86 
MR. HARRIS ON THE TRANSIENT MAGNETIC STATE 
rotating- metallic disc on a freely suspended magnet ; whilst at the same time 
they are in accordance with the opinion of M. Arago, that almost every known 
substance can exert an influence on the compass-needle. Every species of 
matter therefore may be considered to be more or less susceptible of a state 
of transient magnetic energy, arising from induction. 
32. The hypothesis advanced by Mr. Herschel and Mr. Babbage in expla- 
nation of some of these phenomena is extremely simple, and to a great extent 
very satisfactory ; inasmuch as it agrees with all the observed operations of 
magnetic induction, and, as stated by them, supersedes the necessity of ad- 
vancing any new hypothesis in magnetism ; it is nevertheless considered by 
M. Arago as insufficient*. This celebrated philosopher appears to deny, that 
the limit of the motion produced by any force, which can be supposed to re- 
side in the induced poles, even if their existence be admitted, can exceed a 
minute of a degree : whereas, in order to explain the rotation, it should ex- 
ceed 90°. It may be observed however, that a ring of metal, copper for ex- 
ample, immediately surrounding the poles of the needle, can, as first stated by 
M. Arago, diminish the amplitude of its oscillations, and, as seen in the fore- 
going experiments, so fetter its motion, as to reduce the vibrations in a given 
space from 420 to 1 4. We must therefore necessarily conclude the force in- 
duced in each consecutive point of the ring to be very considerable. With 
non-metallic bodies, the force is certainly very much less ; but these do not 
fetter the motion of the needle to anything like so great an extent, nor will 
these substances rotate, as far as I can find, by the influence of a magnet 
revolving in an exhausted receiver without sensible vibration, notwithstand- 
ing the rapidity of the rotation exceeds 700 revolutions in a minute. 
33. After considering with much attention the hypothesis just alluded to, I 
am led to offer a few additional observations, with a view of extending the 
principle it involves, and which may possibly be useful in further elucidation 
of the perceived effects ; whilst, at the same time, they will not be found in 
any way inconsistent with the known laws of magnetic action. 
The facility with which a magnetic development is induced in bodies, may 
be designated by the term “ susceptibility” ; the time during which this effect 
of induction remains in them “ retentive power”. These terms have been 
* Edin. Philos. Journal, vol. v. p. 326. 
