332 M. De la Rive's Researches on the Voltaic Arc. 



when it is in its natural state. Finally, by modifying, through 

 the agency of heat, the molecular arrangement of some metals, 

 such as platinum and brass, I had succeeded in obtaining from 

 them, during the passage of the interrupted current, sounds, 

 which, though feeble, were yet distinct. 



The preceding reflexions tended to confirm me in my 

 opinion, that sounds produced under die influence of mag- 

 netism in the experiments on the voltaic arc, are owing to a 

 molecular modification effected by the action of the magnet, 

 and the more so inasmuch as the voltaic arc may be regarded 

 as produced by a succession of interrupted currents, following 

 each other with extreme rapidity, rather than by a perfectly 

 continuous current. I accordingly took bars of other metals 

 besides iron, as of tin, zinc, lead, bismuth, &c. I placed them 

 on the poles of the electro-magnet, and 1 caused an interrupted 

 current from a Grove's battery of from five to ten pairs to 

 traverse them. They emitted no sound as long as the electro- 

 magnet was not magnetized ; but as soon as it was, sounds were 

 very distinctly heard, composed of a series of strokes corre- 

 sponding to the interruptions of the current, and analogous to 

 that produced by a toothed wheel. The bars were eighteen 

 inches long, and from nine to ten lines square. Rods of 

 copper, platinum, and silver produced a similar effect ; a rod 

 of iron did not emit a much louder sound under the influence 

 of the magnet than it did when not exposed to this action. 



What appeared to me most remarkable was, to find lead, a 

 body so little elastic, yield a sound as powerful as those pro- 

 ceeding from the other metals, when placed under the same 

 circumstances. The position of metallic bars with respect to 

 the poles of the electro-magnet did not in any way modify the 

 result of the experiment ; they might be placed axially, that 

 is to say, in the direction of the poles, or equatorially, that is, 

 across the poles; the effect remained the same, being merely 

 weakened as the distance between the bar and the poles in- 

 creased. In order to hear the sound distinctly, when not very 

 powerful, it was sufficient to establish a communication be- 

 tween the metallic bar and the ear by means of a wooden rod. 

 In this manner the sound was not unfrequently heard prolonged 

 some seconds, though growing constantly feebler, until it ceased 

 entirely, after the source of magnetism had been withdrawn 

 from the electro-magnet. Mr. Faraday has remarked an 

 analogous fact in the action of the transparent medium on the 

 polarized ray, an action which does not cease immediately 

 with the magnetism of the electro-magnet. Is this prolonga- 

 tion owing to the magnetization of the electro-magnet not 

 ceasing in a sudden manner ; or to its return to its primitive 



