EFFECTS OF ROTATION OF MAGNETS. 
69 
fig. 5, which represents in its simplest form the well-known 
apparatus for generating an electric current from the relative 
motion of a magnet and a portion of a circuit; or 
(conversely) for developing a movement of rotation by Fig. 5. 
means of a current introduced into the circuit. It con- 
sists simply of a magnet and a wire, w, the wire being 
so arranged as to be capable of rotating about the 
magnet, with its ends maintained (by convenient means) 
in slipping contact with the polar and equatorial parts 
of the magnet. By this arrangement of the wire, what- 
ever its shape or length, it cuts (as is known) by one 
revolution all the lines of force of the magnet. Here a current 
may be generated either by rotating the wire about the magnet, 
or by rotating the magnet on its axis, in both which cases the 
relative motion of the wire and magnet is the same , and there- 
fore the cause of the current the same. It was here supposed, 
however, in accordance with the hypothesis adopted, that the 
cause of the current was different in the two cases. When the 
wire rotated about the magnet, it was admitted that the current 
was due to the intersection of the external lines of force of the 
magnet by the wire. When, however, the magnet rotated on 
its axis, the current was supposed to be due to a different cause. 
It was here supposed (according to the hypothesis adopted) that 
the rotating magnet intersected its own system of force, becoming 
charged up statically at the polar and equatorial parts, and that 
the wire, w, led off this charge — that this , therefore, was the 
cause of the current in the wire w. This, therefore, it may be 
observed, makes a difference in the mode of action in two cases 
where the relative motion is the same. This, however, need 
not be done, for regarding the system of force about the magnet 
as accompanying the magnet whatever the nature of its motion 
( i.e . the system of force as invariably at rest relatively to the 
magnet) , it follows that when the magnet rotates on its axis, 
its system of force accompanies it and intersects the wire w, 
thereby generating a current in the wire, precisely as it did 
when (conversely) the wire, w, was made to intersect the system 
of force (by the rotation of the wire about the magnet). Thus 
the effects in both cases become correlated under one cause, viz., 
the relative motion of the wire w, and the system of force of 
the magnet. The singular distinction drawn when the magnet 
rotates on its axis is therefore shown not to be necessitated by 
the experimental facts. 
6. Taking now the reverse case, when the current of a battery 
is applied either to cause the external wire, w, to rotate about 
the magnet, or to make the magnet rotate on its axis, the 
battery current being made to traverse the circuit made up of the 
magnet and external wire, w. Here, also, evidently the same con- 
