the Thomson Effect. 445 



In Le Roux's table <r = 2; .-. 0^ = 2-50: cr and </, however, 

 are of opposite sign. Introducing nickel, Le Roux's table 

 becomes: — 



+ 



— 



Sb 64 



Fe 31 



Cd 31 



Bi 31 



Zn 11 



Arg 25 



Ag 6 



Pt 18 



Cu 2 



Ni 2-25 





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 

 carbon, the two ends of the carbon being maintained at con- 

 stant temperatures, and passing the electric current alternately 

 in 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 any accuracy, the rate at which the deflection increased. 



The method of Le Eoux was then tried, of using two strip3 

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

 This method not only doubles the deflection due to the Thom- 

 son Effect, but also greatly diminishes the deflection due to the 

 heat evolved on account of the electrical resistance of the car- 

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

 their physical properties, and the contacts with the faces of 

 the thermopile were the same on each side, the latter deflec- 

 tions would evidently be entirely eliminated. 



Two carpenter's pencils were split longitudinally, the lead 

 being left in one half of the wood. They were then tightly 

 bound, parallel, against each face of the thermopile, and insu- 

 lated from it by thin pieces of mica. Especial care was taken 

 to fasten the carbons firmly, so as to prevent any motion from 

 the passage of the current. The pencils were placed perpen- 

 dicularly, the lower ends in two vessels of mercury, surrounded 

 by melting ice; the upper ends were at the temperature of the 

 air. The upper ends were electrically connected; and the 

 wires from a battery of three Grove cells were placed in the 

 vessels of mercury. The thermopile was connected with a 

 reflecting galvanometer of six ohms resistance. 



