The Top Load of a Tesla Coil acts as the capacitor in the secondary circuit. Instead of

being two parallel plates with a dielectric in between, it connects between the top of

the secondary and uses air as a dielectric to ground. Since it is a capacitor it stores

energy and releases it as a streamer. The Top Load can be a toroid or spherical shape

and although either works, just know that changing the shape or size of the top will

change its capacitance.

 

 

I created three different tops: one toroid and two spherical. For the toroid, I simple put to aluminum pie plates back to back and placed an aluminum duct around. For the spherical shapes, I used two Styrofoam balls bought from a craft store with 4” and 5” diameters. I covered these with aluminum foil and simply placed it on top of my secondary coil. Just remember that the few inches of wire you left at the top touch the top.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

After some testing, I found that I got the best results from a 5” spherical shape. A spherical top load will have this capacitance: