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Naming a coil size, was Re: Big Coil

Original poster: FutureT@xxxxxxx

In a message dated 1/25/05 11:01:57 AM Eastern Standard Time, tesla@xxxxxxxxxx writes:

Original poster: "david baehr" <dfb25@xxxxxxxxxxx>

80" sparks from a 4.5" coil !!! WOW, could you give me a few specs on your coil ? I'm gettin' just over 48" with my 4" coil,....,,,,Hmm, thinking back, my very first coil was about 4" dia. , pvc wire on an ol' carpet roll tube with glass plate caps and single spark gap , discharge was about 4" , Look how far we have come!!

I think that the somewhat common practice of naming coils by
their secondary diameter does not give a true indication of their
spark length capabilities.  Consider for example a 4" coil that is
15" tall, and another 4" coil that is 25" tall.  The taller coil will
be capable of giving much longer sparks before the secondary is
destroyed by racing sparks as the power input is increased.
In fact the taller coil will be able to give sparks that are about
1.67 times longer.  That's because the taller coil is longer by
that factor in the above example.  For example if the shorter
coil can give a 50" spark, then the longer one would give an
83.5" spark.  Of course the taller coil will require (and will be
able to withstand) a higher input power to get the 83.5" sparks.
I'd suggest that naming a coil by its height would give a better
measure of its capabilities.  It's true that a wider diameter
sometimes helps efficiency, but the ability to withstand a
large input power depends more on the height than the diameter
at least up to some point.  Of course it's desireable to keep
the ratio of dia to height within an acceptable range.

In one of my experiments I compared the performance of a
4" x 23" secondary, and a 6" x 24" secondary.  There was no
real difference in performance, and no real difference in power
handling capability, because they were both of similar height,
(or length if you prefer).