EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
161 
137- The latter experiment is analogous to those made by Mr. Barlow with 
a rotating iron shell, subject to the influence of the earth*. The effects then 
obtained have been referred by Messrs. Babbage and Herschel to the same 
cause as that considered as influential in Arago’s experiment -f~; but it would 
be interesting to know how far the electric current which might be produced 
in the experiment would account for the deflexion of the needle. The mere 
inversion of a copper wire six or seven times near the poles of the magnet, 
and isochronously with the vibrations of the galvanometer needle connected 
with it, was sufficient to make the needle vibrate through an arc of 60° or 70 °. 
The rotation of a copper shell would perhaps decide the point, and might 
even throw light upon the more permanent, though somewhat analogous 
effects obtained by Mr. Christie. 
138. The remark which has already been made respecting iron (66.), and the 
independence of the ordinary magnetical phenomena of that substance, and 
the phenomena now described of magneto-electric induction in that and other 
metals, was fully confirmed by many results of the kind detailed in this sec- 
tion. When an iron plate similar to the copper one formerly described (101.) 
was passed between the magnetic poles, it gave a current of electricity like the 
copper plate, but decidedly of less power ; and in the experiments upon the in- 
duction of electric currents (9.), no difference in the kind of action between 
iron and other metals could be perceived. The power therefore of an iron 
plate to drag a magnet after it, or to intercept magnetic action, should be care- 
fully distinguished from the similar power of such metals as silver, copper, &c. 
&c. inasmuch as in the iron by far the greater part of the effect is due to what 
may be called ordinary magnetic action. There can be no doubt that the cause 
assigned by Messrs. Babbage and Herschel in explication of Arago’s phe- 
nomena is true when iron is the metal used. 
139. The very feeble powers which were found by those philosophers to be- 
long to bismuth and antimony, when moving, of affecting the suspended magnet, 
and which has been confirmed by Mr. Harris, seem at first disproportionate 
to their conducting powers ; whether it be so or not must be decided by future 
experiment (73.) X- These metals are highly crystalline, and probably conduct 
* Philosophical Transactions, 1825. p. 317. f Ibid. 1825. p. 485. 
+ I have since been able to explain these differences, and prove, with several metals, that the effect is 
in the order of the conducting power ; for I have been able to obtain, by magneto-electric induction. 
MDCCCXXXII. 
Y 
