402 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



Each of the thermo-junction wires was brought to a mercury cup, made by cutting 

 4 centim. lengths from a piece of thin glass tubing with a bore of about 3 millims., 

 sealing one end of each length, and fastening it, with the sealed end down, in a brass 

 tube 5 centims. long and 3 centims. in diameter, mounted in a block of wood and 

 containing mercury. Ten of these glass tabes nearly full of mercury were arranged 

 in a circle round the bulb of a thermometer graduated in -^ degrees, which was 

 placed in the centre of the brass tube. To diminish loss or gain of heat, the brass 

 tube was wrapped with green baize (fig. 7). The ends of the wires leading to the 

 potentiometer arrangement for measuring the electromotive force in the thermo- 

 electric circuits, could be placed in any two of these mercury cups, and by means of 

 an additional short wire of platinoid, any two thermo-circuits could be placed in 

 series, and the difference of the electromotive forces in the two determined. 



TJieory of Flow of Heat in Discs. 



Let H = heat generated per second in the heating coil. 



h = heat lost per second, per square centimetre per degree excess of tempera- 

 ture of the discs over that of the enclosure. 



v = excess of temperature over that of enclosure. 



t = actual thickness of a disc, plus a small correction, p. (409) to allow for 

 heat lost along thermo-wires. 



r = radius of discs. 



Subscripts C, U, M, S refer to " cover " " upper " " middle " discs,* and disc of 

 substance respectively, the heating coil coming between C and U, and the substance 

 to be tested between U and M (fig. 5). 



When the " steady state " has been attained, the heat received per second by the 

 copper disc M from the disc of substance S, and given up to the air 



Similarly-, the heat received by the disc S, of material tested and given up to the 

 air and to the disc M 



- 9 . 

 r 



If I is the thermal conductivity of the material, the heat flowing through the disc 

 tested 



= 7n ^^L=l^ 



The reason for the choice of these terms will be understood from Part II. 



