404 Miscellanies. 



Oct. 18. — The following extract from a letter, addressed by Prof. 

 Henry, of Princeton, to Prof. Bache, was read, announcing the discov- 

 ery of two distinct kinds of dynamic induction, by a galvanic current. 



" Since the publication of my last paper, I have received through 

 the kindness of Dr. Faraday, a copy of his fourteenth series of exper- 

 imental researches ; and in this I was surprised to find a statement 

 directly in opposition to one of the principal results given in my paper. 

 It is stated in substance, in the 59th paragraph of my last communica- 

 tion to the American Philosophical Society, that when a plate of met- 

 al is interposed between a galvanic current and a conductor, the sec- 

 ondary shock is neutralized. Dr. Faraday finds, on the contrary, un- 

 der apparently the same circumstances, that no effect is produced by 

 the interposition of the metal. As the fact mentioned forms a very 

 important part of my paper, and is connected with nearly all the phe- 

 no'iieiia described subsequently to it, I was anxious to investigate the 

 cause of the discrepancy between the results obtained by Dr. Faraday 

 and those found by myself. My experiments were on such a scale, 

 and the results so decided, that there could be no ro.)m for do.il t as 

 to their character ; a secondary current of such intensity as to para- 

 lyze the arms havin j be< n so neutralized, by the interposition of a 

 plate and riband of metal, as not to be perceptible through the tongue. 

 I was led by a little reflection to conclude that there might exist a case 

 of induction similar to that of magnetism, in which no neutralization 

 would take place ; and I thought it possible that Dr. Faraday's results 

 might have been derived from this. I have now, however, found a 

 solution to the difficulty in the remarkable fact, that an electrical cur- 

 rent from a galvanic battery exerts two distinct kinds of dynamic 

 induction : one of these produces, by means of a helix of long wire, 

 intense secondary shocks at the moment of breaking the contact, and 

 feeble shocks at the moment of making the contact. This kind of 

 induction is capable, also, of being neutralized by the interposition of 

 a plate of metal between the two conductors. The other kind of in- 

 duction is produced at the same time from the same arrangement, and 

 does not give shocks, but affects the needle of the galvanometer ; it 

 is of equal energy at the moment of making contact, and of breaking 

 contact, and is not affected by the introduction of a plate of copper 

 or zinc between the conductors.* The phenomena produced by the 

 first kind of induction form the subject of my last paper, as well as that 

 of the one before ; while it would appear from the arrangement of 

 Dr. Faraday's experiments, that the results detailed in his first series, 



* Since writing the account of the two kinds of induction, I have found that the 

 second kind, although not screened by a plate of copper or zinc, is affected by the 

 introduction of a plate of iron. In the cases of the first kind of induction, iron acts 

 as any other metal. 



