DOUBLE-CURRENT MAGNETO-FARADIC APPARATUS. 273 



develop the phenomena of induction in the thick wire of the 

 central coil. The theory of these phenomena being the same as 

 that of induction in all magneto-faradic instruments, it is unneces- 

 sary to describe it here. 



At the instant when this induction is produced in the thick wire 

 of the central coil, if union of the two ends of the fine wire of the 

 second coil is effected by a complete conductor, a counter current 

 will be developed in this fine wire, in a direction opposite to that 

 of the current of the primary coil. (The possibility of this induc- 

 tive action of the central upon the secondary coil, in electro-mag- 

 netic instruments, has been contested, on theoretical grounds, by 

 MM. Becquerel. I will hereafter show, by experiments, that it 

 takes place in my ajjparatus.) 



As one of the terminations of the wire of the primary coil and 

 of the secondary coil proceeds directly to P, the knob of the rheo- 

 phore, and as the other extremity of these wires communicates with 

 the metallic plate G, which supports the large wheel, it becomes 

 necessary to bring this plate into relation with P', the knob of the 

 second rheophore. The regulator of the intermissions, D, fulfils 

 this condition completely ; because it communicates with the knob 

 F by a copper wire, and it is brought in contact either with the 

 plate G, or with the pins a of the great wheel A, fixed to that plate, 

 by means of the to-and-fro movement effected by the knob D. 



If it be wished to pass rapid currents through the conducting 

 cord of the rheophore attached to the knob P, it is evident that 

 the knob D should bring the spring I into relation with the plate 

 G, which is itself in relation with the other extremity of one of 

 the two wires. But if the spring I only communicates with 

 the plate G by the pins a of the large wheel A, the current 

 will only reach the knob P' four times, or twice, or even once, 

 for each revolution of the large wheel. CDnsequently, the con- 

 ductors of the electrodes attached to the knobs P and P' receive 

 only one, or two, or four intermissions per revolution, when the 

 spring touches the pins; while they receive the rapid current 

 (thirty-two, or sixty-four intermissions per revolution) when the 

 spring touclies the metallic plate G. I therefore call the knob D, 

 which moves the spring, the regulator of the intermissions. 



The commutator of the coils acts in the same way as that of the 

 volta-faradic apparatus. 



For the theory of the graduation of the two currents, I must refer 

 to what has been said on the same subject, with regard to 

 the volta-faradic apparatus. I shall hereafter explain how to 

 administer the most powerful or the most feeble doses with my 

 magneto-electiic instrument. 



