LARGE DOUBLE-CURRENT VOLTA-PARADIC APPARATUS. 253 



and to do so wastes a considerable amount of time. Upon such 

 considerations is founded the usefulness of the commutator of the 

 coils. 



C. — Graduation of the induced currents. 



Although, as I have shown, a very powerful apparatus is neces- 

 sary for the treatment of certain diseases, yet sucli an apparatus 

 would be unfitted for physiological and pathological investigations, 

 and would eveu become dangerous, unless we were able to appor- 

 tion electricity to every single organ in accordance with the degree 

 of excitability that it may possess. 



We owe to Dr. Eognetta a method of graduation whieli be 

 applied to the electro-dynamic apparatus of MM. Breton, freres, 

 and which permits us to control exactly the dose of electric excita- 

 tion. It is effected by pushing in or drawing out the central coil 

 within the external one, so as to produce induction in a greater or 

 less number of the turns of the latter. I had adopted this method 

 of graduation for my first volta-electric apparatus (fig. 51), pre- 

 sented to the Academy of Sciences in 1847, with tlie single differ- 

 ence that I made the external reel movable, while the internal one 

 was fixed to receive the system of the trembler.^ I soon discovered, 

 however, that this method of graduation was not free from incon- 

 veniences ; for the movable coil necessitated the employment of 

 movable connections, which were frequently out of order, which 

 complicated the manufacture of the instrument, and either dimin- 

 ished, by oxydation, the intensity of the currents, or even inter- 

 cepted their passage. 



A fortunate accident led me to discover a means of graduation 

 much more simple, and which I applied to my instruments with- 

 out delay, instead of the method by the mutual influence of the 

 coils. I made this important discovery in the following manner : — 



At the beginning of my investigations, I had covered a powerful 

 coil with copper in order to protect the wire. To my great sur- 

 prise, I found that the coil had lost nearly all its power. I 

 observed also, that, in removing the covering, the more the coil 

 was exposed the more its current increased. The fact was a ray 

 of light ; for, from that moment, the method of graduation by the 

 metallic tube was discovered. I found, after some experiments, 

 that the last half of the tube measured the intensity of the current 

 as exactly as the first ; and that the current increased or dimin- 

 ished, as the tube was replaced or withdrawn, in exactly arithme- 

 tical proportion. 



* M. Dubois-Eeymond, of Berlin, has adopted the latter system for the graduation 

 of his induction apjiaratus. 



