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hot, and a mellow beam of light resembling sunlight is shed down from the lamp as shown in Fig. 66. The sun lamp is very simple and inexpensive, and yields a rich light suitable for picture galleries and shops.
Very powerful arc lights are now produced for light-houses and for exploring fortifications or searching for torpedo boats in time of war. Lights of from 10,000 to 150,000 candles are employed, and great reflectors, such as the Holophote of Siemens, and the Projector of Col. Mangin, are placed behind the arc to throw the penetrating beam in a particular direction. These arc lights are gradually gaining ground in the lighting of streets and cities. In California the entire town of San José is lighted by a few powerful lamps placed on tall iron masts, fitted with umbrella-like reflectors, which shed the light downwards over a large area. The flickering noticeable in single lamps when near them is absent from the blended radiance of several lamps placed at a distance overhead, and it is probably by imitating the sun and other celestial orbs in this way that extensive out-of-door illumination by the electric light will be effected.
In passing from the more powerful arc lights destined for large areas, to the incandescent lamps intended for indoors, we encounter a class of lamp coming between the “arc” and “incandescent sorts, viz., the semi-incandescent lamp of Reynier and Werdermann.
The light-giver or electro-pyre (to coin a special word) is formed of a pointed pencil of carbon A Fig. 67 abutting
FIG. 67. on a block of carbon B: and the current flowing between the pencil and block raises the joint to a white heat. The light is mainly emitted by the
incandescent pencil point, but a tiny arc like a bead is also formed between the two carbons ; hence the intermediate properties of the lamp. The pencil of carbon slowly wastes away and has to be fed forward by means of mechanism.
The Joel lamp is an improved form of the Werdermann, and is illustrated in Fig. 68. In it the block of carbon is replaced by a block of iron E, against which the carbon pencil is pressed by the weights w which hang by cords, passing over pulleys R R, the lowermost of which is carried by the end of the pencil holder. The pencil point is grasped by two metal jaws J J which guide it upwards towards the electrode E. The pencil is guarded by a metal tube n n. A collar c of iron forms the armature of an electro-magnet s, which is wound with fine wire and so arranged that if an abnormally large arc should form between the carbon and the iron E, the electro-magnet s would come into action, lift the weight w, and free the jaws J J, thereby allowing the pencil point to close with the electrode E.
The external appearance of Joel's lamp is very neat, as may be seen from Fig. 69, which represents it as fitted up in the Pompeian Court of the Crystal Palace at the Exhibition of 1882. It is suitable for halls, offices, and shops : and while being more commodious than the arc lamp it is more economical than the pure incandescent lamp. According to Professor W. Grylls Adams, it is capable of yielding a light of 700 candles per horse power.
shaped bulb of glass A exhausted of air by means of an improved Sprengel air-pump, and containing a loop of carbon filament, which is supported by two platinum wires sealed through the glass. The carbon is prepared by taking a thin slip from the skin of the bamboo cane and charring it. The junction with the platinum wire is made by an electro-plating of copper.
When an electric current of sufficient strength is sent through the carbon filament it glows with a soft but brilliant light resembling that of a wax taper, but of a power ranging from ten to twenty candles. The absence of the air in the bulb conduces to the durability of the carbon, which cannot burn away, and such a lamp is