Thermoelectric Position of Carbon. 



119 



with the filament at a temperature of 2400° C. We get the 

 ratio 



Heat due to Peltier effect 3*94 x 2673(144 + 2400) 



Whole heat in filament 



100 x 10 8 



2-7 x 10 7 



10 lu 



„ =-0027. 



The local heating at the negative end of the filament is 

 therefore only about one quarter per cent, of the whole heat. 

 This is too small to have any appreciable influence on the 

 life of the lamp. It does not appear therefore that the Peltier 

 effect accounts for the giving way of the carbon-filament at 

 one end rather than at the other. In the course of these 

 experiments one phenomenon was forced on my attention, 

 the details of which are given below. It seems that con- 

 tinuous heating alters the thermoelectric properties of carbon. 

 This alteration must be the result of molecular change in the 

 material. That this has any bearing on the destruction of 

 the filament that ensues, by keeping it for a long time at a 

 high temperature, my experiments are not extended enough 

 to decide. 



Experimental Details. 



The E.M.F. at various temperatures was determined by the 

 null method known as PoggendorfFs. 



The primary circuit contained a constant cell A, a gradu- 

 ated wire BC, and a box of resistance-coils R. 



On the graduated wire were placed sliding pieces e and /, 

 which served as the terminals of the secondary circuit. In 

 the secondary circuit were a galvanometer Gr, and the therino 

 couple T whose E.M.F. was to be measured, and a key K. 



Fig. 1. 



ik J± 



The arrangement of the apparatus used in the platinum- 

 carbon experiments has now to be described. 



