Prof. Henry’s Contributions to Electricity and Magnetism, 551 
tion has already given rise to so many errors ; further, the resistance of oppo- 
sition, an element hitherto little attended to in England*, which is not merely in 
itself of importance, but to which especial regard must be had in the discussion 
of the two views respecting the origin of voltaic electricity, as it is considerably 
affected by the chemical action. Very generally the heightening of the inten- 
sity of the current by direct chemical attack on the one or other metal of the 
circuit is merely due to the diminution of the resistance of transition, and is not 
a result of the increase of the electromotive force. But it must be proved with 
respect to this force, that it is in direct ratio to the energy of the chemical action 
on one of the metals (or to the difference of the actions on both metals) of the 
circuit, if the chemical theory is to be regarded as founded. It would, however, 
then be requisite to continue to separate and quantitatively to determine the in- 
dividual elements which have any influence on the intensity of the current, in the 
same way as it has been done by Fechner in his work, Maas hestimmungen 
ilher die Galvanische Kette (Leipzic, 1831, 260 pages in quarto). I am con- 
vinced that the English physicists, to whose zeal and ability we are already in- 
debted for so many interesting facts in the field of galvanism, would add con- 
siderably to the extension of our scientific knowledge of this branch of physics, 
and would consider Fechner’s memoir on the Contact-theoryf in a different 
point of view were they more intimately acquainted with that work, based on 
Ohm’s theory, and illustrating and extending it. 
POGGENDORFF, 
* By which, among other things, the recent, important, and interesting dis- 
covery of Mr. Roberts (See L. & E. Phil. Mag. for Feb.) finds its explanation. 
f Besides the article on the Contact-theory, translated in the Phil. Mag., 
might be mentioned two others in Poggendorff’s Annalen, vol. xlv. p. 432, and 
vol. xlix. p. 433. M^hat Schoenbein has brought forward against it does not 
appear to me to have any weight, and indicates his non-acquaintance with 
Ohm’s theory. 
LX XXIV. Contributions to Electricity and Magnetism, No. Ill, 
On Electro-dynamic Induction. By Joseph Henry, LL.D.^ 
Prof, of Natural Philosophy in the College of New Jersey^ 
Princeton. 
[Continued from p. 265, and concluded.] 
Section VI. — The production of induced Currents of the dif- 
ferent Orders from ordinary Electricity. 
98. FARADAY, in the Ninth Series of his Researches, 
remarks, that “ the effect produced at the commence- 
ment and the end of a current (which are separated by an in- 
terval of time when that current is supplied from a voltaic 
apparatus) must occur at the same moment when a common 
electrical discharge is passed through a long wdre. Whether 
if it happen accurately at the same moment they would entirely 
neutralize each other, or whether they would not still give some 
definite peculiarity to the discharge, is a matter remaining to 
be examined.” 
99. The discovery of the fact, that the secondary current, 
which exists but for a moment, could induce another current 
