252 HISTORY OF GALVANISM. 



in formulae of remarkable simplicity and generality. In this Memoii, 

 Professor Wheatstone describes an instrument which he terms Rheostat^ 

 because it brine* to a common standard the voltaic currents which arc 



o 



compared by it. He generalizes the language of the subject by em- 

 ploying the term rheomotor for any apparatus which originates an 

 electric current (whether voltaic or thermoelectric, &c.) and rheometer 

 for any instrument to measure the force of such a current. It appears 

 that the idea of constructing an instrument of the nature of the 

 Rheostat had occurred also to Prof. Jacobi, of St. Petersburg.] 



The galvanometer led to the discovery of another class of cases in 

 which the electrodynamical action was called into play, namely, those 

 in which a circuit, composed of two metals only, became electro-mag- 

 netic by heating one part of it. This discovery of thermo-electricity 

 was made by Professor Seebeck of Berlin, in 1822, and prosecuted by 

 various persons; especially by Prof. Gumming 5 of Cambridge, who, 

 early in 1823, extended the examination of this property to most of 

 the nietals, and determined their thermo-electric order. But as these 

 investigations exhibited no new mechanical effects of electromotive 

 forces, they do not now further concern us ; and we pass on, at present, 

 to a case in which such forces act in a manner different from any of 

 those already described. 



DISCOVERY OF DIAMAGNETISM. 



[2nd Ed.] [By the discoveries just related, a cylindrical spiral of 

 wire through which an electric current is passing is identified with a 

 magnet ; and the effect of such a spiral is increased by placing in it a 

 core of soft iron. By the use of such a combination under the influence 

 of a voltaic battery, magnets are constructed far more. powerful than 

 those which depend upon the permanent magnetism of iron. The 

 electro-magnet employed by Dr. Faraday in some of his experiments 

 would sustain a hundred-weight at either end. 



By the use of such magnets Dr. Faraday discovered that, besides 

 iron, nickel and cobalt, which possess magnetism in a high degree, many 

 bodies are magnetic in a slight degree. And he made the further very 

 important discovery, that of those substances which are not mag- 

 netic, many, perhaps all, possess an opposite property, in virtue of 

 which he terms them diamagnetic. The opposition is of this kind ; 



6 Camb. Trans, vol. ii. p. 62. On the Development of Electro-Magnetism bv Heat 



