EFFECTS OF ROTATION OF MAGNETS. 
67 
is rotated about another axis (fig. 4) ; i.e. that the system 
of force about the magnet obeys one law when the magnet is 
rotated about one axis, but a different law when it is rotated 
about an axis perpendicular to the former; which is evi- 
dently quite untenable. Indeed it must be perfectly clear that 
we have here simply a case of relative motion, or the inductive 
effect is the same as if (conversely) the ring were made to rotate 
about the fixed magnet, when, as a known fact , the ring would 
be charged up in the manner indicated. To assume that a 
magnet when rotating on its axis produces no inductive effect, 
would be to assume that when the magnet rotates inside the 
ring, there is no effect on the ring, but when (conversely) the 
ring rotates about the magnet, there is an effect on the ring, 
i.e. that the result is different while the relative motion is the 
same , or that the result depends on the absolute motion of the 
ring. Now it is an admitted characteristic of electro-dynamic 
phenomena to depend on relative and not on absolute motion* 
Moreover, to suppose that it made no difference whether the 
magnet was rotated or not, would involve the assumption that 
when magnet and ring rotate together with the same velocity, 
and therefore there is relative rest , the effect is the same as 
when the ring only rotates, or there is relative motion. These 
are therefore some of the inconsistent conclusions to which the 
above hypothesis necessarily leads, and which thereby prove the 
hypothesis to be untenable. 
V. From the hypothesis that the system of force of a rotating 
magnet does not partake of the motion of the magnet, it fol- 
lowed as a necessary conclusion that when a magnet rotated on 
its axis, it revolved through its own system of force , producing 
an inductive effect on itself, charging itself statically at the 
polar and equatorial parts with electricity of opposite sign. Thus 
the earth being a magnet, it was considered that the polar and 
equatorial parts of the earth must be charged up with electricity 
of opposite sign, by its rotation on its axis, and the surmise was 
thrown out (merely as a dependent suggestion) that the electric 
phenomenon of the Aurora Borealis might have some connection 
with this (“Phil. Trans.” 1832, p. 177). The same conclusion 
was also arrived at by M. Pliicker and published in “Poggendorfif’s 
Annalen” (1852, p. 357) in a paper — “ Ueber die Reciprocity 
der elektromagnetischen und magnetoelektrischen Erscheinun- 
gen.” M. Pliicker describes a number of experiments with 
currents in support of his view that a magnet charges itself up 
by its own rotation ; but since all his experiments in principle 
resemble the first one described (with the rotating disk), and 
since he has overlooked precisely the same fact as that pointed 
out (the same oversight affecting all his experiments alike), it 
will be unnecessary to refer to these experiments here. 
