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big coil history

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 03:51 PM 3/30/2005, you wrote: 
Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>



Thanks for the feedback on the introduction of Medhurst C to the
Tesla world.  I had a feeling Malcolm had something to do with it.
I wonder, how were people calculating coil resonances just prior
to that?   The inductance is not to hard to deal with, but how was
capacitance estimated?  People couldn't have still been using the
wire length thing at that late stage because their coils worked!
And some of them were quite large, so there must have been another
fairly reliable method in vogue.

I suggest empiricism played a huge role. Remember that there were a lot of "rolled poly" caps back in the 80s and early 90s. The whole capacitance thing was pretty uncertain, so you'd just do cut and try until it worked. There were some rough and ready toroid C estimation formulas around by the late 80s that I ran across in the early 90s. I think people just calculated the approximate C of the toroid and a cylinder, then calculated an approximate fres (or measured it using a signal generator), then wired up a primary and tuned for maximum smoke.

The idea of actually calculating coupling, and then having some theoretical basis for selecting a particular k, for instance, wasn't much evident.

There's no question that some coil builders had a better "feel" for proportions and sizes that would work. Also, it's pretty easy to get a coil to "work" at all, especially if you aren't concerned about efficiency. I've seen a number of big pig powered coils that only produce 5-6 ft sparks, which indicates a pretty non-optimized design, considering that 10 ft should be fairly easy.


I for one would welcome hearing more about the 'early days' from
the old timers.  Why not put a few priceless recollections into
the archives for preservation?    I'd like to know more about the
history of this hobby, its technical developments, and the people.
Who were the movers and shakers back in the 50's, 60's, and 70's?
When did people start building big coils for public performances
and special effects, and how did they design them to work without
too much trial and error?

http://www.ttr.com/ is Bill Wysock's site, and describes his work with coils (which certainly goes back a few years) as well as a lot of information about the Griffith Park Observatory display coil as well as Ken Strickfadden.

On the non-hobby side..
The Aurandt coil at the Griffith Observatory is well over 60 years old, and was used for public demonstrations on a daily basis (and still was as of a couple years ago, with some refurbishing by Bill Wysock). It started with Leyden jar caps (high tech beer bottle caps) went to glass plates, and then used something more modern.

Ken Strickfadden (Frankenstein special effects) used tesla coils, probably from 1931 onwards.

There have always been several HV effects people in Hollywood over the years, but I don't have names for them. They'd typically do all sorts of electrical effects. The one I particularly like is the one where you hold a couple carbon rods in the air over your head, connected to the 110VDC bus on stage and strike arcs to simulate lightning. Wearing a hard hat is recommended to avoid singed hair. Prior to the advent of big xenon flash tube based lights, stage lightning was almost always done by carbon arcs driven from battery banks. I used to work at a place that had a trailer full of batteries, with a gasoline generator for charging, and a big plexiglas box with a plunger that brought a bunch of carbon rods together.

Physicists like Merle Tuve were building coils in the WW II era. Tesla coils are described in Craggs and Meek's book, along with design equations based on coupled LC circuits. Those people would have been using references like Circular 74 or its predecessors.


Given that folks in the 30s and 40s were using the lumped LC analysis approach (and it had certainly been around before that), it would be interesting to see where, when, and why, the 1/4 wavelength of wire idea started to be used.


And where is Ed Harris now?
--
Paul Nicholson
Manchester, UK.
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