128 



ELECTRO-MAGNETISM. 



live, is, according to Gumming bismuth, mercury, platinum, tin, lead, gold, 

 copper, silver, zinc, iron, antimony. When heat is applied to the junction of 

 any pair of these, the current passes from that higher in the list to that lower. 

 Thermo-electric batteries have been made by a combination of pairs in series. 

 Baron Fourier ma*de a hexagon of three pairs of bismuth and antimony : by 

 heating with a lamp or cooling with ice three junctions, he obtained increased 

 effects ; by heating and cooling the alternate junctions at the same time, he in- 

 creased the effect. From experiments by Oersted, " it appears that the thermo- 

 electric current produces a prodigious quantity of electricity, but in a state of 

 very feeble intensity, while the Voltaic current has a very great intensity ;" so 

 that short elements are most advantageous. M. Pouillet found that if the elec- 

 tro-motive power of a constant Voltaic pair were 95, that of a thermo-pair of 

 bismuth and antimony would be 1. Mr. Wheatstone, by his admirable appli- 

 cation of Ohm's law, found the proportion 1 : 94'6. 



