# RE: Top Capacitance Dominant?

```Hi Terry,

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi John,
>
> At 05:03 PM 04/30/2000 -0700, you wrote:
> >
> >Terry -
> >
> >Another possibility is that the toroid loses capacity when it is mounted on
> >the secondary coil. This would mean the coil self capacity is more than 3
> >pf. In an earlier post I said the toroid looses 15 to 20 % of its free space
> >capacity when placed on the secondary.
>
> I am not quite sure how to interprete the program's numbers.  Of course,
> the self C of the coil and the terminal C are place into each other's
> fields so the total is less than the two in free space.  But the resonant
> voltages at the terminal are very high so there is a vast amount of energy
> storage there which is accounting for the high capacitance calculation.  I
> wonder if it is correct but I don't have any reason to doubt it.
>
> >
> >A few years ago I read somewhere that a typical single wound TC coil would
> >lose most of its self capacitance if the windings were spaced more than 31
> >mils apart. Could this be one of the reasons why Tesla built the CS coil
> >with about 3 inch spacing? The Medhurst equation does not even use windings!
>
> I would not think that space winding is a factor.  My big TC has 20.1 mil
> wire wound with 10 mil spacing and medhurst is still very precise.  Since
> self C is a function of a coil's height and diameter, space winding should
> not make that much difference as long as it was not far into the extreme.
> Perhaps someone has a very space wound coil they could check?

I found the formula worked to a couple of percent on that series of
coils I progressively wound over the same length with fewer and
fewer turns last year. I was absolutely amazed! I ended up with a 3
foot length of coil with just half a dozen turns of 0.8mm wire on it. At
that stage, the resonant frequency was showing a definite and
progressive convergence with the expected value for a straight
monopole.
In summary, according to Medhurst, Cself was constant and all I
was doing was changing L (and calculation of L matched well). True
value or not, that is a recipe par excellence! At least it has that going
for it. No other source I have seen can match it for such a wide
range of coils.

Regards,
Malcolm

```