In recent tungsten lamp practice nitrogen gas is introduced in the bulb after extracting the air as fully as practicable. It has been demonstrated that the pressure of the gis re tards the wasting of the filament, more than off setting the heat lost by its introduction to the bulb. As tungsten lamps can be made of 1,000 to 2,000 candle power, and show a life of 1,000 to 2,000 hours, they have naturally become very popular.
Wiring and building regula tions of all large cities contain more or less stringent regulations as to the wiring of build ings, the result of somewhat numerous fires caused by careless wiring in the early stages of introducing electric lighting. Cables carrying street wires are now abundantly insulated, and the individual wires entering buildings are care fully insulated and protected from accidental abrasion. At the point of entering a building, s switch-box is usually placed, with accommo dation for one or more fuse-wires. These are short connections made of easily fusible wire, as a mixture of lead and zinc, so that when by accident a strong current reaches the switch, the fuse is instantly burned out, and the cur rent can go no further, having no wire conduc tor. Wires for incandescent lighting are usu ally covered and boxed in, under certain regula tions of safety, and led to the various points of use—to ceiling fixtures, pendants, electroliers, brackets, portable lamps, etc. The pendant, with flexible cord, that can be swung to any convenient point within its radius, is easily the most popular lamp. The modern idea of inte rior lighting of dwellings involves the use of a semi-transparent inverted half globe, as of thin porcelain, within which the light is hidden, casting its full Flare upward toward a white ceiling, from which a soft radiance is reflected downward over the surroundings. This plan
diffuses the light in a most acceptable manner.
The low value of the common radiants is due to the fact that they are based upon the incandescence of carbon.
If the conversion of coal into light is through the medium of the incandescent lamp, we throw away 95 per cent of theoretical energy, and secure as light only 95/100 or prac tically 1 per cent of the energy of the coal.
However, the incandescent light is almost a perfect light. Since it gives off no waste prod ucts of any kind, it has no deleterious effects upon the atmosphere of the rooms or buildings in which it may be utilized. Its light is white, soft and brilliant, yet not dazzling. The lamp itself is rather ornamental than otherwise, and lends itself readily to external ornamentation without danger of fire, since of course it gives off no flame. Nevertheless, by way of caution it may be noted that, comparatively insignificant as is the heat which the incandescent lamp gives out through the glass bulb it cannot be allowed i to rest for an undue time in touching proximity to inflammable materials, since experiments have shown that when such a lamp is encased in two thicknesses of muslin for about six min utes, and fresh air is admitted to the interior, the muslin has burst into flame. Again, an arti cle of celluloid pressed against the lamp for three minutes has ignited; and a newspaper, under similar conditions, ignited in three-quarters of an hour.