534 Dr. Silvanus P. Thompson on a 



law of force by dividing up the times, and therefore the corre- 

 sponding spaces decribed, into elements so small that the 

 force might be supposed to vary uniformly with the distance 

 along OX in any single element. This is not put forward as 

 a satisfactory proof, but then the object of this note was to 

 show, not how to prove the theorem, but how to convince 

 oneself of its truth after proving it by highly analytical 

 methods. This I found hard in the case of systems like that 

 of Case I., where a slight variation in the initial conditions 

 (viz. the difference of velocity Bv of the points M'K') causes 

 two such systems to separate indefinitely. I could not see 

 how this was compatible with the multiple differential (8x . 8v) 

 remaining constant till I had worked out the above explana- 

 tion. It is a pity that systems with more than one degree of 

 freedom could not be treated by this graphic method, but a 

 similar objection applies equally to the graphic proof of the 

 formula for uniformly accelerated motion of our text-books 

 and to many other valuable illustrations of the principles of 

 dynamics. 



LIII. Note on a Neglected Experiment of Ampere. 

 By Silvanus P. Thompson*. 



IN the year 1822 Ampere, being at Geneva, showed to 

 De la Rive in his laboratory a number of his now 

 classical experiments on the actions of conductors carrying 

 currents upon other conductors and upon magnets. These 

 are recounted by A. De la Rive in the Annates de Chimie et 

 de Physique, xxi. p. 24 (1822). 



Amongst these experiments there is one that has almost 

 been forgotten, but which, had it been followed up, would 

 assuredly have led Ampere to anticipate by some years the 

 great discovery of Faraday as to the induction of currents. 



The experiment was first described by Ampere in October 

 1821, in a letter f to Van Beck. It had been tried in the 

 preceding July. The following is Ampere's own first descrip- 

 tion of it: — 



" Oe que cette experience prouve directement, c'est que la 

 proximite d'un courant electrique n'en excite point, par 

 influence, dans un circuit metallique de cuivre, meme dans 

 les circonstances les plus favorables a cette influence. Yoici 



* Communicated by the Physical Society : read April 26, 1895. 



t Journal de Physique, t. xciii. p. 447, Oct. 1821. Reprinted at p. 212, 

 vol. ii. of the ' Collection de Memoires relatifs a la Physique, publics par 

 la Societe francaise de Physique,' 1885. 



