on various Phcenomena of Induction. 281 



53. It remained to examine whether the presence of a mag- 

 net or of a conductor traversed by a current always induces 

 electricity in a neighbouring conductor, placed in a convenient 

 manner ? To this question the reply must be negative, 



54. We know that if a natural or artificial magnet, or an 

 electro-dynamic helix, is brought near to an electro-magnet 

 of soft iron, the latter, on taking the magnetic power, produces 

 an instantaneous current of induction in its helix. But when 

 the approach is made slowly, the developed current loses its 

 intensity, and by increasing or diminishing the distance of the 

 soft iron from the magnet by very slow degrees, we succeed 

 in completely destroying all sensible induction. 



55. The first experiments were made with a horse-shoe 

 iron magnet, capable of supporting eight kilogrammes; it was 

 suspended by a metallic chain connected with the arbor of a 

 lathe. An electro-magnet was fixed solidly on the table under 

 the magnet, and its helix fastened to the thermo-electric rheo- 

 meter. On turning the winch very slowly, I succeeded in 

 bringing the magnet to within 0^^*004 distance of the electro- 

 magnet without any induction in the helix; but from that 

 point the attraction of the magnets having brought them in 

 contact from the extensibility of the chain, the needle was vio- 

 lently deflected. 



BQ. I then used a magnet formed of seven horse-shoe iron 

 plates, capable of raising nearly forty kilogrammes. The 

 electro-magnet was no other than one of the keepers of the 

 magneto-electric machine, of which the magnet formed part. 

 On employing the same rheometer, I succeeded in bringing 

 to the point of contact, and of separating to a distance of se- 

 veral millimetres, the two pieces to///?ow^ any sensible induction 

 resulting. I operated by means of a screw which moved very 

 slowly the slide upon which the magnet was solidly retained. 

 The needle of the rheometer deviated several turns as soon as 

 I rapidly caused a variation, although of a small arc, in the 

 position of the keeper parallel to the polar surfaces of the 

 magnet, at any of the distances at which the experiment had 

 been made. 



57. I arrived at the same result on employing the voltaic 

 current as the cause of induction. I placed a cylindrical 

 copper ring in some water containing y^^th of sulphuric acid, 

 so that its geometrical axis was vertical. This ring was con- 

 nected with one of the extremities of the wire of the rheometer, 

 and to the other extremity I soldered a very thin plate of 

 amalgamated zinc, cut into the form of a very slender tongue. 

 This plate, being immersed in the direction of the axis of the 

 ring, produced an induced current very nearly null when the 



