February 4, 1910] 



■SCIENCE 



165 



useless infra-red rays than any other 

 opaque material at the same temperature. 

 A polished metal is therefore a more effi- 

 cient light source than the same metal with 

 a black, or even rough surface. This is 

 derived from Kirchhoff's law of radiation 

 and absorption, which was early estab- 

 lished. 



It may seem like penetrating too far into 

 details to consider for a moment the 

 changes in structure and surface which the 

 carbon filament of our incandescent lamps 

 has undergone, but the development of such 

 an apparently closed problem is instructive 

 because it has yielded to such simple meth- 

 ods of attack. The core, or body, of the 

 carbon filament of to-day is made by some 

 one of the processes based on dissolving 

 and reprecipitating cellulose, which are 

 used in artificial silk manufacture. The 

 cellulose solution is squirted through a die 

 into a liquid which hardens it into dense 

 fibers. These cellulose fibers are then car- 

 bonized by being heated, out of contact 

 with the air, at as high a temperature as 

 possible with gas furnaces. All of this is 

 also merely the application of chemistry 

 which was first worked out in some of the 

 German chemical laboratories. This plain 

 carbon filament (the result of this simple 

 process), which might have been satisfac- 

 tory in the early days, would be nowadays 

 useless in a lamp, as its practical life is 

 only about 100 hours at 3 watts per candle. 

 In a subsequent process of manufacture it 

 is therefore covered with a steel gray coat- 

 ing of graphite, which greatly improves the 

 light emitting power. This coat is pro- 

 duced by heating the filament in an atmos- 

 phere of benzene or similar hydrocarbons. 

 The electric current which heats the fila- 

 ment is of such an intensity that the decom- 

 position of the hydrocarbon produces a 

 smooth, dense deposit of graphite. With 

 this graphite-coat the filament now burns 



about 500 hours. But the simple graphite 

 coat can itself be improved. It is improved 

 by being subjected, for a few moments, in 

 the electric furnace, to a temperature of 

 about 3,500°, so that the life now becomes 

 about 1,500 hours under the same operating 

 conditions as before. The product of this 

 treatment is known as the metallized fila- 

 ment, because its temperature coefficient of 

 resistance is by this last step made similar 

 to that of the metals. 



A case is shown on the table which con- 

 tains illustrations of the carbon incandes- 

 cent lamp manufacture in the shape of 

 cellulose solution, squirted cellulose fiber, 

 carbonized fiber, etc. 



Among the incandescent lamps which are 

 before you I have one containing a plat- 

 inum wire filament. You will see, as I 

 turn on the current, that the intensity of 

 its light is not vei-y great, even when the 

 current is sufficient to melt the wire. A 

 much greater luminosity is produced by a 

 plain carbon filament, and a still greater 

 by the graphite-eoated and metallized car- 

 bon, before they are destroyed. In the case 

 of carbon, the useful life of the lamp de- 

 pends much more on the vaporization of 

 the material than on its melting-point, and 

 these lamps, as shown, will operate for a 

 short time at very much greater efficiencies 

 or higher temperatures than is possible 

 when a practical length of life is consid- 

 ered. Thus, besides the physical effect of 

 surface quality, we have evidence of differ- 

 ences in the vapor pressure of different 

 kinds of carbon. It looks as though car- 

 bonized organic matter yielded a carbon 

 of much greater vapor pressure for given 

 temperature than graphite, and that even 

 graphite and metallized graphite are of 

 quite distinctly different vapor pressures 

 at high temperatures. It may be interest- 

 ing to note here that if the carbon filament 

 could withstand for 500 hours the maxi- 



