288 



LOCALIZED ELECTRIZATION. 



the direction of the rotation, indicates the liberation of a constant 

 current in the disc. But this current is of very small intensity ; 

 it is incapable of producing chemical decompositions, contrac- 

 tions, &c. It is the same with the induction currents, which 

 are occasioned by the terrestrial magnetism.^ 



B.— Instrument of PmV.— « To obtain induced currents of 

 somewhat marked intensity, it is necessary that they should be 



developed in wires of 



some length, in such a 

 way that the conductor 

 which reunites the ends 

 of the wire should be 

 moderately good, or at 

 least not much inferior 

 to the wire itself, so that 

 the current may traverse 

 it, instead of passing 

 back again by the same 

 wire in which the in- 

 duction took place. M. 

 Pixii was the first to 

 construct an instrument 

 on this principle." ^ 



" The instrument of 

 Pixii is shown in fig. QS. 

 A B is a bar of soft iron, 

 curved in to tlie form of 

 a horseshoe, and round 

 it is rolled an insu- 

 lated copper M'ire, the 

 extremities of which are 

 shown at X, Y ; this electro-magnet is fixed. Opposite to it is 

 placed a permanent magnet, the poles of which are seen at C 

 and D. This magnet is supported by a vertical axis, M N, on 

 which is mounted a pinion, I, to which a wheel, K, communi- 

 cates movement ; a handle, L, serves to put the instrument in 

 motion. 



" As will be seen, the poles C and D of the permanent magnet 

 alternately approach and recede from the extremities of the 

 electro-magnet ; there is, then, induced, in the wire covering the 

 latter, a current which changes its direction with each half turn. 

 In the original instrument of Pixii a sort of cam was placed at G, 



Fig. 6S. — Apparatus of Pixii. 



De la Rive, Traite d'dedriciU tMorique et appliqu^e, 1854, torn. 

 Ibid., loc. cit., p. 79. 



p. 378. 



