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Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>
However as Robert Jones has pointed out, the
capacitance Medhurst's formula gives is *not* the static isotropic
capacitance of a conductive cylinder. This is easily measured and I
have done it. For one particular resonator I recal measuring
somewhere in the vicinity of 100pF where Medhurst gives (a la
resonant frequency calculations) somewhere around 25pF. Therefore,
Medhurst (and Wheeler) give us a useful tool for coil design but say
little about the actual working mechanism.

For the coils that I have measured, or calculated, the Medhurst capacitance is close to one half of the capacitance of a hollow cylinder to "infinity". And there is a good reason for this, because if the distributed capacitance of the whole coil is split in two, one at each end of the coil, Each of them is at the position of the Medhurst capacitance when the other end is grounded. Of course, this assumes that the grounding at one end doesn't affect much the distributed capacitance at the free end of the coil, what appears to be true.

Examples (calculated):
A coil with 1 m of height and 20 cm of diameter:
Medhurst capacitance: 15.28 pF
Distributed capacitance of a cylinder with this shape: 27.57 pF
A coil with 1 m of height and 40 cm of diameter:
Medhurst capacitance: 21.29 pF
Distributed capacitance of a cylinder with this shape: 38.07 pF

This effect is clearly visible in a Magnifier, where another
capacitance, similar to the Medhurst capacitance, loads the
bottom of the third coil, as part of the capacitance to ground
between the secondary and third coils.

I still think the idea of
coherence on the resonator is not an accurate description of coil
operation *unless perhaps* the coil is fed with pulses and not an

This story of "coherence" is very strange, unless it means the coherence of the two (or more) oscillatory modes of the system, when the energy transfer from the primary is complete.

Antonio Carlos M. de Queiroz