344 



ELEMENTARY LESSONS ON [CHAP. vm. 



bismuth-antimony pair united by a copper wire. Volta's 

 law (Art. 72) concerning the difference of potentials 

 due to contact would tell us that when all are at one 



temperature the dif- 

 ference of poten- 

 tials between bis- 

 muth and copper 

 in one direction 

 is equal to the sum 

 of the differences 

 between bismuth 

 and antimony, and 

 between antimony 

 and copper in the 



Fig. 142. 



other direction, and that hence there would be equilibrium 

 between the opposing and equal electromotive -forces. 

 But when a junction is heated this equilibrium no longer 

 exists and Volta's law ceases to be true. The new 

 electromotive-force set up at the heated junction is found 

 to obey the following laws : 



(i.) The thermo- electromotive -force zs, for the same 

 pair of metals^ proportional (even through con- 

 siderable ranges of temperature) to the excess of 

 temperature of the junction over the rest of the 

 circuit. 



(ii.) The total thermo-electromotive- force in a circuit 

 is the sum of all the separate thermo- electromotive- 

 forces at the varimis junctions. 



It follows from this law that the various metals can be 

 arranged, as Seebeck found, in a series, according to 

 their thermo-electric power, each one in the series being 

 thermo-electrically positive (as bismuth is to antimony) 

 toward one lower down. The following is the thermo- 

 electric series of metals, together with the differences 

 of potentials (in microvolts) which they exhibit with a 

 difference of temperature of iC, lead being regarded as 

 the standard zero metal. 



