RE: Theory vs. Measurement

To: tesla-at-pupman-dot-com
Subject: RE: Theory vs. Measurement
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
Date: Tue, 4 Jun 1996 10:52:00 +1200
Hi Pike and all,
                You asked....
<snip, points noted>
> You've mentioned Medhurst's formula several times and I've looked
> for it but not found it, at least not by that name.

I originally found it in the the AWA Radiotron Designer's Handbook
ed. F. Langford-Smith. 

In simple form :
                 Cself = H x D    where D is the coil diameter in cm
and H is a value depending on coil h/d. H ranges from 0.51 for
aspect 2, to 0.81 for aspect 5 in a more-or less linear fashion. I 
wrote and posted a program to evaluate this once, but don't seem to 
have it handy. Perhaps Dr Rzeszotarski who does have it handy (I 
think) would post it again.
    Ed's formula is buried in my archives. I'll look for it and post
later today or tomorrow.
    Numerical examples? Try any coil (without terminal) you like. 
Measure fr as accurately as possible (no resistance, minimum loading,
maximum isolation and all that), then using the measured or calculated
inductance of the coil, calculate Cself using the standard resonance
formula. It should agree fairly well with the value calculated from 
the formula above.
    I KNOW that we are not dealing with lumped circuits. This is a
recipe that works (even if for the wrong reasons).
    Finally you say :

> Experimental and theoretical work are two sides of the same coin.
> For me, the big high in engineering is carrying through a theoretical
> development, building the widget, and seeing it work as predicted.
> This almost always requires several iterations refining and reconciling
> theory and practice, but eventually things work, and you understand why,
> and you feel a real sense of accomplishment.

Agree totally! If the numbers don't add up, then the model is missing
something IMO. Interestingly enough, the Corum's mention this point at
the start of their analysis and then go on to use some quite 
unrealistic assumptions (e.g. the point about operating at kc is 
clearly false as their own oscillograms show). I am doing what I
can to help with modelling a spark-producing system which is what
most if not all coilers are concerned with.

Regards,
Malcolm