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XXVI. On the Determination of Thermal Conductivity and 

 Emissivity. By N. Eumorfopoulos, B.Sc, Demonstrator 

 in Physics, University College, London*. 



IF a long metallic bar of uniform cross-section be heated at 

 one end, and be left long enough to acquire a steady 

 state, the distribution of temperature along it is usually 

 expressed by -_ 



u x =u e~ xs ^ fe", 



where u x is the excess of temperature over surroundings of a 

 point on the bar at a distance x from the point 

 whose excess of temperature is u , x being mea- 

 sured positively along the direction of decreasing 

 temperatures ; 

 h and k are the emissivity and conductivity re- 

 spectively ; 

 p is the periphery of the bar, and s the cross-section. 



If the bar be of circular cross-section and r its radius, the 

 above expression reduces to 



2^= u e v **• ; 



that is, if two bars have at one point of each the same 

 temperature w , they will also have the same temperature u x 

 at distances x 1 and x 2 respectively, measured from these 

 points, and such that 



x x x/h- = x 2X /A_. 

 V k x r x V k 2 r 2 



If the bars are of different radii, but of the same material, 

 and we accordingly write /i 1 = /< 2 and k 1 = k 2 , we get 



= \/S- 



Xi 



x 2 V r 2 



Some time ago Prof. G. C. Foster suggested to me, as a 

 laboratory exercise, to undertake the verification of this 

 relation. 



The method adopted was to heat the ends of two such rods 

 in steam until they had acquired the steady state, and then 

 by means of two thermoelectric joints (one on each rod) to 

 find a series of isothermal points. 



With two brass rods of radii r 1 = 3'4 and r 2 = 2*6 mm. 

 respectively, the distances at which equal temperatures were 



* Communicated by the Physical Society : read January 11, 1895. 



