its axis, so as to bring
any part opposite to the tangential wire,--still, if afterwards the wire be
moved in the directions indicated, the current of electricity will be from
P to N; or if it be moved in the opposite direction, from N to P; so that
as regards the motions of the wire past the pole, they may be reduced to
two, directly opposite to each other, one of which produces a current from
P to N, and the other from N to P.

[A] By magnetic curves, I mean the lines of magnetic forces, however
modified by the juxtaposition of poles, which would be depicted by
iron filings; or those to which a very small magnetic needle would
form a tangent.

115. The same holds true of the unmarked pole of the magnet, except that if
it be substituted for the one in the figure, then, as the wires are moved
in the direction of the arrows, the current of electricity would be from N
to P, and when they move in the reverse direction, from P to N.

116. Hence the current of electricity which is excited in metal when moving
in the neighbourhood of a magnet, depends for its direction altogether upon
the relation of the metal to the resultant of magnetic action, or to the
magnetic curves, and may be expressed in a popular way thus; Let AB (fig.
25.) represent a cylinder magnet, A being the marked pole, and B the
unmarked pole; let PN be a silver knife-blade, resting across the magnet
with its edge upward, and with its marked or notched side towards the pole
A; then in whatever direction or position this knife be moved edge
foremost, either about the marked or the unmarked pole, the current of
electricity produced will be from P to N, provided the intersected curves
proceeding from A abut upon the notched surface of the knife, and those
from B upon the unnotched side. Or if the knife be moved with its back
foremost, the current will be from N to P in every possible position and
direction, provided the intersected curves abut on the same surfaces as
before. A little model is easily constructed, by using a cylinder of wood
for a magnet, a flat piece for the blade, and a piece of thread connecting
one end of the cylinder with the other, and passing through a hole in the
blade, for the magnetic curves: this readily gives the result of any
possible direction.

117. When the wire under induction is passing by an electromagnetic pole,
as for instance one end of a copper helix traversed by the electric current
(34.), the direction of the