CONDUCTIVITY. 105 



glass disc of known conductivity (as in Fig. 71) on the upper disc, and 

 then a copper plate on the glass, a heating coil on this, and a final 

 copper disc on the coil. The pile thus built up was varnished to give it 

 uniform and known emissivity, and placed in horizontal position in a 

 uniform temperature enclosure. The temperature excesses of the three 

 discs v : v 2 v s were determined by thermo-electric junctions inserted in 

 small holes, and were taken as uniform in each disc. Let the conducting 

 area of each face of the glass disc be a g , that of its emitting surface s g ; 

 let its conductivity be K g , and let its thickness be g. We may take the 

 heat passing through its middle surface as 



K a v i ~ V 2 



9 

 The heat emitted by its surface is 



and that emitted by the lower half is approximately 



hs v i + v 2 



M 9^ 



Hence the heat passing through the lower face is 



which is determinate. 



Now, passing to the copper disc with temperature excess # 2 , let its 

 emitting surface be s c , the heat passing from this with the liquid and 

 ebonite is equal to the heat passing in minus the heat emitted by the 

 disc, or is determinate as 



Q 2 = Q! - hs e v a 



Assuming that the now is vertically down through the liquid, that its 

 conductivity is K, its area a, and its thickness I, the heat passing through 

 it is 



The heat certainly does not flow vertically through the ebonite, but 

 we may put it as 



A(v 2 - 8 ) 



where A is some constant, and we may then put 



fa V 3 ) 



If, then, we can determine A, we can find /c, since Q is known. 



To find A a separate experiment was made, in which the liquid was 

 replaced by air, of which the conductivity is approximately known. 



The results obtained, between 25 and 45, agreed with the formula 



