[Prev][Next][Index][Thread]

Re: The "Medhurst" method for the self-capacitance of an air-core coil

Tesla List wrote:
> 
> Subscriber: mcranfor-at-ichips.intel-dot-com Thu Jan 23 22:17:37 1997
> Date: Thu, 23 Jan 1997 01:19:43 -0800
> From: Micheal Cranford <mcranfor-at-ichips.intel-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Cc: mcranfor-at-ichips.intel-dot-com
> Subject: The "Medhurst" method for the self-capacitance of an air-core coil
> 
<SNIP>

>   I have developed an analytic formula using the Medhurst data that predicts
> the same capacitance values within 1/3 of a picofarad at the given values of
> the M-array in the program.  The formula is a continuous expression - unlike
> the BASIC program, which uses a piece-wise linear technique - so it should be
> more accurate at values that are not exactly on the original data points (this
> would include extrapolating beyond the end of the original data points).
> 
>   I the process of generating the analytic formula I noticed that a plot of the
> original Medhurst data actually shows 2 linear curves having slightly different
> slopes and intersecting at about R=30. I seriously doubt that anything physical
> that could exhibit this effect so I assume that measurement errors are the real
> cause (Medhurst may have switched coil forms or wire size at R=30 to handle the
> larger coil aspect ratios).
> 
>   I tried tracking down the original Medhurst paper by searching for a copy of
> the Radiotron Designer's Handbook (edited by Langford-Smith) only to discover
> that it is out of print.  A local library has informed me that they can get it
> but it will take at least 6 weeks. I found two copies of the Radiotron book in
> a bookstore - a 3rd edition and a 4th edition. The used 3rd edition was priced
> at $30 but it did not have the Medhurst paper in it.  The used 4th edition had
> the Medhurst paper in it but it was priced at *only* $110 (I passed on buying
> it and examined the article while in the store).

Michael,

Could you please share the analytical expression you're using?
Unfortunately, I only have the Third Edition of the Radiotron book, and
none of my other books reference the Medhurst papers. Thanks!

-- Bert --