On the Sounds produced hy the Electric Current. 423 



of the helix, but that in all cases the longitudinal one sub- 

 sists; and that this force, acting in the direction of the axis, 

 exists equally with a transmitted current. It must produce a 

 longitudinal sound, whether it tends to lengthen or abruptly 

 to shorten the bar ; the transversal sound can only arise from 

 the external current, and in an excentric position of the bar. 

 Passing then to the examination of the sounds themselves, 

 M, Wertheim finds their explanation in the mechanical ac- 

 tions which we have just indicated; he does not therefore 

 think that either the magnetization or the transmission of the 

 electric current produces vibrations of a particular kind; but 

 he supposes that the mechanical actions which they engender 

 determine accidentally these longitudinal or transversal vibra- 

 tions, as any other cause might do. He admits, however, 

 that there is in some cases a dull noise {bruit sec), like a kind 

 of crepitation, which is propagated at the moment when the 

 current traverses an iron bar or wire ; and he concludes by 

 remarking, that there are still in this class of phaenomena 

 many obscure points, especially that which relates to the 

 manner in which a current traversing the iron produces in it 

 a mechanical shock. 



M. Wertheim's new observations, of which I have thus 

 given a summary, have induced me to resume my experiments, 

 and to investigate more closely the curious phasnomena in 

 question. 



It appears to me evident now, that the point which I have 

 sought to establish in my first memoir of 1845*, namely that 

 the magnetization or the passage of the electric current pro- 

 duces a molecular derangement, is no longer disputed. I 

 admit, on the other hand, that I have perhaps attached too 

 much importance to the nature of the sounds produced, and 

 to the influence of certain causes, such as tension, which oc- 

 casions them to vary. It may be, in fact, that the wire, once 

 set in vibration by either the external or transmitted cur- 

 rent, a simple friction against any metallic piece next to the 

 monochord may suffice, if not completely, at least in many 

 cases, to account for the variety of the remarkable sounds 

 which are heard, especially when well-annealed iron wires are 

 usedt. 



My attention has therefore been especially directed, in 



* Coviptes Rendus des Stances de I' Academic des Sciences, vol. xx. p. 1287j 

 and Archives de rElectricite, vol. v. p. 200. 



^ The Coviptes Rendus de V Academic des Sciences, vol. xxvii. p. 457, 

 contains tiie extract of a memoir by M. Duhamel on the multiple sounds 

 of bodies, which it appears to me may be well applied to the study of the 

 sounds produced in the iron wires by simple molecular movements. See 

 Phil. Mag., vol. xxxiv. p. 415. 



