18 LOCALIZED ELECTRIZATION. 



ciples that may be found in any treatise upon physics), I think it 

 necessary to state certain general considerations, which must be 

 kept in mind by all who use this kind of electricity in electro- 

 physiological experiments, or in the science or practice of medicine. 

 Addressing myself to a medical public, generally but little con- 

 versant with the technicalities of treatises on physics, I shall 

 avoid the use of these technicalities ; and, in order to be intel- 

 ligible to all readers, shall express myself as much as possible in 

 ordinary terms. 



The different forms of induction apparatus derive their force, as 

 is well known, either from a galvanic pile, or from an artificial 

 magnet. The former are called electro-dynamic, the latter electro- 

 magnetic instruments. Both are composed of a wire of copper, 

 covered with silk, of variable length and thickness, rolled in tight 

 spirals so as to form a coil, in the centre of which is placed either 

 a bar of soft iron or a magnet. In most of the electro-dynamic 

 instruments a second copper wire, finer and longer than the first, 

 is rolled around it so as to form a second coil. The ordinary 

 electro-magnetic apparatus has only a single wire, rolled around 

 an electro-magnet, or around each arm of a magnet. 



It is also well known that, to obtain the action of an electro- 

 dynamic instrument with two coils, we complete the communica- 

 tion between the extremitic^s of a circuit formed by an electro-motor 

 and by the wire rolled round the central bar of soft iron (the wire 

 of the primary coil). At the instant when this circuit is completed, 

 a change is wrought in the electric state of the wire, and also of 

 the soft iron — which latter becomes for tlie time a magnet. If the 

 circuit be opened, a new electric and magnetic change is produced ; 

 the natural electricity of the wire returns to it's normal state, and 

 the iron loses its magnetism. It is only at the moments when 

 these changes are produced that the phenomena of induction are 

 manifested in the primary coil, by the material influence of the 

 spirals and of the temporary magnet upon the spirals themselves ; 

 so that no electro-physiological phenomena are observed in the 

 intervals. (The current produced under these conditions is called 

 the extra-current). If we place in the circuit a living contractile 

 organ, the muscle of a frog for example, at the moment when the 

 circuit is completed the muscle contracts, and then returns to its 

 state of repose. If, then, the circuit be re-opened the muscle again 

 contracts, but this contraction is much less energetic than the 

 former. The same electro-physiological phenomena are produced 

 by the current proceeding from the wire rolled around the first 

 one, and induced by it. And if, in place of a muscle, we put a 

 galvanometer in the circuit of wire, we see the needle deviate at 



