[TCML] Medhurst Internal Capacity Again

[Harvey Norris <harvich@xxxxxxxxx>]

>From Archives;
Re: Medhurst Formula

* To: tesla-at-pupman- dot-com
* Subject: Re: Medhurst Formula
* From: MSR7-at-PO.CWRU. EDU
* Date: Mon, 3 Jun 1996 07:26:36 -0400

Hello Coilers,
>-Ed Harris said
><snip>
>These formulas are, of course, only valid for coils which are
>base driven below the point of corona formation...
This is an important point to remember. One can use these formulas to determine initial coil parameters, but expect the operating resonant frequency to be lower due to extra capacitive effects caused by the corona cloud. For my little coils (3 foot sparks and less), I have to move out 1/2 to 1 turn on the primary coil as a result of this. Plan accordingly.

>Mark R. posted the self capacitance formula due to Medhurst
>in a previous post.
Here is a repeat of that old post:

I use the formula attributed to Medhurst for self capacitance
estimation. It gives values which are fairly close to what I measure experimentally. The formula does not take into account wire spacing, so it may be in error somewhat for spacewound coils. I suspect space winding does not substantially alter the distributed capacitance, however. I have seen no formula for spacewound coils.

Medhurst's formula: C = K x D

where: C = capacitance in picofarads
K = constant which depends on the ratio of the coil height to diameter
x = means multiply K times D
D = solenoidal coil diameter in centimeters
H = coil height 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

Medhurst, R.G. "H.F. Resistance and self-capacitance of single-layer solenoids", W.E. 24.281 (Feb 1947) p 35; and W.E. 24.282 (March 1947) p 80.
W.E. stands for Wireless Engineer, a British journal.
Medhurst claimed 5% accuracy for his data, based on experimental
measurements. He wrote the paper in response to earlier theoretical papers by Palermo from 1937 and 1942. Palermo predicted different distributed capacitance values, based on a mathematical model. He did minimal experimental verification of his results. Following Medhurst's papers, Palermo's work has been pretty much discredited.
Regards,
Mark S. Rzeszotarski, Ph.D. 

I see from the above data that my two 1 ft and 2 ft 4.5 in OD PVC tube secondaries wound with 23 gauge wire should have about 6.5 pf and 9.4 pf internal capacity respectively. In LCR meter measuring the inductance of the 1 ft secondary I find 8.3 mh. I also find that it is said that the TC unloaded secondary resonates at a frequency well in excess to its quarter wave value. Here for the 1 ft 23 gauge secondary if we use the internal capacity of 6.5 pf and inductance of 8.3 mh in the resonance equation, we arrive at 685,550 hz as the solution, but for the quarter wave value using 41.3 turns/inch we find this to be ~ 420,000 hz. This is 63 % higher value for only a 2.6 H/D ratio. Now suppose we hook a scope having 20 pf internal capacitance to the secondary and add this to the amount of capacity in parallel as the measurement. My calculations show that the new resonant frequency should be ~ 340,000 hz. This is within 3 % of the 350,000 hz I arrived at by
 using short neons with a intervening plate area as a wide band rf source where the TC secondaries pick up (clean) signals 12 ft away...
http://www.youtube.com/user/harvich#p/a/u/0/k10BRK-DldA

This first video was made sloppily, but since the frequencies seem to jive with theories I thought it worthy of perusal. I have now advanced the state of this plasma arc transformer considerably, but this is the first peek.
Sincerely HDN



Pioneering the Applications of Interphasal Resonances http://tech.groups.yahoo.com/group/teslafy/


      
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