the induced electric current
is the same in all, namely, from the circumference towards the centre.

* * * * *

231. The law under which the induced electric current excited in bodies
moving relatively to magnets, is made dependent on the intersection of the
magnetic curves by the metal (114.) being thus rendered more precise and
definite (217. 220. 224.), seem now even to apply to the cause in the first
section of the former paper (26.); and by rendering a perfect reason for
the effects produced, take away any for supposing that peculiar condition,
which I ventured to call the electro-tonic state (60.).

232. When an electrical current is passed through a wire, that wire is
surrounded at every part by magnetic curves, diminishing in intensity
according to their distance from the wire, and which in idea may be likened
to rings situated in planes perpendicular to the wire or rather to the
electric current within it. These curves, although different in form, are
perfectly analogous to those existing between two contrary magnetic poles
opposed to each other; and when a second wire, parallel to that which
carries the current, is made to approach the latter (18.), it passes
through magnetic curves exactly of the same kind as those it would
intersect when carried between opposite magnetic poles (109.) in one
direction; and as it recedes from the inducing wire, it cuts the curves
around it in the same manner that it would do those between the same poles
if moved in the other direction.

233. If the wire NP (fig. 40.) have an electric current passed through it
in the direction from P to N, then the dotted ring may represent a magnetic
curve round it, and it is in such a direction that if small magnetic
needles lie placed as tangents to it, they will become arranged as in the
figure, _n_ and _s_ indicating north and south ends (14. _note_.).

234. But if the current of electricity were made to cease for a while, and
magnetic poles were used instead to give direction to the needles, and make
them take the same position as when under the influence of the current,
then they must be arranged as at fig. 41; the marked and unmarked poles
_ab_ above the wire, being in opposite directions to those _a'b'_ below. In
such a position therefore the magnetic curves between the poles _ab_ and
_a'b'_ have the same general direction with the corresponding parts of the
ring magnetic curve surrounding the wire NP