320 



foote: temperature and emissivity 



intersect at a point for any temperature of the lamp. That is, 

 a perfect color match against a black body is possible at all 

 temperatures. Hyde, Cady, and Forsythe 3 have published a table 

 of color temperature versus apparent temperature (X = 0.665m) 



for an untreated carbon filament lamp. If (- — —J is plotted 



against —, using their observations, the relation is found to be 



linear within observational errors, in accordance with equation 

 (5). From the slope of the line, q is found to be +462. Putting 

 this value of q in equation (1) the monochromatic emissivity 

 is seen to decrease with increasing temperature as follows. 



TABLE I 



TEMPERATURE ABSOLUTE 



1500 

 2000 

 2400 



RELATIVE MONOCHROMATIC EMISSIVITY 



1.00 



0.92 

 0.89 



Mendenhall and Forsythe 4 experimentally found the emissivity 

 at 1300 and 2300° absolute to be 0.86 and 0.79 respectively. This 

 is in qualitative agreement with the above table. 



Color temperature of tungsten. A more interesting verification 

 of the correctness of the above equations is obtained by a con- 

 sideration of Hyde, Cady, and Forsythe's 5 data on tungsten. In 

 this paper a table is given of the observed color temperatures, 

 true temperatures, and apparent temperatures (X = 0.665/t) for 

 a tungsten lamp. From the values of the apparent temperature 

 and true temperature the emissivity for X = 0.665/* may be 

 computed according to equation (4). Taking the value of 



322 



c 2 = 14350 it was found that e T represents the tempera- 

 cure coefficient exactly. The coefficient p of equation (1) was 

 found to be +0.0000104 and the general equation for emissivity 

 of tungsten is as follows. 



3 J. Frank. Inst. 181: 420. 1916. 



4 Astrophys. J. 37: 389. 1913. 



5 Loc. cit. p. 419. Also Worthing, idem, p. 417. 



