Musings on Medhurst

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: Musings on Medhurst
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 5 Jun 1998 22:51:02 -0500
Approved: tesla-at-stic-dot-net

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From:  D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
Sent:  Friday, June 05, 1998 10:20 AM
To:  Tesla List
Subject:  Re: Musings on Medhurst

to: Thomas

Many of these mysteries were actually solved long ago by David Sloan who
developed the lineac at Stanford.  He built a resonance transformer similar
to your #4.  It only had 18 turns of secondary coil, was vacuum tube
driven, and developed 1,000,000 volts at 100 kilowatts.  Sadly, he only
published one paper on this data but his research indicates he did
understand the processes involved.  It was a coaxial resonator in a large
metal tank filled with insulating gas.  His failure to publish more data
leaves us wondering what he knew.

DR.RESONANCE-at-next-wave-dot-net


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> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-pupman-dot-com>
> Subject: Musings on Medhurst
> Date: Thursday, June 04, 1998 10:38 PM
> 
> 
> ----------
> From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
> Sent:  Thursday, June 04, 1998 2:45 PM
> To:  tesla-at-pupman-dot-com
> Subject:  Musings on Medhurst
> 
> Malcolm, Terry, and other interested coilers,
> 
> The recent discussions about the possibility of the
> Cself of a secondary being dominated by the capacitance
> between the coil and ground led me to the following
> thoughts. 
> 
> Assume that the same piece of wire of the same length
> is used to build four Tesla coil secondaries...
> 
> A) A single wire stretched out straight would have 
> zero "inter-turn" capacitance and maximum capacitance
> due to exposed surface area. Inductance would be minimal.
> 
> B) A skinny coil would have a large inter-turn capacitance
> and its exposed surface area capacitance would be
> about half that of example (A), since the inside of the
> coil would be shielded. Inductance would be medium.
> 
> C) A large diameter coil would have a larger inductance,
> about the same inter-turn capacitance, (but with a larger
> inductance per-turn than in (B)), and slightly larger exposed 
> surface area capacitance, since shielding is not perfect. 
> 
> D) The final extreme. A single-turn secondary. Almost Zero
> inter-turn capacitance. ALMOST maximum capacitance due to
> exposed surface area capacitance. Fair sized inductance.
> 
> (A) and (D) would be pre-dominantly 1/4 wave devices.
> According to the direction of the recent discussions on this
> List, it would seem that (B) and (C) would not.
> 
> One method used to determine Cself is table-lookup, attributed to
Medhurst:
>         
>         C  = K x D      (D in centimeters)
>         
>         H/D       K
>         5.0     0.81
>         4.5     0.77
>         4.0     0.72
>         3.5     0.67
>         3.0     0.61
>         2.5     0.56
>         2.0     0.50
>         1.5     0.47
>         1.0     0.46
> 
> What is the correlation between H/D and K? Could it be related to
> the fact that as H/D grows smaller the ratio of exposed area
> capacitance and inter-turn area capacitance changes?
> 
> Things to think about...
> Why does Medhurst's method only work if the base of the coil
> is effectively connected to ground?
> Coilers have often noted that certain H/D ratios work best for Tesla
> coil secondaries of a given diameter. WHAT is maximized at these
> particular ratios? Why is there not one single "best" H/D ratio
> that would work for ALL coil sizes?
> What is capacitively different about an operating Tesla secondary
> and one that is just hanging around waiting for the juice to be turned
> on? What is different about huge high power Tesla coils and smaller,
> more efficient but lower power Tesla coils?
> 
> All of the above are bits and pieces of the same puzzle. 
> 
> Hope this helps.
> Fr. Tom McGahee
>