Prof. E. Wartmann's Sixth Memoir on Induction. 277 



171. I afterwards operated on metallic plates placed near a 

 magnetic apparatus, the induction of which could make itself 

 felt successively in all their sections. With this view I pro- 

 cured three discs, one of tempered steel, another of untem- 

 pered steel, and the third of soft iron, m, 198 in diameter and 

 m- 0018 in thickness: they were fixed in turn on a glass bar, 

 kept vertical by a convenient support. The magnet, which is 

 provided with a vertical steel rod situated in the prolongation 

 of the geometrical axis of the bar, and parallel to the branches 

 of the horseshoe, is arranged above the plate, and at a distance 

 variable at will. This rod is terminated by an endless screw 

 in which a toothed- wheel catches, which is worked by a winch. 

 One of the extremities of the wire of the electro-magnet is 

 fastened to the rod, whilst the other is soldered to a copper 

 ring which surrounds an isolating wooden disc carried by 

 the axis. This simple arrangement permits the current of 

 Grove's battery to pass into this wire without interruption, 

 whatever be the velocity of rotation given to the magnet. It 

 suffices for this to employ as voltaic poles two copper springs, 

 one of which presses against the steel axis, and the other 

 against the ring. 



172. The three discs, being of different elasticity, gave a 

 different fundamental sound, notwithstanding the equality of 

 their dimensions. The electro-magnet exerted on them, at 

 a distance of two to six millimetres, so energetic an attraction, 

 that it was necessary to recur to an enormous pressure against 

 their support to overcome it entirely. 



173. After having sprinkled the plate under observation 

 with very dry sand, and determined the corresponding sounds 

 by different modes of vibration indicated by acoustic figures, 

 I endeavoured to ascertain whether the magnetic influence is 

 capable of modifying these sounds. Here, again, all the 

 results were negative. The only difference of action of the 

 magnet, according as it was immovable or turned with any 

 rapidity, was limited to a slight irregularity of the nodal lines, 

 solely produced by the currents of air. 



174. When iron-filings were substituted for the sand, the 

 sonorous phaenomena were still the same, although more sup- 

 pressed. The filings spread in the vicinity of the poles accu- 

 mulated against them as soon as the electric current was 

 established, and soon rendered the rotatory motion of the 

 magnet almost impossible. 



175. These experiments were repeated, rendering the pas- 

 sage of the electricity discontinuous. The molecular shocks 

 investigated by several physicists were then heard. With a 

 mercurial commutator in the circuit of the battery exciting 



