Chapter 12 Electromagnetic Induction
217
You will also notice that a human being
walking across the street passing through
the coil does not trigger a light change. Most
motorcycles are too small to trigger a light
change. There are some motorcycles that have
magnets mounted on them in an effort to
create a large enough magnetic fi eld to acti-
vate the coil and timer.
Factors of Inductance
Look at Figure 12-11. Note that there are
several things that affect the amount of induc-
tance in a coil:
• Number of turns of wire in the coil.
• Type of core.
• Cross-sectional area of the core.
• Spacing between the turns of wire.
The more turns of wire on a coil, the more
lines of fl ux there will be to act on other turns.
That means the inductance will increase as
the number of turns increases. If you had only
one turn of wire in the coil, the inductance
would be very small and could be ignored.
With a large number of turns, a small amount
of inductance from each coil of wire adds up,
creating a large impact on the circuit.
If you use a core material such as soft iron,
you will get more inductance than if you use
hardened steel. A magnetic fi eld can be created
in the core depending on the material from
which the core is made. The magnetic fi eld of
the coils combines with the magnetic fi eld of
the core and creates a greater counter emf than
the coil would have created alone. Inductors
with magnetic cores have greater inductance
than those with air cores or insulating cores.
Soft iron cores have the ability to increase
the fl ux density much better than hardened
steel cores. Steel is produced by using alloys
while it is in the liquid state. After it cools
to a solid state, it is hardened by drawing it
through dies to reduce the size. High internal
stress and hardness caused by drawing it
through dies reduces the magnetic properties
of the steel.
If the core of the inductor has a large
cross-sectional area, it will create more fl ux
lines than a small core. An increase in fl ux
lines means an increase in fl ux density and an
increase in inductance.
Spacing the turns of wire in the coil affects
the amount of inductance in the circuit. If the
turns are widely spaced, the weaker outer
edge of the magnetic fi eld around a turn of
wire would be all that cut through other
turns, or the lines of fl ux from one turn may
not cut through any other turns at all. If either
of these situations occurred, either less voltage
or no voltage would be induced.
Figure 12-11. The amount of inductance in a
coil of wire is affected by the physical makeup
of the coil.
Close Spacing
Wide Spacing
Large Cross-Sectional
Area
Small
Soft Iron Hardened Steel
Large Number of Turns Low Number
High Inductance Low Inductance