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Re: Measuring self-capacitance directly (Re: flat secondary)



Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> Antonio; I nead your councel. I know what distributed capacitance of a coil
> is. Why isnt the shunt capacitance of a plate on the side of a flat coil
> nearly the same as distributed capacitance. No not exacty, but nearly the
> same for practical uses ?

A flat coil with one of the terminals open is just a metal disk if you
measure its capacitance to ground. This happens because the potential
is the same all along the coiled wire. But when in use in a (grounded)
resonator, one of the ends is grounded, and the potential along the
coil is not uniform (zero at the grounded end). Because of this, the
coil stores less energy in the electric field around it than an
insulated disk would store, and so the equivalent capacitance to ground
seen at the top of the coil is smaller than the capacitance of a metal
disk at the same position. The factor is of about 60%.
A solenoidal coil also behaves in this way, with the "self-capacitance"
being about 50% of the capacitance of an insulated metal tube with the
same dimensions. See a post by Paul Nicholson some time ago with
computed capacitance reduction factors for several geometries.

Antonio Carlos M. de Queiroz