the arc is a positive action effected by the current.

Both styles

of lamps give satisfaction, the most notable example of the shunt lamp being the Thomson-Rice, while the most prominent type of the differential is the Brush.

In addition to the elementary features above described, shunt lamps usually have an auxiliary shunt winding of coarse wire that acts in striking the arc. If the fine-wire shunt coil only were employed, the retractile mechanism of the first lamp to operate on starting up would separate the carbons without drawing an arc, since only a very small current can pass through the shunt coils of the other lamps in the series. The carbons will tend to vibrate, like the hammer of an electric bell, until all the carbons are down together, when they can all pick up their arcs. The coarse-wire shunt passes sufficient current around the carbons when apart to maintain an arc between the carbons of any other lamp in the series. As soon as the arc is struck, the circuit of the coarsewire shunt is opened by the armature or an auxiliary magnet or some such device.

Cut-out. In all series lamps it is absolutely essential to insert a device by which a continuous path is provided for the current in case the carbons fail to feed, or if they are totally consumed, or when the lamp is to be rendered harmless for inspection. In a circuit fed by a powerful arc machine the tendency to maintain the current is enormous; and an arc a yard long may be drawn at a potential of two thousand volts or more, unless some positive and reliable means exists for short circuiting it. These devices, known as cut-outs, are contacts connected to the poles of the lamps, and so arranged that they are brought together as required. Cut-outs should operate —

When the carbons fail to feed.

When the carbons are consumed or broken off.

When operated by hand by the trimmer.

The surfaces of the cut-out that make contact should be made of a metal not easily oxidized, such as silver, and with the surfaces vertical so as not to collect dust. A common form of cutout is a silver button on the armature making contact with a fixed button, when the armature is pulled all the way over by the shunt coil. This practice is not to be commended, since it does not operate if the armature itself should stick. A better

arrangement is an auxiliary cut-out in addition to the armature cut-out, which will operate even if the armature sticks. The disadvantage of such a device is that it is liable to operate when not wanted, but this may be overcome by adjusting the auxiliary cutout to act only at considerable increase over normal potential.

The cut-out that comes into play when the carbons are consumed consists usually of a contact attached to the carbon rod or parts moving with it, making connection with another contact on a fixed part of the lamp. To cut out the lamp by hand, a lever is frequently provided that will have the same effect as the descent of the carbon. In any case, the effect of a series cut-out is to dead short circuit the lamp, making it safe to handle.

On lamps that are run two in series on constant potential circuits, the cut-out must be set to introduce a resistance that will produce the same drop as the lamp itself, thus disturbing the other lamp or lamps in the same string as little as possible.

Inclosed arcs when run in series require a similar cut-out, but have none at all when burning singly in parallel on incandescent circuits. In this case the arc breaks when the carbons are exhausted, and the lamp goes out.

Long-range pull of magnets. It is evident that the magnets of all lamps must exert a pull on their armatures just sufficient to maintain a balance, whether the armature be close to or far from the pole. Where the force to be overcome by the armature pull remains constant, the magnet or solenoid must be constructed so as to have long range, that is, an even pull throughout the travel of the armature.

Where, however, the attraction of the coil for the armature is very unequal through the limit of motion, as it is apt to be unless specially provided for, the force of the armature is equalized over the range of the carbon motion by some mechanical device, called an equalizer, such as a pair of cams.

Temperature Correction. Another essential feature of modern lamps is an arrangement whereby the energy at the arc is maintained constant, independent of variations in the temperature, and hence current in the shunt coil. In shunt lamps this is usually effected by the somewhat expensive recourse to a special metal, with a low temperature coefficient, for the wire of the shunt. In differential lamps it is usual to shunt the series coil

itself with a small piece of low coefficient metal. When the shunt resistance increases through heat, weakening its current and pull, it would be overpowered by the series coil were it not for the fact that the copper series coil has also risen in temperature, shunting more current around itself through the low coefficient wire, and thereby weakening its own pull. By proper adjustment of this "temperature" shunt the lamp, whether hot or cold, may be made to maintain constant energy at the arc.

Magnetic Circuits. The magnetic circuits of the shunt and series coils have an important bearing on the sensitiveness of the lamp. If the shunt coil is wound over or under the series coil, but in opposition to it, so that it has no separate magnetic flux of its own, increase in shunt current will weaken the pull of the series coil by a certain number of ampere turns. When, however, the series and shunt have separate magnetic circuits, and pull against each other, an increase in shunt current will draw the armature or core toward the shunt coil, shortening and strengthening its own magnetic circuit, and lengthening and weakening that of the series. This strengthens the pull of the shunt, while it weakens that of the series; the result being that the actions of the two coils are stronger and more positive when each has its own magnetic circuit.

The arrangement for feeding the carbons in arc lamps is usually of the non-focusing type when intended for service where the utmost simplicity of mechanism is essential. All street lamps were formerly non-focusing; lately, however, the care of electrical apparatus has become better understood, so that focusing lamps are quite practicable. The focusing arc lamp has several advantages over the non-focusing type. One of these is that the shadows of the side rods and bottom part of the lamp stay in the same place, and do not increase in size, as occurs when the arc travels downward on the negative carbon. Another advantage is that the heat is generated at the same point, and the globe is not unevenly heated rendering it liable to crack. The focusing arc also permits the use of a very efficient reflector, since the reflector can be placed and maintained close to the arc. (See Fig. 268.) In continuous current lamps this is not of such great importance, because most of the light is thrown downward anyway, but in alternating lamps it is quite essential. For use with holophane

globes a focusing lamp is important, since the holophane is designed to diffuse and distribute the light properly when coming from a certain specified point, preferably an inch or two above the center of the globe. If this point of light should travel downward, as it does in the non-focusing lamps as the negative carbon is consumed, the action of the holophane would be much less regular and satisfactory. The disadvantage of focusing lamps is the complication in mechanism required to feed the lower carbon, and the added difficulty of trimming. The lower carbon is usually drawn up by a chain passing around a wheel in the upper part of the lamp to the carbon-holder carrying the upper carbon. As the upper carbon descends, the lower one is drawn up, the action being regulated by a clutch working on some part of the wheel or chain mechanism. Another disadvantage of this type of lamp is the necessity for using carbons of different sizes. Thus a common combination is a ginch upper carbon 14 inches long, and inch lower carbon 12 inches long, with a life of about twelve hours. If the lamps burn only seven or eight hours, as they do during a large part of the year, the remainder of the carbon is wasted. Whereas, in double carbon non-focusing lamps the same length of time would consume practically all of one pair of carbons. If it consumed more or less than an even pair of carbons, the remaining portion would be used to trim one side of the lamp, and a full set of carbons would be used in the other. Thus, no carbon is wasted, unless the lamps burn so long that more than one and one-half pairs of carbons are required; in this case of course it would be necessary to fully renew both pairs. Again, carrying two sizes of carbons in stock is somewhat of a nuisance for arc-light stations and lamp-trimmers.

Double Carbon Lamps. Originally, in order to gain life, carbons were made very long, and were therefore expensive and liable to break. A pair of carbons some 12 or 14 inches long, of a size that will give good efficiency and steadiness, such as inch, will not last longer than eight hours; and this was found too short for all-night service during the American winter. To meet this need, Brush invented the double-carbon lamp. In this lamp there are two independent sets of carbons, both fed by one mechanism. The Brush clutch, which is shown in Fig. 277, is the device originally used to feed one pair until it was consumed, and then the other.

As will be seen, it consists essentially of a clamp holding two washers, each one encircling one of the carbon rods. The carbon rod is gripped by the tilting of the washer when the clamp is raised, and the clamp is so shaped (one of the jaws being wider and one washer having a larger hole than the other) that one washer is tilted more than the other. This causes one washer to grip its rod before the other. As the clamp is raised, the rod first gripped

Washer

Fig. 277. Original Brush Clutch.

lifts its carbon from the negative, and the arc follows between the next carbon and its negative. In order to keep feeding the carbon last raised, and not the other one, a stop, or release, is so placed that on the descent of the clamp it will come in contact with the washer that has been most tilted, and release its grip on the rod before the other washer has struck the detent. When this carbon rod has descended its full length, the clamp in trying to feed falls still more, finally touching the release, allowing the second carbon rod to slip down and strike the arc. The extreme simplicity of this mechanism found immediate favor, and the short carbons that it was enabled to use materially lessened the cost of arc lighting. Practically the same device is applied at present in a modified form in all double-carbon lamps.

Carbon Feed. Two methods are commonly employed to feed the carbons: In one, the clutch mechanism acts directly on the carbon, and this is termed direct or carbon feed. In the other form, the carbon is gripped by a clamp attached to a long rod, sometimes to a chain passing over a wheel, and the feeding is done by the action of a clutch gripping the rod or wheel. When the rod is used the lamp is known as a rod lamp, or in the case of a chain or band as a chain or band lamp.

Rod Feed. Taking up the rod feed first: The advantage it