Induction, Electro-magnetic.
The inter-reaction of electromagnetic lines of force with the production
of currents thereby.
A current passing through a conductor establishes around it a field of
force representing a series of circular lines of force concentric with
the axis of the conductor and perpendicular thereto. These lines of
force have attributed to them, as a representative of their polarity,
direction. This is of course purely conventional. If one is supposed to
be looking at the end of a section of conductor, assuming a current be
passing through it towards the observer, the lines of force will have a
direction opposite to the motion of the hands of a watch. The idea of
direction may be referred to a magnet. In it the lines of force are
assumed to go from the north pole through the air or other surrounding
dielectric to the south pole.
Two parallel wires having currents passing through them in the same
direction will attract each other. This is because the oppositely
directed segments of lines of force between the conductors destroy each
other, and the resultant of the two circles is an approximation to an
ellipse. As lines of force tend to be as short as possible the
conductors tend to approach each other to make the ellipse become of as
small area as possible, in other words to become a circle.
If on the other hand the currents in the conductors are in opposite
directions the segments of the lines of force between them will have
similar directions, will, as it were, crowd the intervening ether and
the wires will be repelled.
Fig. 200. ATTRACTION OF CONDUCTORS CARRYING SIMILAR CURRENTS.
By Ampére's theory of magnetism, (see Magnetism, Ampére's Theory of,) a
magnet is assumed to be encircled by currents moving in the direction
opposite to that of the hands of a watch as the observer faces the north
pole. A magnet near a wire tends to place the Ampérian currents parallel
to the wire, and so that the portion of the Ampérian currents nearest
thereto will correspond in direction with the current in the wire.