TM 5-4240-501-14&P
THEORIES OF OPERATION
Ignition
Now, we haven’t said much about one thing, the
condenser. The condenser is a sort of safety valve on
the primary circuit. It is connected across the breaker
points to prevent the circuit from jumping the breaker
point gap, arcing, as it is called.
Figure 30A.
Let us explain it this way. Suppose we had a large pipe
through which we forced water at a high rate of speed,
Figure 30A. This corresponds to our primary circuit.
Coming out of the large pipe is a much smaller pipe.
This is our secondary circuit. As long as the large pipe is
unobstructed, the water is free to flow and very little will
flow out through the small pipe.
Figure 30B.
Now suppose we could suddenly shut off the large pipe,
Figure 30B. The water will stop flowing through the large
pipe, but the inertia of the water back in the large pipe
will force the water out through the small pipe at a
tremendous velocity until the pressure is dissipated.
This corresponds to the high voltage in our secondary
circuit.
Figure 30C.
However, suppose our valve could not stand the
pressure and would break. (Figure 30C.) This would
correspond to arcing across the breaker points. The flow
would continue through the large pipe, and very little
would flow through the small pipe.
Figure 30D.
If we put another small pipe near the valve, (Figure 30D)
and over the end place a strong rubber bag, we have the
equivalent to our condenser. Thus, when we close our
valve, the pressure on the valve would be partially
absorbed by the rubber bag, the valve would not break
and water would stream out the small pipe where we
want it to go.
The rubber bag must be of the proper size and strength.
If it is too small, it will not take up enough of the pressure
and the valve will break anyway. If it is too large, it will
hold too much water, and there will not be enough
pressure to force the water out through the small pipe.
Figure 31.
The ’same thing applies to the condenser. The proper
capacity should be about .2 microfarads or 16 to .24.
This is just right to prevent arcing at the points and still
cause the primary current to stop flowing.
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