428 Prof. De la Rive on the Vibratory Movements 



perceptibly weakening it in the steel wires and rods ; lastly, it 

 causes it completely to disappear in the iron tube through 

 which the conducting copper wire covered with silk passes. 

 These effects vary, moreover, with the absolute intensity of 

 the currents employed j and they are easily interpreted on the 

 principles which we have stated. We must, moreover, not 

 lose sight of the fact, that when the continuous and discon- 

 tinuous currents are of equal force, they destroy one another 

 whenever they pass together; so that the discontinuous one 

 no longer acts, and the continuous one becomes intermittent 

 in its action. 



Lastly, permanent magnetization materially modifies the 

 sound which the passage of the discontinuous current causes 

 a soft iron wire or rod to emit. In order to guard, in this 

 experiment, against the mechanical effects due to magnetiza- 

 tion, I placed a soft iron rod on the two poles of an electro- 

 magnet, taking care by means of an interposed sheet of paper 

 to avoid metallic contact between the poles and the rod. 

 I then placed upon it considerable weights, that its position 

 might not be modified by the magnetization, and passed the 

 discontinuous current through the rod ; it gave out a succes- 

 sion of dry metallic sounds, which became much more in- 

 tense and grave at the moment when 1 magnetized the elec- 

 tro-magnet. It is evident that this modification and this 

 strengthening of the sound are caused by the contest which is 

 established between the longitudinal position which the influ- 

 ence of the magnetization impresses on the particles of the 

 soft iron, and the transversal position which the passage of 

 the current tends to give them ; the oscillations of the particles 

 must necessarily have more amplitude, since they take place 

 between these extreme portions. The effect of permanent 

 magnetization, although still perceptible, is less marked with 

 steel rods, and especially with those of tempered steel. 



§ II. Investigation of the vibratory movements 'which ?ion-mag- 

 netic bodies experience under the influence of external and 

 transmitted electric currents. 



I stated in a preceding memoir, that rods, even of a tolerable 

 size, of different non-magnetic metals emitted a distinct sound 

 when, after having placed them under and very near an electro- 

 magnet or in the interior of the axis of a helix, a discontinuous 

 current was made to pass through them. The sound only be- 

 came perceptible, whatever was the force of the transmitted cur- 

 rent, at the instant when the wire of the electro-magnet or that 

 of the helix was traversed by a continuous current. I also ob- 

 served that the effect was still more marked when the metal was 



