Absolute Thermal Conductivity of Crystals fyc. 115 



whence, getting rid of- , we have 



k _ , T'-T _ t'-t 



z x~~ z+ ~dT~ dt 



• (10) 



dx 



dx 



so x i s determined by the two observations. 



6. Although this method requires two experiments while 

 the other (§3) required only one, yet it will probably be more 

 useful than the other, as it is applicable to very thin slices of 

 crystals — in fact the thinner the better, — whereas the other one 

 applies more to substances in the form of a short rod ; for it fails 

 when z becomes very small. This failure of the § 3 method is 

 due to the fact that it depends entirely on radiation going on 

 from the cylinder at the same intrinsic rate as from the rods, 

 and on some appreciable quantity of heat being lost in this 

 way during its passage through the cylinder ; hence of course 

 a certain lengtli of the cylinder is essential. 



Observation of the Curves of Temperature. 



7. We have seen (§1) that, to determine the curves of tem- 

 perature, it is necessary to know the actual temperature of two, 

 or perhaps one should rather say three, points on each rod. 

 Each rod then should have three holes bored to receive the 

 bulb of a thermometer, one near each end, at a distance I from 

 one another, and another in the middle halfway between the 

 other two. Let the temperatures which these thermometers 

 indicate above that of the air at the time be denoted by T , T$, 

 T l7 T 2 , T|, T 3 , their position being shown in the figure. 



Fig. 2. 



Boiling 

 Water. 



Ice or 



JErcezLngMCxtare. 



Denote the distances thus : — 



a=l = b 3, 



1 a = Z 1 = J 2, 



01 = Z=23=Z -Z 1 ; 



then measuring x from for the first rod, we have as the curve 



12 



