Lesson3/Learning Event 2
There are two terminals in the coil assembly for the primary winding. One terminal is connected
to the wire from the ignition switch which connects and disconnects the coil from the battery. The
other terminal is connected to the movable breaker point in the distributor.
When the ignition switch and breaker points are closed, current flows through the primary winding
of the coil. The current flowing through the few hundred turns of the primary winding builds up a
strong magnetic field. This field surrounds the primary and the secondary windings and makes the
iron core a strong electromagnet.
Remember, to induce a voltage into a conductor we must have a magnetic field and relative motion
between the conductor and the magnetic field. We do get relative motion between the field and
the conductors when current starts to flow in the primary windings, but this buildup is too slow to
induce a voltage in the secondary winding that is strong enough to jump the air gaps in the
distributor cap and spark plugs. When the magnetic field reaches its maximum strength, there is
no relative motion between it and the windings, so no current will be induced in the windings.
Suppose we suddenly shut off the current flowing through the primary winding by opening the
breaker points. The magnetic field would collapse and disappear. As it collapses, its lines of force
would cut across the primary and secondary windings at tremendous speed. The lines of force
collapsing across the windings would induce a voltage into each turn of the coil's windings. Voltage
induced into the primary winding is called self-induced voltage because the magnetic field was
created by the primary winding in the first place. Voltage induced in the secondary winding is the
result of what is called mutual induction. The secondary winding did nothing to create the
magnetic field, but a voltage is induced into it because it is "mutually" located with the primary
winding.
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