VOLTAIC ELECTRICITY. 619 



of the motion to these lines. Ampere, on the other hand, supposed 

 the magnet to be made up of transverse electric currents (chap, vi.) ; 

 and had deduced all the facts of electro-dynamical action, with great 

 felicity, from this conception. The question naturally arose, in what 

 manner, on this view, were the new facts of magneto-electric induction 

 by motion to be explained, or even expressed ? 



Various philosophers attempted to answer this question. Perhaps 

 the form in which the answer has obtained most general acceptance 

 is that in which it was put by Lenz, who discoursed on the subject to 

 the Academy of St. Petersburg in 1833. a His general rule is to this 

 effect : when a wire moves in the neighborhood of an electric current 

 or a magnet, a current takes place in it, such as, existing independently, 

 would have produced a motion opposite to the actual motion. Thus 

 two parallel forward currents move towards each other : hence if a 

 current move towards a parallel wire, it produces in it a backward 

 current. A moveable wire conducting a current downwards will move 

 round the north pole of a magnet in the direction N., W., S., E : 

 hence if, when . the wire have in it no current, we move it in the 

 direction N., W., S., B., we produce in the wire an upward current. 

 And thus, as M. de la Rive remarks, 3 in cases in which the mutual 

 action of two currents produces a limited motion, as attraction or 

 repulsion, or a deviation right or left, the corresponding magneto-elec- 

 tric induction produces an instantaneous current only ; but when the 

 electrodynamic action produces a continued motion, the corresponding 

 motion produces, by induction, a continued current. 



Looking at this mode of stating the law, it is impossible not to regard 

 this effect as a sort of reaction ; and accordingly, this view was at once 

 taken of it. Professor Ritchie said, in 1833, "The law is founded on 

 the universal principle that action and reaction are equal." Thus, if 

 voltaic electricity induce magnetism under certain arrangements, mag- 

 netism will, by similar arrangements, react on a conductor and induce 

 voltaic electricity. 4 



There are still other ways of looking at this matter. I have else- 

 where pointed out that where polar properties co-exist, they are gene- 



2 



Acad. Petrop. Nov. 29, 1833. Fogg. Ann. vol. xxxi. p. 483. 



3 Traite de I'JElectricite, vol. i. p. 441 (1854). 



4 On the Reduction of Mr. Faraday's discoveries in Magneto-electric Indue 

 km to a General Law. Trans, of R. S. in Phil. Mag. KS. vol. iii. 37, and vol 

 IT. p. 11. In the second edition of this history I used the like expressions 



