8o 



INFRA-RED EMISSION SPECTRA. 



to 



c 



of his energy curve, corrected for slit-width, occurs at 9.2 ,u, while the 

 value computed from the "displacement law," using the temperature 

 297 abs. and A = 2920, would be found at 9.8 ;x. If we compute the 

 value of this constant from the observed values of >} max and T, then 

 ^ = 2722, which is about 6 per cent lower than for a complete radiator. 

 From experiments on the radiation from plane surfaces, such as a radi- 

 ometer vane, this departure from the radiation of a full radiator is to be 

 expected. Then, too, this form of radiation is complicated by the window 

 before the radiometer vane. In general, the intervening material between 

 the source and the receiver would have no effect. For very delicate radi- 

 ometers, however, the writer found (Carnegie Publication No. 35) that the 

 inequality in the radiation from the radiometer window had a great effect 

 upon the position of the vanes. Just how much this would affect the 

 vane in the present case is unknown. In the present investigation a Ru- 

 bens thermopile (20 junctions of iron-constantan) was allowed to radiate 

 to a copper vessel at liquid-air temperature. A mirror spectrometer and 

 rock-salt prism, described in Carnegie Publication No. 65, were employed 

 to produce the energy spectrum. The only changes introduced consisted 



in replacing the Nernst heater 

 by a short focus mirror, which 

 projected an image of the bot- 

 tom of the vessel at liquid- 

 air temperature, upon the 

 spectrometer slit. The gal- 

 " vanometer used with the ther- 

 mopile had a sensitiveness of 

 i=2Xio -10 amperes and a 

 full period of about 12 sec- 

 onds. The deflections were 

 very unsteady, due to air cur- 

 rents caused by the evapo- 

 ration of liquid air. The 

 thermopile was placed in the 

 position formerly occupied by 

 the radiometer and was cov- 

 ered by the inner metal shield, 

 with a slit 1 by 15 mm. area. 

 The whole was inclosed in a 

 metal tube covered with felt. 

 This formed a more perfect black body than is possible with a radiometer. 

 The receiver to which the thermopile radiated consisted of a thin cylindri- 

 cal copper vessel about 4 cm. diameter and 12 cm. long (covered on the 

 inside with copper oxide and lampblack) , which was suspended in a vessel 

 of liquid air, as shown in fig. 55. The receiver was kept stationary and 



CD 



i. 



CD 

 O 

 CD 



Fig. 55- 



