552 MR A. CRICHTON MITCHELL ON THE THERMAL CONDUCTIVITY 



Temperature Excess. 



Rate of Cooling. 



°C. 



F. 



T. 



M. 



150 



299 



3 02 



302 



160 



319 



327 



328 



170 



336 



353 



356 



ISO 



350 



380 



3815 



190 



364 



4-08 



411 



200 



376 



4-37 



4-4 



210 



389 



4-66 



47 



220 



401 



4-96 



503 



230 



414 



526 



537 



240 



4-28 



556 



5-74 



250 



4-40 



5-87 



612 



260 



452 



6-2 



655 



270 







693 



The area of the " statical curve of cooling " was arrived at by treating the 

 curve in sections, and estimating what amount had to be subtracted from or 

 added to the ordinates of each section in order to make it a logarithmic. The 

 areas were checked by the use of a quadrature formula. When it was required 

 to correct the areas for the change in specific heat with increase of temperature, 

 the arithmetical mean of the two temperature excesses on the statical curve, 

 and which included the particular section, was taken; multiplied b} r the 

 increase per degree and the uncorrected area multiplied by this product with 

 unity added to it.* By plotting a curve, as in Diagram 7, whose ordinates 

 represent the area of the statical curve of cooling beyond different values of the 

 temperature excess, represented by the abscissas, a useful means of checking 

 results arrived at by the methods mentioned is supplied. 



In the case of the two copper bars, the cool end was maintained at a steady 

 temperature by means of a cold water bath. In estimating the area of the 

 statical curve of cooling some allowance must be made for the heat so carried 

 off from the end of the bar. This quantity which has to be added is estimated 

 as follows. Let T : , T 2 , T 3 , &c, be the tangents at three successive increasing- 

 values of x, viz.. x u x 2 , x 3 , &c, and A 1} A 2 , A 3 , &c, the area of the statical 



A — A> 

 curve of cooling beyond the same values of x. Approximately, k— m l _Ht . 



1 2 



When the quantity on the right hand side of this equation is multiplied by 



* The following numbers were used in correcting for change of specific heat as representing the rise per 

 degree Centigrade : — 



Iron, ....... -0014 



Copper (Crown), ...... -00088 



Copper (C), ...... -0009 



German Silver, ...... "0009 



They are based on the results of experiments made in 1868, by the late Mr J. W. Nicol in Prof. Tait's 

 Laboratory; the materials used, with the unfortunate exception of the iron, being portions of the bars 

 tested for conductivity. 



