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. araday 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- 
nomena 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 resulis so decided, that there could be no room for dou t as 
to their character ; a secondary current of such intensity as to para- 
lyze the arms having been so neutralized, by the interposition of a 
plate and riband of metal, as not to be perceptible through the tongue. 
I was led by alittle 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, founda 
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 5 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 
ef the one before; while it would appear from the arrangement of 
Dr. Faraday’s experiments, that the results detailed in his first serless 
ee scala hee 
* Since writing the account of the two kinds of induction, I have found that re 
ind, although not screened by a plate of copper or zine, is affected by os 
of iron. Inthe cases of the first kind of induction, iron ac 
