MODEL T TC'S

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

For the record, I ran a very very crude experiment here last night.  I
took a "Ford coil", adjusted the point tension so it ran fairly
smoothly, and connect a small and adjustable parallel-electrode gap
across it and ran it from a 6 volt battery.  Input current was about 1/2
amp.  I then connected several different capacitors across the gap and
adjusted the spacing until it sparked "fairly well".  Results were:

   C          GAP      C x GAP^2
 
 500 pf      0.062"      ~1.9
  
1000 pf      0.040"      ~1.6

2000 pf      0.025"      ~1.25

Hope you can unscramble that table.  Assuming that the breakdown voltage
of the gap is proportional to the spacing (it probably isn't at these
low voltages) then the "C x GAP^2" value is a measure of the energy
stored in the capacitor and presumably available to excite the primary
of a small coil. IF this reasoning is correct, the energy was greatest
with the 500 pf capacitor, and only a little bit less for the 1000 pf
one.  Based on this brief experiment it appears to me that, for this
coil and these particular operating conditions, a value around 1000 pf
would probably be a good compromise between maximum energy output and
primary inductance.  If the mood strikes me I'll try to wind a suitable
high-indutance primary in the next couple of days and try to excite a
little coil and see what happens.

	These were by no means the longest sparks I could get out of this coil.
By the time honored technique of wedging a piece of wood between the end
of the coil and the vibrator spring and "and adjusting for maximum good"
it is possible to get a lot more power out, at the expense of burning up
the contact points which are probably pretty hard to find these days.  I
plan to hook up a transistor driver later and play with optimizing it.

	Should mention that, because it wasn't oil soaked, this coil is
probably one I bought new from Western Auto Supply for the then
exhorbitant price of almost two dollars (replacement point set cost
about a quarter).  Of course, for a 13 year old kid during the late
depression years that was a LOT of money!

	I have a number of other old coils and will try similar experiments
with them. It's obvious that the output impedance of the coils varies a
lot, and is pretty high.  This suggests that the optimum coil for
driving spark plugs is not the optimum one for driving capacitors.

More later,

Ed