through the solid conductor to raise its temperature to 2000° or 3000° Fahr., and to get from it a rich golden light resembling that of the sun.

g

ARC LAMPS.

Coming now to the arc lamps in general use, we shall consider first the Brockie lamp, which is supplied by the British Electric Light Company, and fed by the currents of the Gramme machine. As in most others, the light is supplied by two carbon rods (Fig. 59, P, N) one of which P is connected to the positively electrified brush of the generator and the other N to the negative brush. The positive carbon P is torn down or consumed by the current at twice the rate of the negative carbon, and in order to keep the arc practically the same, Mr. Brockie arranges the lamp so that every half minute the positive carbon is instantaneously forced forward. In this way the electric wick is periodically trimmed and screwed up. The action causes a momentary blink of the light, but where several lamps are burning together the flicker is not

FIG. 59.

noticeable in the general effect. The trimming is effected in the following manner. The positive, or upper carbon, is supported by a grip clutch g, held by an electro-magnet within the box B, through which part of the main current flows. At intervals of half a minute, however, this by-current is cut off by an automatic key, or commutator," worked independently, and the clutch

being released, the carbon drops down into contact with the lower one. But this state of things lasts only for an instant. The re-closing of the by-circuit by the commutator enables the electromagnet once more to draw up the armature, operate the clutch, and draw back the upper carbon to the distance which gives the proper width

of arc.

The result is a very steady light, as may be seen at Cannon Street Railway Station and elsewhere.

The Brush lamp,

66

P

FIG. 60.

B

which is fed by the Brush machine, is shown in Fig. 60. Like the Brockie, it is a clutch lamp, that is to say,

the positive or upper carbon P is held by a metal clutch actuated by an electro-magnet device excited by the current passing through the carbon and yielding the light. But instead of being regulated by an automatic commutator at regular intervals, it regulates itself whenever there is any need for regulation. The regulating device consists of a hollow coil of wire through which the current passes, and thereby attracts a soft iron core into the hollow. This action operates the clutch and supports the upper carbon at the proper distance to establish the arc. But when the arc widens through the waste of carbon, the current gets weaker in the hollow coil and the soft iron core falls away, therein releasing the clutch and allowing the upper carbon to descend by its own weight until the proper width of arc is restored. A piston moving in a well of glycerine serves to soften the descent of the carbon and graduate the operation of the lamp. All this mechanism is enclosed in the box B. In the Siemens electric lamp, which is fed by the Siemens machine, the regulation is still more perfect and continuous. The current through the carbons is diverted through a pair of hollow coils, one of which has a high resistance and serves as a by-path or shuntcircuit to the other. These coils attract soft iron cores connected to the carbon holders, and when the arc becomes too wide the influence of one predominates over the other, and allows the upper carbon to be fed to the lower. This differential action is, however, best seen in the Pilsen lamp, of Messrs. Rowatt and Fyfe, the horizontal form of which is shown in Fig. 61. Here a spindle-shaped core of soft iron c is placed between two hollow coils of wire м м' so that its points

enter a little into both coils. The core is fixed inside the metal tube G which carries the positive carbon, that is, the carbon connected to the positive pole of the generator. The negative carbon is carried by a metal rod D supported on the coils, whilst the positive carbon holder runs on rollers r r. Now the main current

G

7

D

FIG. 61.

passes through the coil м and the arc, but the coil м', which is of finer wire than м and has a higher resist

ance, forms a "shunt-circuit" or by-path for the main current, and as the arc becomes wider by the wasting of the carbon, more current flows through it than before. The result is that the attraction of the coil m' for the core c becomes greater than before, and when it predominates over the counter-attraction of the coil м for the core, the latter moves towards м', and taking the holder & with it, brings the positive carbon nearer the negative one and shortens the arc to its proper width.

The electric lamp of M. Rapieff, which is illustrated in Fig. 62, was introduced into the Times printing office as early as 1878, and is still giving satisfaction.

B