INCANDESCENT LAMPS Because of their flexibility of use and convenience, incandescent lamps have become the most important of all illuminants. Since the last major development in incandescent lamps, the gas-filled bulb in 1913, there have been many improvements that have in creased their field of application. Their size ranges from the so called "grain of wheat" of a half of a watt consumption, such as used in surgical instruments, to commercial types of 5o kw. lamps and above. In 1885, five years after the introduction of the in candescent lamp, there were approximately a million lamps sold. The lamp consumption in 1928 for the world was more than 1,000,000,00o lamps, an increase of more than i,000% since 1885. In 1928 lamp prices in the United States were only about 40% of the pre-war prices of 1914, in spite of the fact that labour and material costs had nearly doubled. As late as 192o lamp manu facturing was largely a matter of many hand operations. About that time automatic machines replaced a score of hand operations in the assembly of the stem, leading-in-wires and filament support mounting.
Lamp Development.—Important developments on "getters" have been made. These are chemicals, principally of the halogen group, which react to perfect the vacuum in the bulb, and unite with the disintegrated particles of tungsten to produce a white de posit on the inner surface of the bulb, thus helping to maintain the candle-power. Developments in coiling the filament, methods of inspection of wire drawing and coiling processes have accounted for great advances in uniformity of life and efficiency. The tipless process of lamp manufacture, perfected by Mitchell and White of the General Electric Company, not only improved the physical aspect of the lamp, but made for more economic lamp produc tion. Pipkin's invention of the inside-frosting process in 1925 was a major development in so far as it cleared the way for standardization of lamps.
it is radiated, the remaining 8% being conducted away as heat through the leading-in wires and anchors. Of the 92% radiated, only a small part appears as energy within the visible spectrum, corresponding to about 6% of the total energy put into the lamp, so that the lamp as a lighting device may be considered as having an "optical" or actual efficiency of 6%, that is, in a 4o watt 115 volt vacuum lamp only about 21 watts appear in the visible spectrum. In a gas-filled lamp the "optical" efficiency is about 8 to io%. These figures for both the vacuum and gas-filled lamps are only general, the actual values depending on the operating temperature of the filament, which in turn largely depends on the size of lamp. In practice the efficiency of lamps is expressed in "lumens per watt" (LPW). If all the energy put into a lamp came out as a radiated energy within the visible spectrum, the LPW of such a lamp would depend upon the amount of energy in each wave length that would determine the predominating colour of the light.