348 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and when the alternating voltage was applied are contained in 

 Table XI. 



From this table it appears that the thermoelectric voltage when the 

 junction is heated by heat conducted from above, in twelve out of the 

 thirteen cases tried, is opposite to the direct voltage obtained when an 

 alternating current is passed through the junction. When the heat is 

 conducted to the junction from below, through the molybdenite, the ther- 

 moelectromotive force in four cases is opposite to the rectified voltage, 

 and in nine cases is in the same direction as the rectified voltage. In 

 only one case, one point of No. 78, is the rectified voltage in the same 

 direction as the thermal voltage, both when the junction is heated from 

 above and when it is heated from below. 



In all of these cases the heat was applied in the neighborhood of the 

 same junction, and there is no opportunity for heat to get to the other 

 junction (copper-plated and soldered) by conduction, on account of the 

 great distance of the other j unction from the source of heat. To make 

 this absolutely certain this distant junction was in some cases submerged 

 in an oil bath. 



So far as I have been able to learn, this phenomenon of the reversal 

 of the thermoelectromotive force at a thermal junction, conditioned on 

 whether the heat is conducted to the junction through one element of 

 the junction or the other element of the junction, is novel. It may be 

 explained by the assumption of another thermal junction of opposite 

 sign in the molybdenite itself below and in the immediate neighborhood 

 of the copper-molybdenite junction. This assumption is plausible be- 

 cause it has been shown above that the molybdenite with which these 

 experiments are performed is thermoelectrically an extremely hetereo- 

 geneous substance. On the other hand the phenomenon may also be 

 explained on the theory that the thermoelectromotive force is deter- 

 mined by the direction of the flow of heat. 



Whatever the explanation of the dependence of the sign of the ther- 

 moelectromotive force on the manner of applying the heat, it is seen 

 that the thermoelectric effect is usually opposite in sign 17 to the recti- 

 fied effect. 



By applying heat from above and at the same time applying the al- 

 ternating voltage, one can make the thermal current and the rectified 

 current neutralize each other. This opposition of sign of the rectified 



17 In the case of silicon-steel, carbon-steel, and tellurium-aluminum, L. \V. Aus- 

 tin has found that the rectified current generally flows in opposite direction to 

 that produced by heating the junction. In his experiments (Bulletin of the Bu- 

 reau of Standards, 5, No. 1, August, 1908) the heat was applied by conduction from 

 above. 



