Voltaic Induction. 37 
currents along the magnet, when the latter, without a plate or coil, 
was made to revolve upon its axis either in the open air or while 
immersed up to the middle in mercury. Although he does not give 
the particulars as to construction, | am inclined to suppose, that 
in the case where the magnet alone revolved on its axis, a metallic 
communication existed between the magnet and its fixed supports, 
also probably metallic. If such was his arrangement, there is no. 
doubt that one kind of current was generated in the north support, 
and the opposite in the south one, and that both these currents 
passed into the magnet to neutralize each other. It does not 
seem even necessary to suppose that the circuit became com- 
plete by the union of these forces at their opposite extremities out- 
side of the magnet, but it is probable that a metallic communication 
enabled them to do so. It is equally apparent, that when the mag- 
net was made to revolve in mercury, the currents originated in this 
metal as well as in the support at the other pole, and that the magnet 
became the line of communication for the opposite currents. Viewed 
in any other light than that furnished by the hypothesis proposed, I 
am inclined to believe that these peculiar cases of concentric rotation 
would appear paradoxical. The deduction seems to establish the 
rule, that when a flat coil or plate is made to revolve, centrally, upon 
any point around a magnetic pole, the tendency will be to form cur- 
rents, moving to or from the centre of motion, in the line of radius. 
It will be intelligible, from what has already been said, why cur- 
rents cannot be generated, by any kind of motion, when the same 
side of a flat coil or plate is exposed to the simultaneous action of op- 
posite magnetic poles, for counter currents must ensue in the same 
portion of metal. Directly the reverse of this will follow, however, 
if we expose opposite sides to the opposite forces. When a coil or 
plate is introduced edgewise between the poles of a horse-shoe 
magnet, it obtains this position, and must necessarily be exposed to 
forces, which, from their proximity, gain a power of action, by mu- 
tual induction, far superior to that of any others situated elsewhere. 
Accordingly this interpolar position is the most favorable for genera- 
ting voltaic currents. It has its disadvantages, however, for-only a 
portion of the coil can be operated upon atonce. If we exceed this, 
the motion being the same, counter currents will arise. When we 
introduce the coil between the poles, so that, its centre lies in the 
plane of the magnetic axis, about one half will be in action; and the 
removal of it, in the same plane, will produce an opposite current to 
