carrying an electric current.

235. If the second wire _pn_ (fig. 40.) be now brought towards the
principal wire, carrying a current, it will cut an infinity of magnetic
curves, similar in direction to that figured, and consequently similar in
direction to those between the poles _ab_ of the magnets (fig. 41.), and it
will intersect these current curves in the same manner as it would the
magnet curves, if it passed from above between the poles downwards. Now,
such an intersection would, with the magnets, induce an electric current in
the wire from _p_ to _n_ (114.); and therefore as the curves are alike in
arrangement, the same effect ought to result from the intersection of the
magnetic curves dependent on the current in the wire NP; and such is the
case, for on approximation the induced current is in the opposite direction
to the principal current (19.).

236. If the wire _p'n'_ be carried up from below, it will pass in the
opposite direction between the magnetic poles; but then also the magnetic
poles themselves are reversed (fig. 41.), and the induced current is
therefore (114.) still in the same direction as before. It is also, for
equally sufficient and evident reasons, in the same direction, if produced
by the influence of the curves dependent upon the wire.

237. When the second wire is retained at rest in the vicinity the principal
wire, no current is induced through it, for it is intersecting no magnetic
curves. When it is removed from the principal wire, it intersects the
curves in the opposite direction to what it did before (235.); and a
current in the opposite direction is induced, which therefore corresponds
with the direction of the principal current (19.). The same effect would
take place if by inverting the direction of motion of the wire in passing
between either set of poles (fig. 41.), it were made to intersect the
curves there existing in the opposite direction to what it did before.

238. In the first experiments (10. 13.), the inducing wire and that under
induction were arranged at a fixed distance from each other, and then an
electric current sent through the former. In such cases the magnetic curves
themselves must be considered as moving (if I may use the expression)
across the wire under induction, from the moment at which they begin to be
developed until the magnetic force of the current is at its utmost;
expanding as it were from the wire outwards, and consequently being in the
same relat