produced in Metals by Electric Currents. 545 



amplitude of the vibrations is not the same for different parts 

 of the wire subjected similarly to the reel. M. de la Rive 

 found this by the comparison of the sounds obtained. These 

 phaenomena result from the attraction exerted upon the wire 

 by the parts of the coil which are the nearest to it : they cause 

 a distinct class of sounds. But there exists another cause of 

 vibrations in the wire, the effect of which is more or less in- 

 dependent of this lateral attraction. Longitudinal vibrations 

 are produced in it, with which correspond sounds of a pecu- 

 liar character. If the axis of the reel was identical with that 

 of the wire, supposing it exactly rectilinear and cylindrical, a 

 transverse deviation would no longer take place. But even 

 then, the molecules on which the electro-magnetic action is 

 exerted are attracted right and left of the centre of the axis of 

 the reel towards this central point, as a steel needle is seen to 

 be drawn into it as soon as it is introduced into the hollow 

 of the helix. It is this internal vibration which, by the dis- 

 continuity of the electric current, is rendered periodical in 

 two opposite directions, that determines the second class of 

 sound. 



Let us now pass to the case of the current transmitted by 

 the wire. In order to study it, I substituted for the mirror the 

 spherical and perfectly polished bulb of a small mercurial ther- 

 mometer. The optical axis of the telescope, passing through 

 the intersection of the crossed wires, was directed on the bril- 

 liant image of a luminous point reflected very obliquely at the 

 upper part of the convexity. This arrangement discovers any 

 change in form of the wire, even in the direction of its length. 

 I was not able to perceive any elongation of the wire under 

 the electric action, although it gave a very distinct sound. I 

 attribute the principal cause of this sound to the polary ar- 

 rangement which the molecules undergo in order to give pass- 

 age to the electricity. This arrangement is manifest in many 

 cases, and I have elsewhere pointed out a very great number 

 of them*. It is the result of a struggle between the molecu- 

 lar forces which constituted the primitive state of equilibrium 

 of the body and the new activity which the dynamical con- 

 dition of the fluid excites. If the flow of the latter is conti- 

 nuous, this struggle is instantaneous, and the noise which it 

 occasions is null or nearly so ; but it recommences with each 

 closing of the circuit if the flow is periodical. 



It is already known from the experiments of M. Peltier f, 



* Memoir on the Electric Diathermansie of Voltaic Pairs : Archives dc 

 P Electricitc, vol. i. page 74. 



t Comptes Rendus des Seances de V Acad, des Sciences de Paris, Jan. 6th, 

 1845. 



