118 Mr. J. Buchanan on the 



of the " Peltier effect." The local excess of temperature thus 

 produced might possibly account for the observed result. In 

 any case the determination of the position of the carbon line 

 on the thermoelectric diagram was of interest. I am not 

 aware that this has been done before. 



Result. — For the specimen of carbon I have tried, the 

 Thomson effect is of the same sign as in copper, and about 

 twice the value for that metal given by Prof. Tait, as quoted 

 in Everett's " Units and Physical Constants," §186. The 

 thermoelectric power of carbon is given by my experiments, 



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



where t denotes, as usual, the mean temperature of the junc- 

 tions. The neutral point of carbon with lead is therefore 

 about - 209° C. 



Thermo-couple of Platinum- Carbon. — As I have mentioned, 

 the series of experiments described below were undertaken 

 with the object of finding the magnitude of the Peltier effect 

 in incandescent lamps. I therefore examined specially the 

 behaviour of a thermoelectric couple of platinum-carbon. 

 The platinum was a piece of ordinary commercial wire pro- 

 cured from Messrs. Johnson and Matthey. The carbon was 

 in the form of rods 12 millim. diam., such as are made for use 

 in arc lamps. The direction of the current in such a couple 

 is from platinum to carbon across the hot joint. The thermo- 

 electric power at the mean temperature t is 



ep*_c= +566 + 3-94* C.G.S. units. . . (2) 



Due, therefore, to the " Peltier effect/' heat will be gene- 

 rated at the negative end of the carbon filament of an incan- 

 descent lamp when the current is passing. We can now 

 calculate what proportion the heat thus generated locally 

 bears to the whole quantity of heat generated in the filament. 



If T denote the neutral point of a thermo-couple, then the 

 Peltier effect for the absolute temperature at one joint is 

 given by 



7r=(K 1 -K 2 )(T-^)^, (3) 



where Kj and K 2 are the numbers that express for each 

 material the ratio that the Thomson effect bears to the abso- 

 lute temperature. (See Taifs ' Heat/ §415.) 

 Hence for a platinum -carbon couple, 



tt = 3-94 (144 + 0)0 (4) 



As a specific example, lake the case of a 100-volt lamp, 



