Figure 11-21. Left-Handed Rule.
This effect of permitting lines of force to pass through
easily is called permeability. Wrought iron is 3,000 times
more permeable than air. In other words, it allows 3,000
times as many lines of force to get through. With this
great increase in the number of lines of force, the
magnetic strength of the electromagnet is increased
greatly, even though no more current flows through it.
Practically all electromagnets use an iron core of some
11-15. Electromagnetic Induction.
a. Current can be induced to flow in a conductor if it
is moved through a magnetic field. In figure 11-22 the
wire is moved downward through the magnetic field
between the two magnetic poles.
As it moves
downward, cutting lines of force, current is induced in it.
The reason for this is that the lines of force resist cutting,
and tend to wrap around the wire as shown. With lines
of force wrapping around the wire, current is induced.
The wire movement through the magnetic field produces
a magnetic whirl around the wire, which pushes the
electrons along the wire.
b. If the wire is held stationary and the magnetic
Figure 11-20. Electromagnetism in a Wire Coil.
field is moved, the effect is the same; that is, current will
be induced in the wire. All that is required is that there
f. The left-handed rule also can be used to
be relative movement between the two so that lines of
determine the direction that lines of force circle a wire-
force are cut by the wire. It is this cutting and whirling, or
carrying current if the direction of current is known. This
wrapping, of the lines of force around the wire that
is done by circling the wire with the left hand with the
produces the current movement in the wire.
thumb pointing in the direction of current flow (negative
to positive). The fingers will then point in the direction
c. The magnetic field can be moved by moving the
that the magnetic field circles the wire.
magnet or, if it is a magnetic field from an electromagnet,
it can be moved by starting and stopping the current flow
g. The strength of an electromagnet can be
in the electromagnet. Suppose an electromagnet such
increased greatly by wrapping the loops of wire around
as the one shown in figure 11-20 has a wire held close to
an iron core. The iron core passes the lines of force with
it. When the electromagnet is connected to a battery,
much greater ease than air.
current will start to flow through it. This current, as it
starts to flow, builds up a magnetic field. In other words,
a magnetic field forms because of the current flow. This
magnetic field might be considered as expanding (like a
balloon, in a sense) and moving out from the
electromagnet. As it moves outward, its lines of force
will cut through the wire held close to the electromagnet.
This wire, therefore, will have current induced in it. The
current will result from the lines of force cutting across
the wire. If the electromagnet is