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CHAPTER VII. 



CONSEQUENCES OF THE ELECTRODYNAMIC THEORY. 



IT is not necessary to state the various applica- 

 tions which were soon made of the electro- 

 magnetic discoveries. But we may notice one of 

 the most important, the Galvanometer, an instru- 

 ment which, by enabling the philosopher to detect 

 and to measure extremely minute electrodynamic 

 actions, gave an impulse to the subject similar to 

 that which it received from the invention of the 

 Leyden Phial, or the Voltaic Pile. The strength of 

 the voltaic current was measured, in this instru- 

 ment, by the deflection produced in a compass- 

 needle; and its sensibility was multiplied by making 

 the wire pass repeatedly above and below the 

 needle. Schweigger, of Halle, was one of the first 

 devisers of this apparatus. 



The substitution of electro-magnets, that is, of 

 spiral tubes composed of voltaic wires, for common 

 magnets, gave rise to a variety of curious apparatus 

 and speculations; but on these and other subjects 

 of the same kind I shall not dwell (E). 



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 cir- 

 cuit, composed of two metals only, became electro- 



