Re: Self capacitance and Medhurst

To: tesla-at-poodle.pupman-dot-com
Subject: Re: Self capacitance and Medhurst
From: Tesla List <tesla-at-stic-dot-net>
Date: Thu, 27 Mar 1997 23:20:18 -0600
Approved: tesla-at-stic-dot-net
Subject: 
        Re: Self capacitance and Medhurst
  Date: 
        Thu, 27 Mar 1997 20:39:29 +0000
  From: 
        "John H. Couture" <couturejh-at-worldnet.att-dot-net>
    To: 
        Tesla List <tesla-at-pupman-dot-com>


At 05:48 AM 3/26/97 +0000, you wrote:
>Subject: 
>        Re: Self capacitance and Medhurst
>  Date: 
>        Tue, 25 Mar 1997 07:25:55 -0500 (EST)
>  From: 
>        msr7-at-po.cwru.edu (Mark S. Rzeszotarski, Ph.D.)
>    To: 
>        Tesla List <tesla-at-pupman-dot-com>
>
>---------------------------   Big snip
>
>
> Coil #  6
> Coil Height in Inches:       3.500
> Coil Diameter in Inches:       8.500
> Height to Diameter Ratio:         .41:1
> Form Material(s):  ACR
> Wire A.W.G. Gauge:        26.
> Wire Diameter in Inches:       .0159
> Approximate Number of Turns:      200.
> Coil D.C. Resistance in Ohms:     18.40
> Coil Self-Resonant Frequency in kiloHertz:        482.
> Inductance in millihenries using Wheeler's Formula:        9.40
> Measured Inductance in millihenries:        9.25
> Difference between LWheeler and Lmeasured:      .15
> Percent deviation:      1.62%
> Predicted Distributed Capacitance (Medhurst Formula) in pF:    11.60
> Distributed Capacitance Necessary to Resonate at Fself Using Lmeas:   
>11.79
> Difference between CMedhurst and Ccalculated:     -.19
> Percent deviation:     -1.59%
>
> Inductance measurements were made with a  BK Precision Model 878 LCR
>meter
>with 1% accuracy.
>
> A Digital Frequency counter with 5 digit accuracy was used to measure
>resonant frequency.
>
> Coil Form Materials:
> PVC means 3/16 inch thick polyvinyl chloride pipe.
> ACR means 1/4 inch thick clear acrylic tubing.
> PU means the unit was coated with 3 coats of polyurethane both before
>and
>after winding the coil.
> CDB means 1/8 inch thick cardboard tubing.
>
> The test coil was placed at least 3 feet from any metal or test
>equipment.
>No top capacitance was added.  The base of the coil was driven by a 50
>ohm
>output impedance signal generator.  Resonance was observed using an
>oscilloscope with the probe placed free in air 3 feet from the coil.  
>The
>top of the coil was left floating.
>
> Apologies to those folks in the world who use real (S.I.) units of
>measurement.
>
>Flames, comments welcolmed,
>Mark S. Rzeszotarski, Ph.D.
>
>--------------------------------------------------

  Mark -
  
  Many thanks for posting the data on the 6 coils that you tested. To my
knowledge you are the first coiler who has ever tested a coil (#6) with
an
L/D under a 0.5 ratio except no doubt Medhurst. A agree with you that
the
Medhurst equation using the coil radius and length is better than using
the
wire length to determine the coil self capacity.

Because a large self capacity coil is undesireable, a large radius coil
which would have a large self capacity according to Medhurst would be
undesireable for a Tesla coil. Yet why did Tesla use a large radius (50
foot) coil at Colorado Springs with a 0.4 ratio when he was aware of the
coil self capacitance problem.

There is obviously more to consider in the design of the TC secondary
circuit. One possible consideration is that Tesla used a large wire
spacing
(4 inches) for his coil. This could have the effect of lowering the coil
self capacity which is not considered in the Medhurst equation.

I would be interested in your comments.

 John Couture