HISTORY, &c., OF INDUCTION INSTRUMENTS. 311 



It has been seen, however, that I have applied the system of 

 Page to my magneto-faradic instrument (with the difference, that 

 instead of the magnet, it is the soft iron that I have made movable), 

 but that I have em})loyed it only as a moderator of the currents (see 

 E, p. 282), that is to say that its purpose is to convert a very power- 

 ful magneto-electric apparatus into one that is more or less feeble, 

 the stronger or the weakest doses of which I measure by metallic 

 graduated cylinders (see D, p. 282). This system of graduation 

 by cylinders is only applicable to coils rolled around the magnet. 



2. — Electro-Dynamic Insteuments. 



A. — Double Induction. — The differential electro-physiological ac- 

 tion of the currents of the primary coil (extra current), and of the 

 secondary coil (induced current), is incomparably more marked in 

 the volta-electric than in the magneto-electric instruments. More- 

 over, we obtain from the former, in small bulk, a much greater 

 power. These are the chief reasons which render the volta-electric 

 instruments the most useful for medical purposes, and which have 

 gained for them the preference in practice, and especially in the 

 Paris hospitals, where they are more common than magneto-elec- 

 tric instruments. 



When I commenced my researches (towards 1840), the electro- 

 dynamic instruments had usually only an extra current ; and those 

 which possessed double induction, like the apparatus of Rognetta, 

 constructed by MM. Breton /reres, used only the induced current. 

 But, since I have shown that the extra current and the induced 

 current exert different actions (which compelled me to write that 

 they possessed different physiological properties, an expression so 

 much and so unfairly criticized by the MM. Becquerel), the makers 

 have endeavoured to construct instruments which should yield one 

 or the other current, at pleasure, according to the indication to be 

 fulfilled. 



It is necessary to know that, in order to obtain an extra current 

 of sufficient intensity to be useful, the wire of the coil must be of 

 sufficient length. I have not found in any electro-dynamic ap- 

 paratus, even the most bulky, a sufficient differential action 

 between the currents of tlie two coils, which evidently depends 

 upon a fault of construction (a defect in proportion between the 

 length and the diameter of the wire) ; in some instruments, I have 

 even found no difference — which is to be explained by a fraud that 

 I proceed to describe. These instruments have but a single coil, 

 composed of a thick wire (about half a millimetre in diameter), of 

 sufficient length, and continued by a wire that is relatively very 

 fine (from a sixth or a seventh of a millimetre in diameter). Two 



