaters and is practically the only kind admitted on the stage, or for stage illuminating purposes. Only a very few houses now use arc lamps for general illumination.

The operation of hand-feed lamps[1] is ordinarily quite simple and will be fully treated under the head of “Projection”, so that we may now consider only the automatic lamps. At the present time these are used mostly, if at all, for the illumination of the exterior of the theater.

[1] Full diagrams and descriptions are in another work of the authors’, entitled “Electricians’ Operating and Testing Manual,” so that no space need be given to these here, save in a very general way.

The operator should first familiarize himself with the construction and principles upon which the mechanism of his lamp is based. For this purpose he should remove the outer jacket, thus exposing the working mechanism; turn on the current; and endeavor to learn the significance of each part. It is of course necessary that the operator understand the hazards due to manipulating live wires and that he should be very careful not to make short circuits or grounds which might destroy parts of the lamp.

Automatic-feed lamps are usually trimmed in the following manner: Bring the lamp within reach; remove the globe; take out the lower electrode; let down the upper electrode rod and thoroughly clean it with crocus cloth. This upper electrode rod is the principal thing that concerns the lamp trimmer; it must be perfectly straight and care must be exercised not to bend it accidentally; it must be clean so that the clutches may properly grip it; it must not be greasy. If it grows dirty or greasy, it will soon become pitted from the current that passes from the contacts to it.

The next operation is to remove the upper electrode and place it in the lower electrode holder. (The length of electrode necessary should be known. The lower one generally burns out first—it being shorter—and if the arc reaches the lower electrode holder, will begin to consume it; if the lower carbon is too long, the arc is liable to reach the upper electrode holder and destroy it.) The upper electrode may then be placed in position and aligned with the lower. To do this it is best to turn it about and try it until it aligns in all positions. The two electrodes should form a straight line, up and down, no matter which way the upper is turned.

In some forms of enclosed lamps, the clutch grips the electrode direct. In such a case all of the upper electrode must be carefully examined to see that it is straight and free from burs, and that it can pass freely into the opening at the top of the inner globe. The successful operation of enclosed arcs depends upon the confinement of the gas in the inner globe. This globe must, therefore, be kept as tight as possible without interfering with the operation of the electrodes which pass through it.

With enclosed arcs, the care of the inner globe is of great importance, because impurities are cast off which soon coat the inner globe and absorb much of the light.

The care of the outer globes in general is also an important matter. A dirty globe looks very unsightly and absorbs much light.

The following points should be carefully considered in handling and trimming lamps:

(1) Be sure that you understand your system and know whether it is a constant-current or a constant-potential system of distribution. With constant-current systems, the current is constant and the voltage over the arc is regulated; while with constant-potential systems, the voltage is constant and the current through the arc is regulated.

(2) With constant-current or series lamps, the line must never be opened, but must be shunted around the lamp if a lamp is to be cut out.

(3) With constant-potential lamps, the lamp must never be shunted but the circuit must be opened.

(4) In all cases each lamp should be controlled by a double pole switch.

(5) Constant-potential lamps cannot be operated without resistance in the circuit; this resistance may be in the lamp itself or outside.

(6) Never handle high tension lamps without insulating yourself from the ground; and handle live wires only with one hand at a time.

(7) Provide spark arresters for all open-arc lamps in the vicinity of inflammable material.

(8) Never leave a lamp without globes where the wind can strike it. It will be blown out or feed often, thus consuming the electrodes very fast and at the same time yielding a very poor light.

Green light emitted by the lamp will indicate that the electrode holders are burning. Strong shadows cast upwards indicate a lamp burning “upside down”. The positive electrode retains heat longer than the negative. The quality and size of electrodes has much to do with successful operation. Always use the kind of electrodes recommended by the maker of the lamp.

Direct-current arc lamps do not require much in the shape of reflectors as they naturally throw most of the light downward, when the upper electrode is positive. They should as a rule be suspended high.

Alternating-current arc lamps throw most of the light from the upper electrode slightly below the horizontal and that from the lower electrode somewhat above. If the light is wanted in a downward direction, suitable reflectors must be provided.