12 JACOBI ON TIIK APPLICATION OF ELECTRO-MAGNETISM 



We are acquainted with the striking and jjcrfect analogy exist- 

 ing between magnets and electro-dynamic cylinders. There is a 

 very beautiful experiment performed by Dr. Roget, and which 

 might sei-ve as a confirmation to these arguments ; I relate it 

 in his own words : " It occun-ed to me, soon after hearing of 

 Ampere^s discovery of the attraction of electrical currents, that 

 it might be possible to render the attraction between the success- 

 ive turns of a heliacal coil very sensible, if the wires were suffi- 

 ciently flexible and elastic ; and, with the assistance of Mr. Fara- 

 day, this conjectm-e was put to the test of experiment, in the 

 laboratory of the Royal Institution. A slender hai-psichord 

 wire, bent into a helix, being placed in a \^oltaic current, instantly 

 shortened itself whenever the electrical cuiTent was sent through 

 it, but recovered its former cUmcnsions the moment the current 

 was intermitted." I hope soon to be able to communicate to 

 the Academy the result of the experiments I shall institute on 

 this subject, conjointly with its illustrious member M. Sti-uve, 

 who has promised me his assistance, these experiments requiring 

 the exactitude and delicacy of micrometrical observations. When 

 Me consider the electro-dynamic cylinder, its total effect increases 

 by the reciprocal attraction of separate coils, whose action be- 

 comes less oblique. The same is the case with soft iron, the 

 magnetism of which increases up to a certain limit, l)y the action 

 of contraction. Heat everj' where presents itself as the enemy of 

 magnetism ; perhaps it is because the two forces encounter each 

 other in opposed molecular actions. For the present I abstain 

 from following up these tUscussions, and all connected with 

 them. 



32. 



Although the remarkable effects occurring at the instant when 

 voltaic contact is completed or interrupted, beai* a striking ana- 

 logy to the actions of magneto-electric currents, there exists, 

 however, a marked difference. If a conducting wire, covered 

 with silk, be bent back on itself, so that the direction of the 

 current is opposed in the adjacent parts, (Faraday, 9th Series, 

 Art. 1096) there is neither discharge nor spark at the mo- 

 ment of the disjunction, the opposite currents of induction ar- 

 ranging themselves in equihbrium to annul these effects. If 

 we remove the adjacent parts, the discharge and sjjark take 

 place. This experiment made me think that it was also pos- 

 sible to destroy the effects of a magneto-electric current, by 



