DOUBLE-CUERENT MAGNETO-FARADIC APPARATUS. 283 



very excitable organs, or for very delicate experiments, unless we 

 could employ a moderator of the currents, analogous to that of the 

 volta-faradic instruments. 



I have found in the apparatus itself the elements of an excellent 

 moderator. 



(a). The armature and its motor system can be moved away 

 from the magnet over a range of two centimetres and a half, by 

 means of the screw N, called the regulator of the armature, and, 

 at this distance, the influence of the soft iron is considerably 

 diminished. This simple moderator suffices for all operations 

 upon man, and, by combining its action with that of the graduator, 

 permits the administration of extremely feeble doses. 



(&). But there are researches or experiments still more delicate, 

 — such, for example, as may be practised upon the frog, — which 

 require the employment of doses that are still smaller, on account 

 of the degree of excitability of the organs of that animal. I have 

 contrived the following arrangement for bringing the electro- 

 magnetic apparatus infinitely more under control. By turning from 

 right to left the small screw placed over the base of the spring 

 8' (one of the springs which produce the intermissions of the 

 current of the central coil upon the teeth of the rheotome B), I 

 prevent the current of this coil from passing through the commu- 

 tator. It will be understood that induction can then only take 

 place by the influence of the soft iron upon the magnet. I do not 

 know whether this experiment has ever been made under the 

 same conditions ; but I can say that the induction of the magneto- 

 faradic apparatus, thus deprived of the simultaneous intermissions 

 occurring in the wire of the coil, which immediately covers the 

 magnet, is so feeble that the current of my powerful instrument is 

 only appreciable by placing the rheophores between the lips. If, 

 then, after having placed the soft iron at its greatest distance 

 from the magnet, we suspend, by the above described method, the 

 intermissions which occur through the rheotome B, the instrument 

 will produce only an induction infinitely feeble, which may still 

 be divided in degrees smaller and smaller, by sliding the graduator 

 tubes over the coils. It is thus that the same instrument, which 

 is capable of affording currents of great intensity, may be used to 

 test the relative excitability of the muscles of the frog. 



§ IV. — The manner of setting the instrument in action. 



The employment of the magneto-faradic apparatus is, fortunately, 

 as we shall see, a matter of extreme simplicity. Without know- 

 ledge of its construction or of its theory, it is enough for the 

 physician, in order to use it, to understand the brief precepts that 



