

Method of Determining Thermal Conductivity. 589 

 this difference is given by 



KA&£^-^ — vApsSx — . 

 dx i ax 



But assuming Newton's Law of Emission and reckoning 

 all temperatures from the room taken as zero, we may also 

 express the heat radiated by the section abed by the expres- 

 sion YipdSx, where p is the perimeter of the bar and E the 

 coefficient o£ emi^sivity. Equating we obtain : — 



KA a^~^dx= E ^' 

 The general solntion of this different! J equation is: — 



where A and B are constants of integration. 



Taking now 6 U wl , 6 2 as the temperatures above the room 



at the positions x = ; # = ■= ; a? = L respectively, we obtain 



2 _/^v v V 3S-»vi, vie 



vp * L *Vr, 



i = ,-4Ky-*-( e 4i) • . (1) 



ft 



_ «WL" E/>L S 



where K = ' T ^ 9 h T i-r-. 



4K 2 KA 



If, however, E is small this reduces to 



-EpL 



2 e -J m =e** + ^, .... (2) 



and if E could be neglected we should have 



— e ... 



6>a — ti\ 



(3) 



It is hoped that with the aid of high-vacuum jacketed 

 vessels equations (2) and (3) will be found useful when 

 applied to liquids, but from the present point of view a 

 particular case of equation (1) is of more importance. 



