192o] 



on Low Temperature Studies 



268 



Comparison with Radiation Laws. 



As already stated, the observed proportions between the manometer 

 readings in some unit period were the basis on which the relative 

 amounts of radiation transmitted were estimated. In order to test 

 the validity of this interpretation, the results so obtained were com- 

 pared with the theoretical values resulting from applying the fourth 

 power law of Stefan. The ratios from black bodies at T x and T 2 

 respectively into a liquid oxygen "sink" at 90° Abs., are thus expressed 

 TV - 90 4 



a ay - 9o 4 ' 



Such ratios for certain common values of T\ and T 2 are set out 

 in Table I. Typical temperatures are given in columns 1 and 2, 

 and the calculated ratios in column 3, while column 4 contains the 

 average values obtained from the thermoscope measurements. These 



Table I.— (TV - 90 4 )/(T„ 4 - 90 4 ) gives the Relative Energy Flux from 

 Two Temperatures, T l and T 2 , into Liquid Oxygen " Sink." 



ratios may be deduced directly from such a curve as that given in 

 Fig. 13, where the values of (T 4 — 90 4 ) are given as orclinates with 

 T as abscissa up to T = 373° Abs., the higher position of the curve 

 being shown separately with scale of ordinates to the right. 



For test observations all stray radiation into the cell was either 

 reduced as far as possible by screens or else directly measured and 

 corrected for, by covering the vessel with a basin containing liquid 

 air and making a " blank " exposure. For ordinary measurements 

 of transmissivity through various materials a little stray internal 

 radiation was not regarded as serious, as it had all to pass through 

 the material under examination before being registered. The pro- 

 portional transmission of the whole mixed radiation was therefore 

 recorded. The principal temperatures used for the Leslie cube (or 

 its equivalent) above the vessel (for the test measurements) were 

 boiling water, room temperature of 10° to 15° C, melting ice and 

 solid carbonic acid finely pounded, moistened with alcohol and 



