CONSEQUENCES OF THE ELECTRODYNAMIC THEOEY. 25 J 



Df the most important, the Galvanometer, an instrument whica, 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 instrument, 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, some of which I shall hereafter mention. 



[2nd Ed.] [When a voltaic apparatus is in action, there may be 

 conceived to be a current of electricity running through its various 

 elements, as stated in the text. The force of this current in various 

 parts of the circuit has been made the subject of mathematical inves- 

 tigation by M. Ohm. 1 The problem is in every respect similar to that 

 of the flow of heat through a body, and taken generally, leads to com- 

 plex calculations of the same kind. But Dr. Ohm, by limiting the 

 problem in the first place by conditions which the usual nature and 

 form of voltaic apparatus suggest, has been able to give great simplicity 

 to his reasonings. These conditions are, the linear form of the con- 

 ductors (wires) and the steadiness of the electric state. For this part 

 of the problem Dr. Ohm's reasonings are as simple and as demonstra- 

 tive as the elementary propositions of Mechanics. The formulae for 

 the electric force of a voltaic current to which he is led have been 

 experimentally verified by others, especially Fechner,' Gauss, 3 Lenz, 

 Jacobi, Poggendorf, and Pouillet. 



Among ourselves, Mr. Wheatstone has confirmed and applied the 

 views of M. Ohm, in a Memoir 4 On New Instruments and Processes 

 for determining the Constants of a Voltaic Circuit. He there remarks 

 lhat the clear ideas of electromotive forces and resistances, substituted 

 by Ohm for the vague notions of quantity and intensity which have 

 long been prevalent, give satisfactory explanations of the most impor- 

 tant difficulties, and express the laws of a vast number of phenomena 



1 Die Galvanische Kette Mathematiscli bearbeitet von Dr. G. S. Ohm, Berlin, 

 1827. 



9 Masa-bestimmengen itber die Galvanische Kette. Leipzig, 1831. 

 3 Remits of the Magnetic Association. * Phil. Trans. 1843. Pt. 11. 



