EFFECTS OF ROTATION OF MAGNETS. 
63 
external conductors is precisely the same as if these conductors 
were themselves translated with the same relative velocity past 
the stationary magnet, or the effect simply depends on relative 
motion. It would therefore seem a necessary inference that the 
effect by the rotation of a magnet also depended on relative 
motion, or that the inductive effect on any external conductor 
would be the same whether the conductor were moved past the 
periphery of the magnet, or the periphery of the magnet were 
(conversely) moved with the same relative velocity past the con- 
ductor (by the rotation of the magnet on its axis). 
3. Faraday, however, appears to have considered that experi- 
mental results opposed this inference, for he concluded (as stated 
in the above quotation) that while the rotation of the conductor 
about the magnet produced an inductive effect on the conductor, 
the same relative motion of the magnet to the conductor (when 
the magnet rotates on its axis) produced no inductive effect on 
the conductor. The experiment on which this conclusion was 
grounded was as follows (“Phil. Trans.” 1832, p. 183): — A 
circular disk of copper d (fig. 1) was rotated on its axis above 
the pole of a cylindrical bar magnet m. A wire w 
(in whose circuit a galvanometer — not shown — was 
placed) had one end maintained in sliding contact with 
the edge of the disk, and the other with the centre of 
the disk. It was found that the current given off by the 
rotation of the disk, and passing through the wire w , 
was precisely the same whether the mag-net in addition 
was rotated on its axis in either direction, or remained 
stationary. Hence it was concluded that the rotation of the 
magnet on its axis produced "no inductive effect on the disk, or 
(generally) that a rotating magnet could produce “ no inductive 
effect on conductors exterior to it.” Faraday adds, after a second 
perfectly similar experiment to the above with a cylindrical copper 
cap instead of a disk (p. 184) — “Thus a singular independence 
of the magnetism and the bar in which it resides is rendered 
evident.” In other words, it was inferred that the system of 
force about the magnet could not be regarded as partaking of 
the motion of the magnet and crossing external conductors, 
when the magnet rotated on its axis. It will be observed that 
this necessarily involves the conclusion that, although the system 
of force emanating from the periphery of a magnet admittedly 
shares the motion (so as to cross external conductors) when the 
periphery is moved in the act of translation of the magnet, yet 
that it does not do so when the periphery is moved in the act of 
rotation of the magnet on its axis. It is difficult to see how 
this conclusion could be theoretically consistent, for what dis- 
tinction can be logically drawn between a motion of rotation 
and one of translation in this case, as by the rotation of a mag- 
Fig. 1. 
_ w 
< 2 — =» 
TK 
