As much as I would have liked a really nice looking smooth surface, I could not afford a custom made spun aluminium example. I therefore used an aluminium extractor fan liner and bent this around a central wooden 'wheel'. I call it a wheel because it was turned on the lathe to have a depression around its central area, much like a car wheel has for the inner tube to sit in. This serves the purpose of locating the outer aluminium liner squarely onto the wood.
The wheel shape was turned to an exact size so that the outside liner would fit snugly around the outside. This snug fit is important for my method to work successfully.







Before the liner was put around the wooden 'wheel' I placed an elastic strap inside the liner and hot glued a plastic locating collar into one end (circled). The elastic was then pulled tight by the wire you can see and the two ends secured to one another.
It is a rather fiddley job trying to secure it, because you need to have your hands inside the liner to tie the knot, whilst at the same time the action of tying the knot is pulling the two ends of the liner together and trapping your hands.


Once you have managed to secure the elastic strap it causes the two ends to butt hard against one another, while at the same time they are located by the collar. I then slipped the joined liner over the central wooden 'wheel' much like you would 'pop' a bicycle tyre onto its rim.








The parcel tape wrapped around it is just a temporary measure!

The central areas were then covered with heavy duty aluminium foil and given a layer of varnish to protect them. This varnish coat offered some protection but very soon became quiet pitted as breakout occurred from its surface. With hindsight the foil would have been better left just plain.

I later made a second toroid with the intention of giving it a smooth surface by covering with aluminium foil tape. I used the same initial method to make the wooden centre as employed on the first one, except I had to mill the shape circular as it was slightly too large to fit on my Myford lathe.






Once the ducting was put in place it was then covered with ordinary wall filler and allowed to dry.





After several layers of filler and plenty of rubbing down, I applied two coats of varnish to protect the delicate surface and to give a 'key' for the tape to stick too.





Applying the tape turned out to be a fiddley and frustrating job. Originally I had drawn the toroid in a 3D CAD program (Rhino) with the aim of cutting out individual segments to apply. I soon realised that unless they all aligned absolutely correct, with respect to one another, the result would look dreadful. I also encountered problems in making a suitably accurate template for this method as well.

Because of these developments I soon realised that individual two inch wide strips laid axially across the toroid was the way to proceed. These still have be aligned and applied very carefully if you wish to avoid undue creases though. You must realise that creases are bound to occur to some extent though, as you are putting a 2 dimensional flat surface, onto a curved 3D surface.
Fortunately minor creases can be removed by burnishing the tape once it has been applied, with something like the back of a spoon, or a curved piece of plastic.





Making an 8.5 x 34.5 inch Toroid is on Page 2


Toroid Height & Size

The toroid not only acts as a capacitor to hold the HV charge, but also serves the purpose of offering some electrostatic shielding to the top part of the secondary. Without shielding you would get excessive corona or even break-out from the top of the secondary coil windings. So placing it too high will mean you loose the benefit of any shielding.
On the other hand placing it too low will mean it will act like a shorted turn at the top of the secondary, which will waste power. So getting it the optimum height is fairly important.

Basically you want to place it as high as you can without the top of the secondary breaking out.
A good upper starting point would be with the toroid positioned so that height 'X' = 0.8 of diameter 'X' - see below.

The final position (probably a bit lower) is only really found by trial and error. This is best done by running the coil in the dark and photographing it to detect any breakout from the top of the secondary.

My own 8 inch 'Phoenix' coil running on a 6 x 24 inch toroid suffered break-out from the secondary at a recent Teslathon, and following some advice I lowered it and quickly solved the problem. When I measured it later though I found I hadn't even followed my own advice above, as the height 'X' prior to adjusting had been about 1.2 times diameter 'X'. Dumb or not?





The actual size of the toroid will depend on the size of the secondary coil. There is it seems no hard and fast rules, though it is generally accepted that the minor diameter will not be larger than the diameter of the secondary coil, in general though on smaller and medium coils the ratio is usually about 0.6 to 0.8. The overall toroid diameter would normally be 2.5 to 4 times the secondary diameter. Other people may aim for a maximum diameter that is about 80 to 90% of the actual winding height though.


If the toroid is too small it will not be able to hold sufficient charge before breakout occurs, so the streamers will be shorter and weaker. A correct sized is one will allow a healthy charge to build up before break-out occurs. You will then get a nice solid streamer that is longer than that produced by a smaller toroid.
A toroid that is too big, although offering increased electrostatic protection, will not have any breakout at all unless you add a 'breakout point' (a small rod or tack sitting on the toroid's surface). Another consequence of being too big is that it can very occasionally cause what are called 'racing sparks'. This is where sparks shoot vertically up and down the secondary coil surface. The usual culprit behind racing sparks however is that the coupling is too tight, so always check that aspect first. To alter the coupling you would need to either raise the secondary (& toroid) in respect to the primary, or else lower the primary in respect to the secondary.