Lesson3/Learning Event 1
On some distributors, the adjustable point is moved in the support to make the adjustment. In
other applications, the support is moved. Let's see just what happens with these parts when the
distributor is in operation.
While the low side of the cam is toward the rubbing block, the breaker arm spring holds the
contact points closed. At this time, current can flow from the batteries to ground, then from the
grounded contact to the insulated contact. From here it will flow out through the primary windings
of the ignition coil. This causes a strong magnetic field to build up around the coil. From the
primary of the coil, current returns through the closed ignition switch to the battery.
Current will continue to flow as long as the circuit is closed. When the cam turns enough, a lobe
on the cam will contact the rubbing block, then push the contact points open. This opens the
primary circuit, and the magnetic field rapidly collapses around the windings in the coil. As the
cam continues to turn, the lobe will move from under the rubbing block. At this time, the spring
will again close the points. As each lobe in turn strikes the rubbing block, the above action takes
place.
So far, we have seen how the magnetic field builds up in the ignition coil while the points are
closed. Also, when the cam lobe opens the points, the circuit is broken and the coil's magnetic
field collapses. This induces a very high voltage in the secondary winding of the coil. This high
voltage forces current to jump the air gap at the spark plug electrodes and ignite the fuel-air mixture
in the cylinder.
In practice, however, creating the high voltage in the coil is not quite so simple. Actually,
electricity, like anything else in motion, tries to remain in motion. It resists any effort to change or
stop its flow. Also, the magnetic field in the coil is collapsing around the primary windings as well
as the secondary. This builds up voltage in the primary windings. The end result is that current will
arc across the points. This arcing will cause the points to burn and be destroyed in a very short
time. It also causes the magnetic field in the coil to collapse more slowly. Remember, we get a
high voltage induced into the coil's secondary windings only if the magnetic field collapses really
fast.
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