type is known as an electromagnet or a solenoid.
Electromagnets can be in many shapes. The field coils
of generators and starters, the primary winding in an
ignition coil, the coils in electric gauges, even the
windings in a starter armature, can be considered to be
electromagnets. All of these components produce
magnetism by electrical means.
The North Pole of an electromagnet can be
determined, if the direction of current flow (from
negative to positive) is known, by use of the left-hand
rule (fig. 1-21). The left hand is around the coil with the
fingers pointing in the direction of current flow. The
thumb will point to the North Pole of the
electromagnet. This rule is based on current, or
electron, flow from negative to positive.
The left-hand rule also can be used to determine
the direction that the lines of force circle a wire-
carrying current if the direction of current is known.
This is done by circling the wire with the left hand with
the thumb pointing in the direction of current flow
(negative to positive). The fingers will then point in the
direction that the magnetic field circles the wire.
Figure 1-21.Left-hand rule.
The strength of an electromagnet can be increased
greatly by wrapping the loops of wire around an iron
core. The iron core passes the lines of force with much
greater ease than air. 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 greatly increased, even though no
more current flows through it. Practically all
electromagnets use an iron core of some type.
Current can be induced to flow in a conductor if it
is moved through a magnetic field. In figure 1-22, 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 line of force resists
cutting and tends 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.
If the wire is held stationary and the magnetic field
is moved, the effect is the same. All that is required is
that there be relative movement between the conductor
and the magnetic lines of force to produce enough
voltage to move the electrons along the conductor.
Moving the magnet can move the magnetic field
or, if it is a magnetic field from an electromagnet,
starting and stopping the current flow in the
electromagnet can move it. Suppose an electromagnet,
such as the one shown in figure 1-20, has a wire held
close to it. When the electromagnet is connected to a
battery, current will start to flow through it. This
current, as it starts to flow, builds up a magnetic field.
Figure 1-22.Electromagnetic induction.