Trowbridge and Penrose — The Thomson Effect. 



ends of the spac 



r(^-^)=K- 7 SD 



the temperatures of the ends of the space covered by the pile 



and the similar expression for nickel : 



^-tf)=K|s,D ; 



£-£ ?' P.' T 



o~d SD 



Equation I then gives the relative value of the coefficient of 

 the Thomson Effect at any temperature d. 



S = -095 S. = -108 D = 8-9 D =8-3 d=0'97 d'=l'15 



In Le Roux's table <r=2 . 

 of opposite sign. Introducin 



Al 0-1 

 Sn 0-1 



The Thomson effect in carbon was next investigated. The 

 carbon used was the graphite of the common carpenter's lead- 

 pencil. The pencils which gave the best results were Faber's. 



Attempts were first made to measure the direction of the 

 Thomson Effect in the same way as in the case of nickel, that 

 is, by placing a face of the thermopile on one surface of the 

 ^irhon: the two ends of the carbon being maintained at con- 

 stant temperatures, and passing the electric current alternately 

 m opposite directions. This method was unsuccessful from the 

 fact that one Grove cell heated the carbon to such a degree 

 that in one minute the spot of light was thrown off the gal- 

 vanometer scale; thus rendering it impossible to measure, with 

 a «y accuracy, the rate at which the deflection increased. 



The method of Le Roux was then tried of using two strips 

 of carbon, each face of the pile being in contact with one strip. 

 ' '"> method not only doubles the deflection due to the Thom- 

 son Effect, but also -ivatlv diminishes the deflection due to the 

 '■'■"'t evolved on account of the electrical resistance of the car- 

 Don. If the two strips of carbon were exactly the same in all 

 t j i, '>!" physical properties, and the contacts with the faces of 

 topile were the same on each side, the latter deflections 

 would evidentlv be entirely eliminated. 



