134 Mr. 8. Tolver Preston on some Electromagnetic 
were revolved together (as one whole). Hence he seems to have 
reasoned that because the deflection remained the same, the 
conditions must have remained the same; and therefore that 
the revolving copper disk, even when attached to the magnet 
(and revolving with it), still continued to cut through the 
magnetic lines of force of the magnet, precisely as it would 
have done if the disk alone had revolved and the magnet been 
fixed. Hence Faraday concluded that the lines of force could 
not possibly partake of the rotatory motion of the magnet*; but 
that the magnet revolved “ amongst its own forces.” 
But I believe it will be apparent here, on inspection of the 
diagram of the apparatus, that the same deflection would be 
equally consistent with the opposite supposition (viz. that the 
lines of force partake of the rotatory motion of the magnet, 
in the same sense as they partake of the translatory motion 
of the magnet). For, admitting that the lines of force revolve 
with the magnet, then they will intersect the galvanometer-loop 
circuit when the magnet revolves on its axis; and this will 
evidently produce a current of the same direction and mag- 
nitude as under Faraday’s singular assumption, which he 
thought himself forced into by the experiment. This, I think, 
is clearly one of those rare cases of a deceptive double mean- 
ing to an experimental fact, which on account of their rare 
occurrence are unexpected, and may consequently sometimes 
escape the notice of even skilful experimenters. I venture to 
believe that few will doubt what the true explanation here is 
after the above elucidation ; and only in this way does the 
contradiction to Ampere’s theory (of the similarity of the 
properties of the helix and magnet) vanish, as also the great 
theoretic difficulty of drawing a distinction between trans- 
latory and rotatory motion in regard to the behaviour of the 
magnetic lines of force. 
I conclude, therefore, that the inferred inductive charge 
produced at the poles and equator of the revolving globe of 
the earth (as a rotating magnet), by “ revolving amongst its 
own forces,’ certainly does not necessarily followy from this 
* After a second perfectly similar experiment to this, with a cylindrical 
copper cap instead of a disk, Faraday adds ‘‘ Thus a singular independence 
of the magnetism and the bar in which it resides 1s rendered evident.” 
(Page 184.) 
Tt On the other hand, another quite different cause may conceivably 
conduce to an electric disturbance on the earth’s surface, or, rather, on 
parts of it which are not rigidly fixed. The tides, namely, form a circular 
band or ring of water through which the earth revolves. These great 
water elevations are not therefore jived (relatively to the revolving earth) 
as the globe of the earth itself is; and consequently the magnetic lines of 
force carried along with the revolving globe can intersect the tides, and 
