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# How to calculate a Tesla coil semi spherical top load capacitance?

I want to know if you can calculate the capacitance a Tesla coil semi spherical top load by calculating what it would be then halving it?

Also if that is possible could you third it or quarter it because I am going to buy metal bowls for it and most aren't quite a semi sphere (a lot only go about 120 degrees)?

## Discussions

It doesn't scale like that unfortunately, because of the way the electric field sets up around an object. On a sphere, that field is entirely uniform and forms concentric shells about the sphere. In anything not spherical, all bets are off.

Steve

That's what I thought would you know any way to calculate something like that?

It CAN be done, but it takes some extremely expensive software unfortunately. May as well just make it, then measure it.

How can I measure it multimeter's don't quite go low enough for measuring that kind of capacitance?

A classic method is to make it resonate with an air-cored coil, and measure the resulting resonance.

Steve

Unfortunately for me that can't be done because I am in the process of making a Tesla coil and I need to know a rough estimate of my secondary coil before I make the primary. I am going to make and measure the capacitance of my tank capacitor in the next few days.

Well guess then, from the spherical case.

I think I will make an excessively large inductance for the primary so I can move around the tap to tune it.

DO that, then rewind the coil when you find the sweetspot, to reduce losses.

Why can't I leave the windings there?

They can create addtional losses.

I'm curious, what will you do when you have a value for the capacitance calculation ?

Maybe test measure it ?

Here's the formula for the capacitance of an isolated charged conducting sphere,

C = 4(pi)e

_{0}RC = capacitance in Farads

pi = 3.14159 ratio of circumference to diameter of a circle

e

_{0 }= 8.854×10^{-12}permittivity of spaceR = radius of sphere in Meters

A

I will calculate the resonant frequency of my Tesla coils secondary.

You could calculate C

_{Large}with the Largest Radius of your semi sphereand also calculate C

_{small}with the smallest radius,Then average the capacitance C

_{avg}= ( C_{Large}+ C_{small}) / 2A