22 LOCALIZED ELECTEIZATION. 



With electro-magnetic instruments it is different. If the magnetic 

 curient be established by the juxtaposition of the soft iron with 

 the poles of the magnet, the interruption takes place only at the 

 moment when the iron is transverse to the poles. If the soft iron 

 slo\\ly describe the arc of a circle in order to assume this position, 

 the magnetic current established between the magnet and the iron 

 will diminish gradually until it ceases. 



But if the iron revolve very rapidly the passage of the current 

 from its maximum to zero takes place in a time so short as to be 

 nearly equivalent to a sudden interruption, as in the volta-electric 

 instruments, and to produce a powerful induction. We should 

 lience imagine that the inductive force of an electro-magnetic 

 instrument would be greater the more rapid the rotation of its 

 soft iron. It is found, however, that in these instruments the 

 power diminishes when a certain rapidity of intermission is ex- 

 ceeded ; as happens also in electro-dynamic instruments furnished 

 with Masson's wheel. This phenomenon, which has long perplexed 

 physicists, is produced by a mechanical cause, and dejjends on 

 a vice of construction that I will explain hereafter. 



Prior to my researches, the electro-magnetic instruments had 

 only a single helix. I caused to be constructed an apparatus of 

 double induction, in which the two helices, superimposed and fixed 

 upon a magnet possess, as I will show hereafter, differential pro- 

 perties like the helices of electro-dynamic instruments with double 

 current. The theory and description will be given in the fourth 

 chapter. 



The foregoing observations not only show that induced elec- 

 tricity cannot furnish a continuous current, being essentially 

 intermittent or instantaneous, but also that each intermission is 

 composed of two currents opposite in their direction. It has also 

 been shown that the current at the end of each intermission is the 

 only one that exerts a powerful physiological action. Lastly, 

 the ordinary electro-magnetic instruments have no action except 

 in rapid intermission ; while the electro-dynamic instruments 

 furnish intermissions either rapid or slow at pleasure. 



§ I. — Different physiological action of the current of tlie first helix, 

 and of the current of the second helix. 



The excitation of the skin by induced electricity, however long 

 continued and however intense, produces no organic action beyond 

 erection of the papillae and a little erythema. There are persons, 

 however, in whom the skin is liable to inflammation under the 

 influence of the slightest cause ; and in them the current of 



