Thermoelectric Position of Carbon. 125 



Hence the E.M.F. of the lead-platinum couple and the 

 thermoelectric power of platinum are, respectively, 



E = -004815 x 1-75 x 10 8 ^j^(*-a) (W^); 



.-. E = 2-07(*-a)(85 + ^i?) C.G.S units, 

 .-.e pt = +176 + 2-07< C.G.S. units. 



The current would be from Pt to Pb across the hot junction. 

 Using this value for e pt , we find for the thermoelectric 

 power of carbon 



e =-390-1-87* C.G.S. units. 



e 



This is the result quoted in equation (1) above. 



In the table of thermoelectric powers given by Everett 

 (' Units and Physical Constants/ § 186) there are given 



forZn, e zn = -234-2*40*, 



„ Ag, ^=-214-1-50*. 



The value for e c shows that the place of carbon in the table 

 is between zinc and silver. In the thermoelectric diagram it 

 is easy to see that for temperatures below 50° C, the line 

 for carbon lies below that for cadmium, taking the value 

 for e c d from Everett's table (loc. cit.). 



Concluding Remarks. 



The disintegration preceding complete destruction of the 

 carbon filament in an incandescent lamp, described by Mr. W. 

 H. Preece, F.R.S., in a paper read before the Royal Society 

 on March 26, 1885, is doubtless the result of molecular 

 changes in the structure of the filament produced by the 

 continued maintaining of the filament at an extremely high 

 temperature. In my experiments, keeping carbon at a 

 moderately high temperature altered the molecular condition 

 of this materia], the alteration being manifested as a change 

 in the thermoelectric power of the carbon. 



Physical Laboratory, University College, 

 London, June 1885. 



