Experiments of Faraday and Pliicker. 135 
experiment. Whether there are other independent grounds 
for such an assumption, I am not aware, and Faraday does 
not state them. 
Tt will be seen clearly that Faraday’s view involves the 
strange assumption that although (as an admitted fact), when 
a copper disk is revolved parallel to and above the pole of a 
magnet, an inductive effect is produced on the disk [so that 
its centre and periphery become statically charged with elec- 
tricity of opposite sign |, yet when, conversely, the magnet is 
made to revolve with the same relative velocity below the fixed 
disk, no effect is produced; yet the relatwe motion is really 
the same in both cases. In fact, Faraday’s aspect of the case 
involves the supposition that the parts of a body at rest (rela- 
tively to the body itself, when it revolves) can produce an 
inductive effect on the body. 
The following experiment (Phil. Trans. 1832) may possess 
an interest, as it was considered by Faraday to confirm his 
view of the static charge produced at the equator and poles 
of a magnet by its own revolution, and which also led 
(Pliicker especially) to speculations over the cause of the 
aurora borealis (Pogg. Ann. 1852, p. 357). A cylindrical 
bar-magnet, m (fig. 3), was set by Faraday in axial rotation, 
and the galyanometer wires were maintained in sliding con- 
tact with the equator and one pole of the magnet respectively, 
as the dotted lines in the diagram indicate. A current was 
observed; and the cause of this current was referred (in the 
same way) to a static charge produced by the magnet cutting 
through its own (internal) lines of force, these lines, or the 
internal field of force, having been supposed to remain rigidly 
at rest, while the magnet revolved independently through it. 
But it will be seen that the current may be simply referred 
(in an analogous way to the previous example of the copper 
disk) to the lines of force moving with the revolving magnet, 
and intersecting the external circuit formed by the galvanometer 
wires. This explanation has the additional advantage of 
bringing the current formed, whether by the rotation of 
the magnet on its axis, or by the (inverse) revolution of the 
wire about the magnet, under the same cause (and not under 
different causes). 
so cause a disturbance of the electric equilibrium on the earth’s surface 
of the character imagined by Faraday, and assigned by him toa quite 
diferent cause. Also the trade winds, which, somewhat similarly, form 
a kind of air-belt through which the earth revolves, can conceivably con- 
duce in the same way to an electric disturbance, especially as these winds 
near the equator are laden with moisture, so as to become in some sense 
conductors. 
