not be affected. This resistance is connected in series with a cut-out controlled by the shunt coil in exactly the same way as the starting resistance in the series lamp. The lamp is also supplied with a regulating resistance in series with the arc. Each lamp of the set is adjusted to consume its proper proportion of the voltage. For example, with two lamps in series on a 220 volt circuit with an arc voltage of 80, each lamp should consume 110 volts, so the regulating resistance should be adjusted for 30 volts.

Another form of series multiple lamp is the same as the multiple, except that it has a small equalizing weight attached to the clutch mechanism, in such a way as to neutralize any tendency which one lamp in the series may have to take more than its share of voltage.

MINIATURE AND INTENSIFIED ARC LAMPS. In the last few years many types of arc lamps burning small carbons (about 4-inch in diameter) have been put on the market. These lamps are the result of the demand for a high efficiency lighting unit, and are intended primarily for interior lighting. They are all multiple lamps and most of them are made for D. C. circuits only, but there are several A. C. lamps of this type. The regulating mechanism of most of these lamps resembles that of the multiple lamps already described. One lamp of this type, however, is radically different. Three carbons are used. The two upper carbons are of small size inclined towards each other and touching at their lower ends. The arc is thus maintained without the aid of any regulating mechanism. The lower carbon is of larger size and is fed upward by means of a series coil, maintaining the arc in a fixed position. The two upper carbons are burned alternately. The lamps of this type are called Miniature, Small Arcs and Semi-Enclosed by different manufacturers. They Owe their high efficiency primarily to the small diameter of the carbons used. This causes a relatively high current density and a high

degree of incandescence at the tip of the carbon. Heat is not as readily carried away by the small carbons as by the larger ones of the enclosed arc. The arc is steadier and does not wander over the end of the carbons as in

enclosed lamps. Cored carbons are used. The light obtained from these lamps is pure white and is one of the best known substitutes for daylight. The bluish tinge noticeable in the enclosed lamp is entirely absent. This makes the lamps particularly well suited for lighting stores and other places where color matching is important. The manufacturers have spent much time in designing the case and globe equipment and supply their lamps in most attractive forms which would add to the decoration in any building. A special feature of one of these lamps is that it is provided with a terminal so that it may be screwed into an incandescent lamp socket.

These lamps may be procured for 110 volt D. C. and A. C. and for 220 volt D. C. circuits.

GLOBE AND REFLECTOR EQUIPMENT. This will naturally be governed by general conditions. The standard equipment consists of a light opal inner and clear outer globe. The advantage of the light opal inner globe is that the opal globe becomes luminous and forms a secondary source of diffused light which eliminates shadows, and at the same time gives a more effective light due to the better distribution. For interior work, there is the further advantage that the violet rays are partly absorbed, thus making a whiter light.

Two clear globes should not be used except for photographic work where a light is required which is high in actinic value. Special conditions sometimes call for other combinations, such as clear inner and opal outer, etc. In a number of cases no outer globe is needed.

For A. C. lamps a reflector is usually used, either with or without an outer globe. A considerable portion of the light from an A. C. arc is thrown upward, and if no reflector were used, this light would be lost.

General Rules for Operation and Care

of Enclosed Arc Lamps

Only those rules which apply to all types of arc lamps can be given. Further information on any particular type can be obtained from the instruction books of the manufacturers.

CARBONS. In order to obtain satisfactory operation of enclosed arc lamps, only high grade carbons should be used. They should be straight, round and of uniform diameter, being free from blisters and dirty spots. Should these spots be present, they should be removed with sand paper before the carbons are inserted in the lamp. COLUMBIA carbons, manufactured by the National Carbon Company, Cleveland, Ohio, have given excellent satisfaction in practically all cases where they have been tried. Their manufacture is closely watched and controlled by an elaborate testing system which eliminates the possibility of crooked, blistered, or dirty carbons reaching the trimmers' hands. The raw materials used are of the highest obtainable purity and are constantly kept at this standard by frequent chemical analyses.

Carbons should be stored in a dry place. Do not place boxes of carbons for any length of time on a cement floor having an earth foundation. If they must be stored in such a place, put boards under them so as to allow free circulation of air.

Trimmers should use a covered bag to protect the carbons from dust and dampness. Burned carbons, cleaning cloths, or other material should not be allowed to come in contact with the carbons.

For D. C. lamps solid carbons should be used almost invariably, with the exception of the miniature and other small lamps, which use as a rule two cored carbons. Most enclosed arc lamps are designed to burn upper carbons 12 inches long and in practically all cases the stub left over from the upper carbon is used for the lower of the

next trim. This should be from 4 to 6 inches long, as specified by the manufacturer. About 3 to 4 inches of lower carbon is consumed per trim, so it is evident that the number of hours burning per inch of lower carbon is high. To a trimmer cutting off carbons, 4-inch will not appear of much importance, but in a lamp burning 150 hours it will mean a life of about 10 to 12 hours. Some arrangements should be made for cutting off carbons the proper length. This can readily be done at the station. If the lower carbon is too long it is liable to cause breakage of the inner globe or melting of the gas cap. A large percentage of inner globe breakage occurs during the first 15 minutes of burning after retrimming. This is when the lower carbon is at its maximum length, with the arc near the top of the globe, and at the same time the globe is undergoing a large temperature change. On the other hand, if the lower carbon is too short, it will burn down into the carbon holder.

In cutting the old upper carbon for the lower of the next trim, the end which has been in the upper holder should be cut off and the carbon placed in the lamp with the burned end to the arc. This brings the lamp to full candle power in a minimum length of time and sometimes prevents flaming and possible globe breakage.

In the D. C. lamp the upper carbon should always be positive, since it is consumed more rapidly. If the lower carbon were positive, it would be consumed in a short time, the carbon holder would be destroyed and the globe melted. With the positive carbon at the bottom, most of the light is thrown upward and not downward. То test for the polarity of a lamp, allow it to burn for a few minutes, then turn it off. The hotter carbon, i. e., the one remaining red longer, is the positive.

For A. C. lamps one cored and one solid carbon are used, but on circuits where the voltage and frequency vary considerably, two cored carbons will often give better results. The lower carbon should be from 51% to 7 inches

long. In this type of lamp both carbons are consumed at about the same rate.

The A. C. lamp, as a rule, uses a 91⁄2 or 12 inch upper carbon.

In trimming an arc lamp the upper carbon should be pushed up as far as it will go in order to make good contact with the holder. Carbons with slightly beveled ends will facilitate this operation. The carbon should slide through the clutch and the bushing of the gas cap freely. After trimming, the clutch should be able to separate the carbons, about 1 inch in the case of the multiple lamps and 1⁄2 inch in the series. If the carbons cannot be separated far enough, it will cause high current in the multiple lamps, which may blow the fuse or possibly burn out a coil, while the series lamps will operate with a short arc which will cause globe blackening. To test a lamp after trimming lift the clutch by pushing on the rod leading to the magnets. Lifting the carbons is only half a test.

GLOBES. The inner globe should be thoroughly cleaned at every trimming. Most of the large operators of arc lamps are doing this work at a central globe cleaning plant. Duplicate sets of inner globes and, with lamps with detachable holders, of lower holders are provided. All globes are washed at the station. Soap is not required and, if it is used, the globes should be thoroughly rinsed in clean water. The grease from the soap if left in the globe may cause blackening. If the number of globes to be washed each day is large, the work can be facilitated by the use of a revolving brush, running under water, or with a continuous stream of water supplied to it. The lower carbons are cut to size at the station and whenever possible the lower carbon and inner globe are assembled in the holder. The globes are then packed in partitioned baskets or boxes, which the trimmer carries around in a wagon. In trimming he has simply to supply a new upper carbon and replace the dirty globe with