The higher the temperature at which an incandescent lamp filament can be operated, the more efficient it becomes. The limit in temperature is reached when the material begins to evaporate rapidly, which blackens the bulb. The filament becoming thinner more quickly, thus rupturing sooner, shortens the life. If, therefore, the evaporating temperature can by some means be slightly raised, the efficiency will be greatly improved. This was accomplished by Dr. Irving Langmuir in the research laboratories at Schenectady, N. Y., by operating a tungsten filament in an inert gas. Nitrogen was first used. The gas circulating in the bulb has the disadvantage of conducting heat away from the filament so that the filament was coiled. This presented a smaller surface to the currents of gas and thereby reduced this loss. The lamps were commercially introduced in 1913 and a patent was granted in April, 1916. [Illustration: GAS FILLED TUNGSTEN LAMP, 1923. This is the form of the lamp as at present made. For 110-volt circuits the sizes range from 50 to 1000 watts.] An increased amount of electrical energy is required in these lamps to offset the heat being conducted away by the gas. This heat loss is minimized in a vacuum lamp, the filament tending to stay hot on the principle of the vacuum bottle. This loss in a gas filled lamp becomes relatively great in a filament of small diameter, as the surface in proportion to the volume of the filament increases with decreasing diameters. Hence there is a point where the gain in temperature is offset by the heat loss. The first lamps made were of 750 and 1000 watts for 110-volt circuits. Later 500- and then 400-watt lamps were made. The use of argon gas, which has a poorer heat conductivity than nitrogen, made it possible to produce smaller lamps, 50-watt gas-filled lamps for 110-volt circuits now being the smallest available. In the present state of the art, a vacuum lamp is more efficient than a gas-filled lamp having a filament smaller than one consuming about half an ampere. Thus gas-filled lamps are not now practicable much below 100 watts for 220 volts, 50 watts for 110 volts, 25 watts for 60 volts, 15 watts for 30 volts, etc. From the foregoing it will be seen that the efficiency of these lamps depends largely on the diameter of the filament. There are other considerations, which also apply to vacuum lamps, that affect the efficiency. Some of these are: the number of anchors used, as they conduct heat away; in very low voltage lamps having short filaments the relative amount of heat conducted away by the leading-in wires becomes of increasing importance, etc. The 1000-watt lamp for 110-volt circuits is now made for nearly 20½ lumens per watt; the 50-watt lamp a little over 10 l-p-w.