ON ELECTRO-DYNAMIC INDUCTION. 309 



25. It must be recollected that all the experiments with these coils 

 and helices, unless otherwise mentioned, are made without the reaction 

 of iron temporarily magnetized ; since the introduction of this would, 

 in some cases, interfere with the action, and render the results more 

 complex. 



SECTION II. 



Conditions which injiuence the production of Secondary Currents. 



26. The secondary currents, as it is well known, were discovered in 

 the induction of magnetism and electricity, by Dr Faraday, in 1831. 

 But he was at that time urged to the exploration of new, and appa- 

 rently richer veins of science, and left this branch to be traced by 

 others. Since then, however, attention has been almost exclusively 

 directed to one part of the subject, namely, the induction from mag- 

 netism, and the perfection of the magneto-electrical machine. And I 

 know of no attempts, except my own, to review and extend the purely 

 electrical part of Dr Faraday's admirable discovery. 



27. The energetic action of the flat coil, in producing the induction 

 of a current on itself, led me to conclude that it would also be the 

 most proper means for the exhibition and study of the phenomena of 

 the secondary galvanic currents. 



28. For this purpose coil No. 1 was arranged to receive the current 

 from the small battery, and coil No. 2 placed on this, with a plate of 

 glass interposed to insure perfect insulation ; as often as the circuit of 



Fig. 3. No. 1 was interrupted, a 



^^^""^ — 'Sisj «? powerful secondary current 



/ ^ — fssa ^ was induced in No. 2. The 



^^ ,-— — ^--,^— _-x^ J arrangement is the same as 



^___^_j%^^^^^^^^_____,.y''''^ that exhibited in Fig. 3, with 



'^^^^''^*""^^^^^^^^;:::iS2i!a* theexception that in this the 



a represents coil No. l,i helix No. 1, and c,d, handles for COmpOUud hcHx is rcpre- 

 receiving the shock. . , . . , . 



sented as receivmg the m- 

 duction, instead of coil No. 2. 



29. When the ends of the second coil were rubbed together, a spark 

 was produced at the opening. When the same ends were joined by 



