102 



HEAT 



The method was used for various other solids and the results obtained 

 were in close agreement with those obtained by another and more exact 

 method which he devised later, now to be shortly described. 



Lees's Disc Experiments* The substance to be tested was formed 

 into a disc X (Fig. 70), say 4 cm. in diameter and 2 or 3 mm. thick. This 

 was placed between two copper discs G l C 2 of the same order of thickness, 

 continuity being ensured by a negligible layer of glycerine. Against 

 one copper disc was laid a flat coil through which a current could be 

 passed so as to supply heat at a determinate rate, and on the coil was a 

 third copper disc 3 . Fig. 70 represents diagrammatically the pile of 

 discs thus made, suspended in a constant-temperature enclosure. The 

 surfaces of the pile were varnished to give them the same emissivity, 



h say, so that the rate of heat 

 emission from a square centi- 

 metre v degrees above the 

 enclosure would be hv. The 

 copper discs would each be of 

 uniform temperature through- 

 out to a near approximation, 

 and their temperatures were 

 taken by thermo-electric junc- 

 tions inserted in small holes 

 drilled in at their edges. Let 

 their temperatures, measured 

 as excesses over the tempera- 

 ture of the enclosure, be as 

 indicated on the right hand 



Uniform temperature enclosure 



FIG. 70. Lees's Disc Experiments on Conduc- 



tivity. CiC 2 C 3 , copper discs ; X, disc to be of the figure, and let their 



tested; Viv 2 v 3 , temperatures above the en- emitting surfaces have areas 



closure; ., , lWfc emitting surfaces. ag indicated on the left hand> 



Let the rate of heat supply by 



the coil be H. Then taking the mean temperature of X as the mean of 

 the temperatures of 



and 2 , we have 



+ Jis 2 v 2 



whence h could be determined, since H was known electrically. 

 The heat flowing through X may be put as 



and it may be taken as equal to the mean of the heat flowing into it 

 from Gj and that flowing out of it to C 2 . But the heat flowing from C a 

 into X is equal to that emitted from X and C 2 , and that flowing from X 

 into 2 is equal to that emitted from 2 . Whence we have 



n^= l ! 



d 21 



-. 



v, + 



* Phil. Trant., A., 1898, p. 399. 



