498 Dr. Lees and Mr. Chorlton on the Thermal 



condition for continuity of flow at the surface of contact of 

 the material and the lower plate, i. e. 



v- 



. t .B=(tf 1 -0 o )(A 1 + V). 



qk 

 Hence in the material under test 



e-e o ={0-e o \{cosh /K M.x+ A±V sinh m a < 



L V 2* Jc /ph V qk J 



V qk 



If b is the thickness of the material, at iv = b we must have 

 6 = 6 2 , the temperature indicated by the thermometer in the 

 upper brass plate, i. e. 



^-^o = (^i~^o)(cosh A /Z\5 + A±V >sinll .MA 

 \ V qk k /ph V qk J 



V q k 



The maximum value of \ /PA . b for the materials experi- 



V qk 



mented on is "1 ; hence the hyperbolic functions can be 



expanded in ascending powers of \ / P— . b, and terms invol- 



V qk 



ving cubes can be neglected. Thus : — 



,,-,., W -,.,( 1+ g, + A±| v /g. s ), 



v qk 



or #2 — 01 & (i , 7 f . ph j\ 



The last term in the bracket never exceeds 4 per cent, of the 

 first two terms, and the final result will be affected only to a 

 very small extent if for h, /i A is substituted. 

 Hence we have 



which enables Jc to be calculated from the result of the expe- 

 riment if A, has been previously determined. 



Determination of Ji 1% — To determine the value of this quan- 

 tity the two plates were separated by a layer of air 3 mm. 

 thick, enclosed by a ring of badly conducting material upon 



