move ROUND a current, and why a compass needle tries to set itself
at right angles to a current, as in the original experiment of
Oersted. The needle, having two opposite poles, is pulled in
opposite directions by the lines, and being pivoted, sets itself
tangentically to them. Were it free and flexible, it would curve
itself along one of the lines. Did it consist of a single pole, it
would revolve round the wire.

Action and re-action are equal and opposite, hence if the needle
is fixed and the wire free the current will move round the magnet;
and if both are free they will circle round each other. Applying
the above rule we shall find that when the north pole moves from
left to right the current moves from right to left. Ampere of
Paris, following Oersted, promptly showed that two parallel wires
carrying currents attracted each other when the currents flowed in
the same direction, and repelled each other when they flowed in
opposite directions. Thus, in figure 32, if A and B are the two
parallel wires, and A is mounted on pivots and free to move in
liquid "contacts" of mercury, it will be attracted or repelled by
B according as the two currents flow in the same or in opposite
directions. If the wires cross each other at right angles there is
no attraction or repulsion. If they cross at an acute angle, they
will tend to become parallel like two compass needles, when the
currents are in one direction, and to open to a right angle and
close up the other way when the currents are in opposite
directions, always tending to arrange themselves parallel and
flowing in the same direction. These effects arise from the
circular lines of force around the wire. When the currents are
similar the lines act as unlike magnetic poles and attract, but
when the currents are dissimilar the lines act as like magnetic