498 Al. H. Weber on the Heat-conducting Power 



With the aid of equations (2), if for abbreviation we put 



we accordingly obtain 



TT^iff 



/HP 



Vkq 



;^/HP « + J 

 KQ 



If we introduce into this expression the mean values of Wq, u^, 

 U^ H, K, the result is 



A = 39^-23 C. 



Consequently, if one surface of an infinitely large iron plate of 

 one millimetre thickness be kept at the temperature of 39° C, 

 and the other at 38^ C, through one squaj^e millimetre area 14*85 

 heat-units will pass in a second from one surface to the other*. 

 If, further, a body consists of iron the surface of which has the 

 same constitution as our iron rod, and if this body be constantly 

 kept at the temperature of 39° C. while the temperature of the 

 surrounding air is 38° C, from everi/ square millimetre surface 

 0'00266 thermal unit will be given up to the air in one second. 



The surface of the iron rod had not the highest polish. In 

 order to try whether the quality of the surface exerted a percep- 

 tible influence on the value of K, the previous experiments were 

 repeated with the same iron rod after its surface had been evenly 

 coated with soot from a gas-flame. Tiie thickness of the coat- 

 ing was such that the metallic surface was just perceptible 

 through it. Here the duration of the periods amounted to 10 

 minutes, in order to make the observations in even and odd pe- 

 riods alternately, not consecutively as before. 



II. Iron rod coated with soot. 

 Even periods. 



K = 14-80 H=:000333 



K=15-21 H = 0-00337 

 Odd periods. 



K = 14-65 H = 0-00319 



K = 14-50 H = 0-00322 



Mean values. 



K= 14-79 H = 0-00328 



We consequently obtain approximately the same value for K, 

 which seems to indicate that, in this method of determining the 

 heat-conducting power, the heat given up to the environs does 



* By multiplyiDg the above by 0*6 we obtain the thermal conducting- 

 power in the units used by Angstrom. 



