RADIATION FROM A BLACK BODY. 



321 



point.^ Both curves show very rapid increase in intensity of the emis- 

 sion lines with rise in temperature. 



RADIATION FROM A BLACK BODY WHEN HEATED TO IOO° C. 



Since the temperature of the cell could not be determined accurately 

 it was highly desirable to compare its radiation with that of a solid 

 whose temperature 

 could be varied and 

 determined more ac- 

 curately. To this end 

 the radiation from a 

 thin-walled, blacken- 

 ed copper vessel was 

 found when filled 

 with water which was 

 heated electrically. 

 The escaping vapors 

 from the hot water 

 caused such a varia- 

 tion in the tempera- 

 ture of the room that 

 the radiometer be- 

 came very unsteady, 

 hence only the region 

 of 4.75 fjL was exam- 

 ined. In figure 152, 

 curve a gives the 

 emission curve of this 

 vessel for diflferent 

 temperatures of the water. The curve is very similar to those found by 

 Paschen. Of course, the temperature of the outside of the vessel is less 

 than that of the water, but not sufficient to debar a comparison with 

 the hot vacuum-tube. Mention has already been made of the fact that 

 the constricted part of the vacuum-tube is the hotter at high pressures, 

 while the regions surrounding the electrodes are the hotter at low 

 cathode rays pressures. At the latter pressure the deflections for the 

 hot cell were from 8 to 10 cm. The vessel of hot water, under similar 

 conditions gave a deflection of 8.5 cm. for a temperature of about 96°. 

 At a temperature of 70° the deflection is about 3 cm. 



As a whole, the results show that the radiation from the cell walls is 

 due to a rise in temperature which is not much, if any, greater than 

 that of the vessel containing water. The temperature of the gas in the 

 vacuum-tube is an entirely diflferent question, which must be consider«d;1^\G^i /'^J 

 separately. /^J^ ^^^ •^•'" 



