INDUCED ELECTIIICITY. 29 



vention of magnetic force ; that is, by producing induction by the 

 initial voltaic current alone, and waiting to test afterwards the in- 

 fluence of temporary magnetism as an initial force also producing 

 induction. 



a. — In my electro-dynamic instruments ivith double current, the 

 difference in diameter and length of the two tvires which ^jro- 

 duced the extra current {current of the first helix) and the current 

 of the second helix, did not alone explain the different action of 

 those currents on the sensihility. 



Experiment. — I constructed two helices, nine centimetres in length, of 

 copper wire, covered with silk. In one the wire was one millimetre in 

 diameter, and 200 metres in length ; in the other a sixth of a millimetre 

 in diameter, and 1000 metres in length. In the centre of the helices was 

 a space, intended to receive a bar of soft iron or a bundle of iron wire, but 

 left empty during the i^resent experiment. 



In this state, the current of a pair of Bunscn's elements, circulating with 

 intermissions through each of the separate helices, gave origin to two instan- 

 taneous currents of unequal power. That which was developed in the thicker 

 and stouter wire (the extra current of authors) exerted a physiological action 

 infinitely greater than that of the fine wire. 



I then equalised the physiological force of the two currents, by placing the 

 moist rheophores one in each hand, or both in the same hand, or on the plane 

 of the muscles, and by then causing the current of the thick wire to pass 

 through a tube filled with distilled water of such a length that the shocks 

 produced were equal to those of the current of the fine wire. 



If, after thus equalising the physiological power of the two currents, their 

 action on the cutaneous sensibihty was compared, it was found that the fine 

 and long wire did not excite this sensibility more acutely than the thick and 

 short. 



It follows from this experiment that an extra current developed 

 in a helix of very long and slender ^ire (1000 metres in length, 

 and one-sixth of a millimetre in diameter) does not excite the 

 cutaneous sensibility more actively than an extra current developed 

 in a wire much shorter and thicker (200 metres in length, and a 

 little less than a millimetre in diameter). 



It is also evident that, if the action of a current ujDon the 

 cutaneous sensibility bore any direct proportion to the degree 

 of tension, the extra current developed in a iine wire should act upon 

 the skin much more powerfully than that developed in a wire 

 thicker and shorter. Tliis does not require to be demonstrated. 



b. — The fine and long wire of my electro-dynamic apparatus of 

 double current only acquires its special action upon the cutaneous 

 sensibility when its current is induced by a thicker wire. 



If the same helix of fine wire, that was used in the foregoing 

 experiment, was exposed to the inductive influence of the helix 

 of thicker wire, the current circulating in its coils instantly acquired 

 the special property by which the sensitiveness of the skin is 



