Experiments of Faraday and Pliicker. 133 
its own plane, the inductive effect depends only on the relative 
motion between the circular current and neighbouring con- 
ducting bodies, and does not depend on whether the circular 
current or the conducting bodies move. When, for example, a 
piece of wire ab (PI.II. fig.1) is placed near a revolving solenoid 
or simple circular current, s s, it is quite indifferent, as regards 
the inductive effect, whether the circular current revolves 
about its centre (or axis), or whether, conversely, the wire ab 
is made to revolve about the fixed circular current, in the 
direction of the dotted line, so long as the relative motion 
between the two is the same. I only suppose that in reference 
to inductive effects with currents we have to do with relative, 
and not at all with absolute motions. If that is true, and also 
if the magnet is to resemble the solenoid in its chief properties 
(according to the well-known theory of Ampére), then I 
contend that a magnet in the above case must behave in prin- 
ciple as a solenoid or circular current. Faraday, however 
(and with him Pliicker), holds the opposite. Faraday ex- 
presses the view that it is not indifferent whether the magnet 
or the external conductor moves, that the effect is not dependent 
on relative motion ; in short, that “ no mere rotation of a bar- 
magnet on its axis produces any inductive effect on circuits 
exterior to it. The system of power about the magnet must 
not be considered as revolving with the magnet”’ (loc. cit. 
supra). 
I will now consider the experiment whence Faraday derived 
the above singular (or exceptional) view in regard to a rotating 
magnet. A copper disk & (fig. 2) was cemented upon the end 
of a round bar-magnet m, with insulating paper intervening 
—or, in the words of Faraday :— 
“The magnet and disk were rotated together, and collectors 
(attached to the galvanometer) brought in contact with the 
circumference and the central part of the copper plate. [In 
the diagram, the dotted line shows this outer circuit, with the 
galvanometer g.| The galvanometer-needle moved as in former 
cases, and the direction of motion was the same as that which 
would have resulted if the copper only had revolved and 
the magnet been fixed. Neither was there any apparent 
difference in the quantity of deflection. Hence rotating the 
magnet causes no difference in the results, for a rotating and 
a stationary magnet produce the same effect upon the moving 
copper.” (Phil. Trans. 1832, page 183.) 
Here Faraday observed (beforehand) a certain deflection of 
the galvanometer when the copper disk was made alone to 
revolve through the lines of force of the fixed magnet. He 
observed the same deflection when the magnet and copper disk 
