OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
89 
36. When, therefore, a disc of any substance is put into a state of rotation 
under a suspended magnetic bar, opposite poles are induced in that part of the 
disc immediately under the bar ; these induced poles may be supposed to pass on, 
and being retained for a short portion of time, will, if the motion be more rapid 
than the time during which the impression remains, be transferred toward the 
opposite poles of the magnet, and exert upon them a repulsive action up to 
that point of distance (35) at which the poles of the magnet again reverse the 
transient poles impressed on the disc, and substitute opposite poles, to be 
again reversed as before. 
3 7. Thus if NsSn (fig. 12) be a metallic disc revolving in the direction 
N s S n under a magnetic bar N S free to move upon a central point C, and 
of which N is the north pole, and £ the south, the effect of this rotation will 
be, to impress upon the semicircle Ns S a south polarity, and upon the semi- 
circle S n N a north polarity, in consequence of the points sss and n n n having 
passed under the poles N and S. Now if the time required to restore the ori- 
ginal magnetic distribution of the plate, be less than that necessary to disturb 
it, these impressed polarities remain for a small portion of time, and hence 
there will always be an attractive force in advance of the poles of the bar, 
and a repulsive force in arrear of them ; consequently the bar becomes driven, 
as well as dragged in the direction of the revolving disc by the resolved portion 
of the oblique actions acting for the most part near the extremities. And there 
is little doubt, that any substance in the least degree susceptible of a transient 
magnetic state, might cause a magnet to rotate ; provided that the rapidity of 
the motion be greater than the time necessary for the restoration of the original 
magnetic distribution of the body acted on ; supposing such rapidity of rota- 
tion possible. 
38. A rotating disc, therefore, circumstanced as above (37), may be consi- 
dered as a circular magnet such as that already mentioned (5), one of the 
semicircles having a north polarity, and the opposite semicircle a south pola- 
rity ; and which polarities eventually neutralise each other about one of the 
diameters ; the only difference being, that the magnetism of the revolving disc 
is transient, and constantly changing its position ; so that the neutral points 
are always near the poles of the suspended bar. The bar therefore by a well- 
MDCCCXXXI. 
N 
