THERMO-ELECTRIC COUPLES' 

 A. W. Davison 



Seebeck discovered the phenomenon of thermo-electricity in 

 182L He arranged a series of the metals, the current proceeding 

 through the cold junction in the following order: Antimony to 

 iron, zinc, silver, gold, tin, lead, copper, platinum, bismuth. 

 He also determined that when three or more metals. A, B and 

 C, are in the series, E being the voltage generated, E^^c = -^'ab + 

 Ebc- In other words, the electromotive force remains the 

 same when a circuit is opened, and a third metal is inserted, pro- 

 vided the temperatures of its ends are the same. 



Gaugain found that for any couple, the relationship between 

 the electromotive force and the temperatures can be repre- 

 sented graphically. Kelvin proved these curves to be parabolas, 

 and since the equation for a parabola is ?/ = ax + hx^, in the case 

 under consideration, E = bt -\- ct^. 



Inversion. Since these curves are parabolas, a maximum 

 point, the neutral point, as A (fig. 1), will be reached. For tem- 

 peratures above this point, the electromotive force will decrease, 

 until U and ^2 are equidistant from, and on opposite sides of, the 

 neutral point, when the electromotive force is zero. 



If U is now increased, the electromotive force will be reversed, 

 and ''thermo-electro inversion" takes place, the current flowing 

 in the opposite direction. This was proved experimentally by 

 Cumming. Hence from the equation the electromotive force is 

 zero when ^i = ti, or when (^i + ^)/2 = -b/2c. 



Thermo-electric power. If we take a lead-metal (Pb — M) 

 couple, whose El = bt -\- ct^, and raise t to {t -{- dt), then El'^^^ 

 = b {t + dt) -\- c (t -{■ dty. Now the small electromotive force 

 generated, dE = El"^^^- El = b . dt + 2ct . dt. 



Thus the rate of increase of electromotive force with tempera- 

 ture dE/dt = b -\- 2ct, is called the thermo-electric power of the 



^ A thesis presented for the Fletcher O. Marsh Prize in Physics, Denison Uni- 

 versity, June, 1910. 



245 



