This is the principle of the galvanometer. A number of methods of
memoria technica have been proposed to remember it by.
Thus if we imagine a person swimming with the current and always facing
the axis of the conductor, a magnetic needle held where the person is
supposed to be will have its north pole deflected to the right hand of
the person.
Fig. 201. REPULSION OF CONDUCTORS CARRYING OPPOSITE CURRENTS.
Again if we think of a corkscrew, which as it is turned screws itself
along with the current, the motion of the handle shows the direction of
the lines of force and the direction in which the north pole of a needle
is deflected. This much is perhaps more properly electro-dynamics, but
is necessary as a basis for the expression of induction.
If a current is varied in intensity in one conductor it will induce a
temporary current in another conductor, part of which is parallel to the
inducing current and which conductor is closed so as to form a circuit.
If the inducing current is decreased the induced current in the near and
parallel portion of the other circuit will be of identical direction; if
increased the induced current will be of opposite direction.
This is easiest figured by thinking of the lines of force surrounding
the inducing conductor. If the current is decreased these can be
imagined as receiving a twist or turn contrary to their normal
direction, as thereby establishing a turn or twist in the ether
surrounding the other wire corresponding in direction with the direction
of the original lines of force, or what is the same thing, opposite in
direction to the original twist. But we may assume that the
establishment of such a disturbance causes a current, which must be
governed in direction with the requirements of the new lines of force.
The same reasoning applies to the opposite case.