METAL FILAMENTS. 



9 1 



Except for the lowest temperatures the value of a obtained by this method 

 is about 6.4. The surface became roughened, which lowers the value of 

 this "constant." The weakness of this method lies in the difficulty of ob- 

 taining the values of the emissivities, particularly the maximum of the 

 energy curve. Since the filaments are so narrow that the prism-face is 

 only partly covered, the height of the ordinates will depend upon the cone 

 of energy falling upon the prism face. For this reason the energy curves 

 of the different substances can not be compared. The value of a for a short 

 tungsten filament (see fig. 61) was found to be 7.3 for one energy curve. 

 Further experiments on another filament gave a value of a =6.88 for the 

 mean of 18 computations and 5 energy curves. The values of a for an 

 untreated carbon filament, for various values of energy consumption, are 

 plotted in fig. 62. As with osmium, the temperature was raised from a 



5<* 



TO 

 10 



C 



o 

 O 



c 

 o 



<? 



1234-56789 Watts 

 Fig. 62. Radiation constant (a) of untreated carbon filament. 



low red to the normal condition. Apparently the emissivity constant, a, 

 drops from 6.4 at about 900 to 5.3 at 1800 to 2000 C. Paschen found 

 no such variation for the specimens of carbon examined. Neither did he 

 find a variation in a for the oxides of copper and of iron. On the other 

 hand, Lummer and Kurlbaum, by measuring the total radiation emitted, 

 found that with rise in temperature the emissivity of these oxides approached 

 that of a complete radiator. This difference may, of course, be possible, 

 just as in the present examination of the Nernst glower the value of which 

 was found to vary, while Mendenhall and Ingersoll, 1 by using a total 

 radiation method of comparison with platinum, could not establish with 

 certainty any variation of a with temperature. An inspection of figs. 58 

 and 59 shows that this may be possible, especially in the latter where the 

 energy curve appears to have two maxima. For the "Helion" filament, 

 which is a carbon filament upon which is a deposit of silica, the constant 

 a=8.3 when new, and decreases to 6.3 after aging. For tantalum the 

 value of a is 6.5. From the energy curve of the acetylene flame published 

 by Stewart (Phys. Rev. 16, p. 123) the value of a is about n. A plati- 

 num strip, 50 X 1.5 X 0.02 mm. inclosed in a glass bulb with a fluorite 

 window, was also examined. Estimating temperatures from the position 



1 Mendenhall & Ingersoll: Phys. Rev., 25, p. 1, 1907. 



