CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



LI. — A METHOD OF DETERMINING THERMAL 

 CONDUCTIVITY. 



By F. A. Laws with F. L. Bishop and P. McJunkin. 



Presented December 13, 1905. Received December 15, 1905. 



Among the various methods for determining thermal conductivity, the 

 wall method, a modification of which is here employed, has been per- 

 haps most frequently used. It has been open to the serious objection 

 that the tenqieratures of the surfaces of the metal may not be the same 

 as those of the heating and cooling baths which have frequently been 

 taken as the temperatures of the metal surfaces. This difficulty was over- 

 come by Professor E. H. Hall by plating the surfaces of the wall with 

 another metal and using the E. M. F. of the thermo-electric junctions 

 so formed to determine the temperature difference. There are also diffi- 

 culties in insuring a perfectly uniform flow throughout the section under 

 investigation. 



The method here employed is to liberate electrically a known quantity 

 of heat inside a spherical shell of the substance, and maintain the outside 

 surface at a constant temperature. The difference of temperature thus 

 produced is measured by means of thermo-electric junctions of which one 

 metal is that of the shell, the other metal being electroplated upon it. 

 This method, then, will insure (1) uniform flow of heat, (2) absence of 

 radiation corrections, (3) exact determination of the temperature differ- 

 ence, (4) an exact determination of the quantity of heat passing through 

 the wall, and (5) independence of the density and specific heat of the 

 substance. 



Let the quantity of heat be represented by Q ; the difference of tem- 

 perature of the surfaces of the shell by t 1 — t 2 ; the radii of the shell by 

 p 2 and p x . 



Then K= WP'-Pd 



4 7r(<! — t 2 )p l p a 



