THERMAL AND ELECTRICAL CONDUCTIVITY. 405 



and the section B, taken at equal distance from the points n i 

 and w, is traversed by a quantity of heat 



The difference 



represents a quantity of heat received in each unit of time by the 

 portion AB of the bar; one part is used in heating it, and the 

 other is lost by the external surface. 



The quantity of heat employed in unit "time to heat a portion 



/ of the bar having its centre at the division /, is ylq ^ , y being 



at 



the specific heat of unit volume ; the total quantity for the length 

 AB is then 



6 representing the mean temperature of this bar. 



Lorenz assumes that the heat lost by the external surface is a 

 function /(#) of the mean temperature. We have then 



If the heating be suddenly stopped, the difference B sinks 

 rapidly and acquires a value 8', taking the value 6' ; we have 

 then 



if the temperatures 6' and are equal at the time in question, 

 we get 



(56) 



