This discussion of points ignition capaciters has fascinated me. I too
believed the purpose of the capaciter was to prevent the points from
arcing. Now I know that's not the case. This discussion inspired me to
dig out my 1935 edition of Dyke's Automobile and Gasoline Engine
Encyclopedia (love those old books). Here's what it says about
capaciters on pages 190-192:
"The condenser assists in demagnetizing the [coil] core. Therefore, if
we could absorb this spark at the interpupter points (which comes from
the self-induced current in the pirmary winding) in some way, and thus
cause a more rapid demagnetization of the iron core, the lines-of-force
would then pass back to the iron core more quickly and would thus cut
the secondary turns of wires at a still greater rate of speed. In other
words, it is the secondary winding into which we wish to induce a very
high voltage. Therefore, by placing in the coil a condenser which is
connected across the interrupter points (P, Fig 3), we can absorb or
suppress the spark across the interupter points of the primary circuit
and stop the flow of self induced current which is still trying to build
up lines-of-force in the core, and thus aid the primary current to fall
to zero value more quicly. Hence the reason for placing the condenser
across interrupter points of the primary circuit.
Purpose of the Condenser
1. To *absorb* the self-induced current of the primary, thereby
allowing the magnetic field to collapse with the greatest possible
speed, and eliminating the spark at the contact points.
2. To *discharge* in an oscelating (alternating) manner back and forth
into the primary circuit, thus completely neutralizing or demagnetizing
the iron core and thereby preparing it for repeated action.
...As this is written out, or as it is read, it seems a rather long
action. The proces, however, does not require on-thousanth of a second,
inasmuch as the current alternates at an extremely high frequency,
sometimes having as many as 100,000 cycles per second."
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