PEIRCE AND WILLSON. — THERMAL CONDUCTIVITIES. 27 



100° 



80° 



60° 



40^ 



20' 



o 



2/ 

 Figure 2. 



3/ 



U 



If one is to measure the quantity of heat that passes through a portion 

 of the disk, lying within a cylindrical surface of revolution of relatively 

 small radius co-axial with the disk, it is desirable to make the ratio of a 

 to I so large that possible changes in the edge temperatures shall not 

 sensibly affect the temperatures at any point within the portion in ques- 

 tion. It will be sufficient for our purpose to consider the temperatures 

 at a distance / from the axis in a homogeneous disk for which a = 5 /. 

 It is evident that the greatest effect of temperature changes on the edge 

 of the disk will apppear at those points on the inside cylindrical portion 

 nearest the edge, tliat is, farthest from the axis. 



Taking the formula 



e = ^ (1 - 2 n - 2 7;_,) + 2^0 • T'.-. + 2 ^, . t; , 



and using the values of T' given in Table VI., we see that, if Q^ = 100° C- 

 and $1 = 0° C, and, if the whole edtre is kept at the temperature 0° C., 



