540 Dr. Silvanus P. Thompson on a 



Nous pretentions a ce cercle un fort aimant en fer a cheval, 

 de inaniere qu'un de ses poles se trouvait au dedans et l'autre 

 au dehors du cercle. Des que nous faisions communiquer 

 avec la pile les deux extremites du fil conducteur le cercle 

 etait attire ou repousse par l'aimant, suivant le pole qui 

 repondait a l'interieur du cercle : ce qui demontrait P existence 

 du courant electrique qui y etait produit par ^influence du 

 courant du fil conducteur. La decouverte que vient de faire 

 M. Faraday des courans electriques produits par Pinfluence 

 d'un aimant, courans qu'avait obtenu Fresnel en 1820, mais 

 qu'il n'avait pas era suffisamment constates par ses experiences, 

 rn'a porte naturellement a employer le galvanoscope multi- 

 plicateur, dont l'illustre chimiste anglais a fait usage, pour 

 constater de nouveau et etudier dans toutes ses circonstances 

 la production d'un courant electrique par un autre courant/' 



Ampere seems to have considered that the oxidation at first 

 alleged by Fresnel was caused by induction currents arising 

 from the diurnal variations of the magnetism of the stationary 

 magnet. But it is evident that the electromotive forces due 

 to any such cause would be of an order of magnitude utterly 

 inadequate for the purpose. Further, as such currents must 

 necessarily have been in alternate directions during the rise 

 and foil of temperature, the total chemical action at the end 

 would in no way depend upon the intermediate charges, but 

 only upon the difference between the initial and final tem- 

 peratures. Hence the total electromotive impulse tending to 

 chemical action must have been infinitesimal compared with 

 that due to simply removing the magnet once from the 

 circuit ! The third version differs from the former in describing 

 the position of the magnet, and saying that the magnet (which 

 in the original Ampere experiment was brought up after the 

 current had been turned on) was placed in position before the 

 circuit was completed. 



From that time forward Ampere's experiment seems to have 

 passed into oblivion. One reference to it, however, appeared 

 in 1872, which gives a fourth version, differing not only from 

 the three former, but accompanied by a picture and description 

 of a very different form of apparatus. This description, from 

 the pen of the usually accurate M. Verdet, is to be found at 

 p. 357 of his well-known Conferences de Physique. It runs as 

 follows : — 



" 209. Experience oV Ampere et De la Rive. — Longtemps 

 avant l'epoque ou Faraday fit la decouverte de ^induction, 

 Ampere et De la Eive * avaient fait une experience relative 



* Bibliotheque Universette, septembre 1822, et Annates de Chimie et de 

 Physique [2], t. xxi. p. 47 (1822), et t. xxv. p. 272 (1824). 



