Experiments of Faraday and Pliicker. 139 
the shell had been rotated in the opposite direction about the 
magnet (or, on my view, the charge depends on the relative * 
motion between the magnet and shell, and not on which 
revolves). When the shell revolves about the magnet, the - 
existence of the charge upon it is admitted, and is dependent 
on well-known principles ; but when the magnet revolves, the 
existence of the charge upon the shell would have been denied 
by Faraday—although the relative motion (between magnet and 
shell) is the same in both cases. For Faraday considered that 
“no mere rotation of a magnet on its axis produces any induc- 
tive effect on circuits exterior to it.” But Faraday thought 
that the rotation of a magnet on its axis could produce an 
inductive effect on itself, or that the magnet could become 
statically charged by “‘revolving amongst its own forces.” 
Although I contend that Faraday’s experiments do not prove 
this static charge, I think I can show how (from another cause) 
such a static charge could come on the magnet in the present 
special cuse (fig. 6) without revolution “amongst its own forces.” 
For it is apparent that if a static charge is produced on the shell 
by the axial revolution of the magnet (whose lines of force par- 
take of its motion and intersect the shell), then this charge 
will act by static electric induction—like a Leyden jar—across 
the layer of air, and call forth an opposite static charge (or 
distribution of electricity) on the corresponding parts of the 
magnet facing the shell. Unless this fact (deducible before- 
hand from the considerations here set forth) be kept in view, 
this peculiar static charge may mislead, and make one think 
that the magnet could here have become charged in no other 
possible way but by “ revolving amongst its own forces.”’ 
Since the first draft of this paper was written, I have 
observed that this question is touched upon in the very com- 
plete work of Prof. Wiedemann (Die Lehre vom Galvanismus), 
where there are also some references, viz. to Beer, Pogg. Ann. 
Bd. xciv. 8.177 (1855); Nobili, Pogg. Ann. Bd. xxvi. 8. 421 
(1833), &e. But on referring to these papers, I find that they 
practically leave the question open (or have done nothing to 
decide it), and I find no mention of the possible double (or 
ambiguous) meaning of Faraday’s fundamental experiment, 
upon which it may be said the whole question hinges. And 
* It is curious here to remark that, on Faraday’s view (that the “system 
of power about the magnet must not be considered as revolving with the 
magnet”), it would follow that the inductive effect on the shell produced 
by revolving it concentrically through the lines of force of the magnet 
would not be altered if the magnet itself were put in motion at the same 
rate as the shell (so that the two were in relative rest); or it would be 
necessary to conclude from this view of Faraday’s that the inductive 
effect between two bodies is the same by relative rest as by relative motion. 
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