﻿190 Mr. Horace II. Poole on the 



serious differences of temperature cannot occur in the copper 

 vessel. 



Conduction of Heat along the Leads. 



Two possible sources of! error must be considered, namely, 

 the possibility of heat entering the calorimeter by conduction 

 along the leads of the couple and the heater, and the possi- 

 bility that the heat-loss from the calorimeter is unduly aug- 

 mented by conduction along the leads of the heater during 

 the periods when the latter was heated by a current. As 

 regards the first error, consider a very long wire of radius i\ 

 and conductivity k { encased in a protecting tube of radius r 2 

 and conductivity Jc 2 and buried in ice, one end being kept at 

 a temperature 6 . If k r is large compared with k 2 the tem- 

 perature w r ill be sensibly uniform across any cross-section of 

 the wire, and the flow of heat through the sheath will be 

 approximately radial. With these assumptions we readily 

 find that *=*„«— 



where „ 2fa 



h i'i 2 log c r 2 ji'i 



6 being the temperature at a point distant x from the end. 

 The actual leads consist of twin 35/40 " flex " enclosed in a 

 single rubber tube. The total cross-section of the twin leads 

 is approximately equal to that of a single wire 1 mm. in 

 diameter, though the loss of heat to the ice, and consequently 

 the fall of temperature with distance along the leads would 

 evidently be more rapid from the twin leads. The external 

 diameter of the thick-walled rubber tube encasing the leads 

 is about 1 cm. If we assume 



/■, = l, /^SxlO- 4 , 



we find for a wire 1 mm. in diameter a = 0'32, hence for a 

 point 30 cm. from the end 6 is given by 



log, e /$ = 30 x 0-32 = 9*6, or 6 [} \6 = 15,000 about. 



so if e ^io°c, e = -oorc. 



As several feet of the leads is buried in the ice, and part of 

 this length is enclosed in copper tubes to facilitate the radial 

 flow of heat, it is evident that the leads near the calorimeter 

 must be exceedingly near zero and far below the temperature 

 of the calorimeter. 



As the ice in the outer vessel melted, the length of lends 

 buried in the ice decreased, but on repacking the outer vessel 



