[HENDERSON] 



TEMPERATURE GRADIENTS 



79 



K = Ko(l + «^) 



is not sufficient to express the variation of thermal conductivity with 

 temperature over the range employed (0° to 500°). 

 A formula of the form 



K = Ko(l + «0+/?^-) 



would probably be needed to express approximately the variation of 

 K with 6 over this range, but with the uncertainty of the given values 

 of a it would be futile to evaluate the coefficient /?. 



Summing tip the results of measurements of temperature gradients 

 over a wide range, it may then be stated that a two power formula 

 seems necessary in order to express approximately the variation of 

 thermal conductivity of air with temperature. Owing to the errors 

 involved in evaluating a correction for radiation, only upper limits to 

 the first coefficient can be obtained from the present measurements. 

 These values are given in the column headed a in Tables I, II and 

 III above. 



A formula of the form 



K = Ko((9/0o)n 



was also found to express approximately the variation of K with d. 

 The values of n obtained are given above. They are, however, not 

 independent of the temperature range and can be regarded only as 

 upper limits owing to the uncertainty in the radiation corrections. 



Temperature Co-efficient Between 0° and 100°C. 



If the temperature range be decreased, the radiation corrections 

 become smaller and errors in them have less effect on the temperature 

 co-efficient. Accordingly, experiments were carried out over the range 

 0° to 100°. 



The electrically heated plate was replaced by one of copper, 

 over which steam was made to circulate. Both silvered and blacked 

 plates were used. The results obtained are given in Table IV. 



Table IV 



The mean of the given values of a is -00261, 



