REQUISITES FOR ELECTRO-MEDICAL INSTRUMENTS. 239 



intermissions ; as when we endeavour to excite the membrana 

 tympani, in the treatment of deafness. 



To sura up, such considerations show abundantly that faradiza- 

 tion by rapid intermissions has numerous useful applications in 

 physiology and therapeutics ; but that in certain cases, when 

 applied to the excitation of muscles, it is attended by dangers 

 arising from the accidents or the deformities which it may occasion, 

 and that then it may be advantageously replaced by currents of 

 slow intermission. 



§ III. — In a great number of cases the apparatus used for faradiza- 

 tion cannot he too i^oiverfid. 



What should we understand by tlie power of an induction 

 apparatus? and how should this power be measured? 



An apparatus is powerful when it acts with energy upon the 

 muscular contractility, upon the sensibility of the skin, of the 

 muscles, of other subcutaneous organs, and of the retina ; and 

 when it is able to traverse a considerable thickness of tissue, so 

 that its recompositions may occur in deeply seated organs. 



We are now aware, from the facts and considerations already 

 set forth (Art. III., Cha}). I.) that the different actions of induction 

 currents that are applicable in practice are only found combined 

 in an apparatus of double induction, the coils of which are con- 

 structed in certain proportions, and that the energy of these 

 different actions, specially exerted, some by the first coil (the 

 extra current), the others by the second coil (the induced current), 

 are in direct ratio to the length of the wires. It therefore follows 

 tliat the power of an induction apparatus is necessarily propor- 

 tionate to its size and weight. Unfortunately \\e have as yet 

 discovered no means of producing a powerful induction by means 

 of a short length of copper wire. 



We may, however, be deceived by appearances if we judge of 

 the power of an apparatus by its size and weight alone, and without 

 actually testing it. And, for the latter purpose, the time has 

 passed by at which it was sufficient to take a rheophore in each 

 hand. It is necessary, if we wish to know the exact degree of 

 power exerted upon this or that function, to direct and limit one 

 or the other current of the apparatus upon its organ, and to 

 proceed with such experiments in the manner that I have already 

 laid down. 



I now proceed to prove, by a few examples, the truth of the 

 statement which forms the heading to this Section, that in certain 

 cases the induction apparatus cannot be too powerful. 



