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Self capacitance and Medhurst

Subject: 
        Self capacitance and Medhurst
  Date: 
        Wed, 19 Mar 1997 06:13:34 -0500
  From: 
        Steve Falco <sfalco-at-worldnet.att-dot-net>
    To: 
        tesla-at-pupman-dot-com


I have seen two methods of finding the self capacitance of a coil.  One
is closed-form:

        V = L/R  (in inches)
        CD = 5.08*R*(.0563*V+.08+.38*SQR(1/V)) 'uufd.

And the other 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

These two methods agree at the low end of aspect ratio, but begin to
diverge at the high end, with the closed-form yielding the smaller
values.

Does anyone on the list have a feel for which approach is more
accurate?  Does anyone know where the closed-form originated?

Note: I have played around with gnuplot, and can easily come up with a
closed-form which matches the table points much more closely.  But I'd
like to know if the closed-form above is better than the table, or is
merely an approximation of the table.

        Steve Falco
        sfalco-at-worldnet.att-dot-net