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

Hello All,
        The formula for self capacitance of a coil, as studied by Medhurst, is
rather simple for typical tesla coil secondaries, where the height:diameter
ratio is normally between 2.5:1 and 5:1.  The values Medhurst published are
described in my earlier post, which is repeated here interested parties:
---------------------------------
       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

The data is from Medhurst in Wireless Engineer V24.281 (Feb 1947) 35, and
V24.282 (May 1947) 80 (printed in England).

Regards,
Mark S. Rzeszotarski, Ph.D.