374 THERMO-ELECTRICITY. [SECT, xxxiv. 



It has already been observed, that three bodies are requi- 

 site to form a galvanic circuit, one of which must be fluid. 

 But, in 1822, Professor Seebeck, of Berlin, discovered that 

 electric currents may be produced by the partial application 

 of heat to a circuit formed of two solid conductors. For ex- 

 ample, when a semicircle of bismuth, joined to a semicircle 

 of antimony, so as to form a ring, is heated at one of the 

 junctions by a lamp, a current of electricity flows through 

 the circuit, from the antimony to the bismuth ; and such 

 thermo-electric currents produce all the electro-magnetic 

 effects. A compass needle, placed either within or without 

 the circuit, and at a small distance from it, is deflected from 

 its natural position, in a direction corresponding to the way 

 in which the electricity is flowing. If such a ring be sus- 

 pended so as to move easily in any direction, it will obey 

 the action of a magnet brought near it, and may even be made 

 to revolve. According to the researches of M. Seebeck, the 

 same substance, unequally heated, exhibits electrical cur- 

 rents ; and M. Nobili observed, that in all metals, except 

 zinc, iron, and antimony, the electricity flows from the hot 

 part towards that which is cold. That philosopher attributes 

 terrestrial magnetism to a difference in the action of heat on 

 the various substances of which the crust of the earth is 

 composed; and, in confirmation of his views, he has pro- 

 duced electrical currents by the contact of two pieces of 

 moist clay, of which one was hotter than the other. 



M. Becquerel constructed a thermo-electric battery of one 

 kind of metal, by which he has determined the relation be- 

 tween the heat employed and the intensity of the resulting 

 electricity. He found that, in most metals, the intensity of 

 the current increases with the heat to a certain limit, but 

 that this law extends much farther in metals that are diffi- 

 cult to fuse, and which do not rust. The experiments of 

 Professor Gumming show that the mutual action of a magnet 

 and a thermo-electric current is subject to the same laws as 

 those of magnets and galvanic currents ; consequently all 



