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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)?

## Comments

Best Answer 9 years ago

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

Answer 9 years ago

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

Answer 9 years ago

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

Answer 9 years ago

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

Answer 9 years ago

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

Steve

Answer 9 years ago

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.

Answer 9 years ago

Well guess then, from the spherical case.

Answer 9 years ago

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

Answer 9 years ago

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

Answer 9 years ago

Why can't I leave the windings there?

Answer 9 years ago

They can create addtional losses.

Answer 9 years ago

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

Answer 9 years ago

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

Answer 9 years ago

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