As a lamp burns the filament vaporizes, condensing on and darkening the inside of the bulb. This darken ing cuts off some of the light given by the lamp. Chemicals called "getters" are put inside the bulb to convert this deposit into a lighter shade so that there will be less absorption of the light. There is a further reduction in lumens of a multiple lamp during its life due to the evaporation of the filament, which becomes thinner as the lamp burns. This increases the resistance of the filament so that it consumes less watts, hence operates at a lower temperature and so gives less light.
The wattage input in a lamp is greater than the amount radiated due to the energy in the heat conducted away by the leading-in wires and anchors; the rate at which energy is radiated from the filament at these points, due to their lower temperatures, is less than that from the main body of the fila ment. In a gas-filled lamp an even greater wattage input is neces sary, due to the heat conducted away by the gas. Thus, in order that a filament may operate at a certain temperature, its surface area must be taken into consideration. There is but one possible filament length and diameter which has the right surface to operate at the desired temperature. The 50-watt, 115 volt lamp has a fila ment length of 17.4 inches and a filament diameter of o.00f 6 inches.
It is desirable to oper ate the filament at as high a temperature as possible in order to get the most light for the energy put into the lamp. The limiting temperature is, of course, the melting point of the filament mate rial. However, the material evaporates at temperatures below the melting point, the lower the temperature the lower the rate of vaporization. The filament becomes thinner as its surface vapor izes until it finally burns out if it has not been broken beforehand. Thus the higher the temperature the shorter the life of the fila ment, so the operating temperature must be considerably below the melting point in order to obtain a reasonable life. Filament temperatures range from
K to
K for lamp sizes from Lc) 1,000 watts (K =Kelvin, and K
=
The Vacuum.—If the bulb were not exhausted, the oxygen of the air in the bulb would chemically combine with the hot tung sten filament. This would produce a white smoke and the fila ment would immediately burn out. The absence of a gas in the bulb eliminates the loss of heat by convection and conduction. Inert Gas Bulbs.—By filling the bulb with an inert gas which will not chemically combine with the filament, the rate at which the filament material evaporates is retarded. The use of gas in
creases the loss of energy put into the lamp owing to the conduc tion and convection of heat by the gas from the filament. In order to reduce the amount lost by conduction, the gas used should have as poor a heat conductivity as possible. Originally nitrogen was used, but argon with a small percentage of nitrogen is now used. Bulb Temperatures.—The melting point of glass is within the range of 2,200 to
F depending on the kind of glass. The temperature at which an ordinary bulb will soften is about
F. The size of the bulbs used for lamps of various wattages must therefore be such that they will radiate the heat from the filament and operate at sufficiently low temperature so that they do not soften. The actual operating temperature of the bulb is of the order of
to
F depending on the lamp wattage.
Since 1926 most bulbs up to and including the 500 watt size have been regularly supplied inside frosted. This process accomplishes diffusion of light from the intensely brilliant filament with little sacrifice in efficiency. In the past it was neces sary to resort to outside etching, spray coating or special white glass bulbs with a loss of 5 to 25% of the light by absorption and at the same time such lamps collected dirt readily and were hard to clean. Above ioo watt lamps are supplied with either clear or white bowl bulbs. The latter differ from clear bulbs only that the lower part of the bulb is sprayed with a superficial coating of white enamel. Coloured lamps are made either in natural coloured bulbs or with coatings consisting of finely-ground pigment mixed with a permanent vehicle and sprayed on the bulbs, either on the outside or in more recent practice on the inside. Lamps may be obtained also with special blue-green glass bulbs producing a whiter quality of light and are designated as
lamps.
In the United States the matter of base standardization was taken up early so that to-day there remain but four standard types in general use aside from those used for min iature lamps. These are designated as mogul, medium, inter mediate and candelabra, indicating the type of socket in which each base will fit. Mogul bases are used on lamps above 200 watts, medium bases used on all sizes below 200 watts. The intermediate and candelabra are used only on certain types of small wattage lamps for special decorative service. (W. M. SK., C. E. W.)