of Magnetic and No7i'magnetic Bodies. 431 



by the continuous current more energetic than the disconti- 

 nuous one, then exerts a preponderating influence. 



With respect to the difference in the nature of the sound, 

 according as the continuous current passes in the same direc- 

 tion as the discontinuous one, or in a contrary direction, I 

 can only explain it by admitting that the influences of the two 

 magnetisms, opposed and unequal in intensity, diminish the 

 amplitude of the oscillations which the particles of the wire 

 traversed by the discontinuous current make around their 

 natural position of equilibrium, — an amplitude which, on the 

 contrary, is more considerable when the two magnetisms act 

 in the same direction. 



I placed a tube of soft iron between the two helices; an 

 increase in the intensity of the sound took place when the two 

 currents proceeded in the same direction, — a diminution and 

 even a complete annihilation when they went in a contrary 

 direction. If the soft iron tube is longer than the internal 

 helix, its presence completely intercepts the influence of the 

 external helix traversed by the continuous current : this is not 

 the case if the tube is slit lengthwise ; its presence then in no de- 

 gree modifies thephaenomena. On placing the tube, or a massive 

 cylinder of soft iron in the axis of the internal helix, and no 

 longer between the two helices, the sound is perceptibly in- 

 creased, especially when the continuous current traverses the 

 internal helix, and the discontinuous one the external helix; 

 the increase takes place even when there is only a discontinu- 

 ous current. In all cases the nature of the sound does not 

 change ; its intensity alone is modified. 



A tube of copper or of any other metal produces no effect 

 when placed in the axis of the internal helix. This is not the 

 case if it is between the two helices ; it then causes a new 

 sound: this sound is evidently attributable to a current of in- 

 duction which circulates around the copper tube ; for if this 

 tube is slit in its whole length, there is no longer any sound, 

 even when, to increase the magnetic force of the helix, a cy- 

 linder of soft iron is placed in its axis. 



It is not necessary that the conductors traversed by a dis- 

 continuous current should have the form of wires or that of a 

 rod, to give out a sound under the influence of a magnet or a 

 continuous current. A brass disc, 12 centimetres in dia- 

 meter and 1 millimetre in thickness, was placed horizontally 

 in the centre of a helix, by means of a vertical metallic support 

 which was fixed in the centre of the disc. Care was taken to 

 avoid any contact between the sides of the disc and the in- 

 ternal surface of the helix ; the discontinuous current was 

 directed from the centre to the circumference, or from the 



