d-SO Prof. De la Rive on the Vibratory Movements 



extremities. If the ambient continuous current proceeds in a 

 contrary direction to the discontinuous one, the sound is 

 weaker than in the preceding case, but stronger than when 

 the exterior helix does not act; it produces likewise a differ- 

 ent impression : it resembles the noise of water on the boil, 

 whereas before it seemed to resemble that which a succession 

 of strong sparks would have occasioned. We must remark, 

 however, that if the discontinuous current is powerful enough 

 to cause of itself a sufficiently intense sound to be easily heard 

 at some feet distant, the influence of an external continuous 

 current, when it passes in a contrary direction, diminishes the 

 intensity of the sound. It always increases it, and in all cases, 

 when the two currents pass in the same direction. 



The continuous current may also be transmitted through 

 the internal helix, and the discontinuous one through the 

 external helix, contrary to what took place in the preceding 

 experiments. In this case, if the two currents pass in the 

 same direction, the sound resembles a succession of shocks; 

 and when they pass in a contrary direction, the noise is only 

 slightly increased, unless the discontinuous current is produced 

 by a weak battery, for example, by a couple of Grove's cells : 

 then the increase takes place also ; only the noise resembles 

 that of water on the boil. 



It is easy to understand how a helix traversed by a continu- 

 ous currrent, and placed externally or internally to the helix 

 traversed by the discontinuous current, increases the sound 

 which this latter emits. In fact, a permanent magnet is in 

 this case created, the action of which is added to that of the 

 temporary magnet which the discontinuous current creates 

 when it passes through the helix. Moreover, in passing the 

 continuous current through the wire of another helix than that 

 which transmits the discontinuous one, the molecules of the 

 conducting wire do not at first receive the position which 

 they take only by the effect of the passage of the disconti- 

 nuous current; this it is that causes them to oscillate freely 

 around their natural position of equilibrium. The same re- 

 sult could not \)e obtained when the two currents passed 

 through the wire of the same helix, as we have already re- 

 marked. 



When the currents pass in contrary directions in the two 

 helices, it is evident that the result is a diminution in the total 

 magnetism of the whole of the two helices placed in juxta- 

 position : this is the reason that the sound is not generally 

 increased ; and it is only so when the discontinuous current 

 being very weak, the magnetism of the helix which it traverses 

 is so likewise. The magnetism of the other helix, traversed 



