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For the Tesla coil that I've built, I'm using a hollow metal sphere as the top (capacitive) load for the secondary circuit. But more often you'll see a hollow metal toroid used.

I'm getting streamers about 2 ft. in length from my coil now using the sphere. Could I expect to see better performance (even larger streamers) by using a toroid? And if so, why?

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  • \$\begingroup\$ Surface area. You can make a toroid shape very large diameter without being too high like a sphere of equivalent surface area would be. \$\endgroup\$
    – Dwayne Reid
    Commented May 5, 2015 at 5:24
  • \$\begingroup\$ Bottom of a sphere will offer a "shorted turn" at the top of the Tesla coil, warping the fields somewhat. Best might be a wide hemisphere above. Meld a half-donut to the edge below the hemisphere. The top edge of the coil itself is then placed in the over-large donut-hole, to shield it from destructive corona. (Note how some large VandeGraaff terminals are shaped, w/lower torus, upper hemisphere. Expensive though!) Best: shape your terminal in order that the surface-field is everywhere equal. This gives Nikola Tesla's design-goal: the entire surface breaks down simultaneously, wo/hotspots. \$\endgroup\$
    – wbeaty
    Commented Apr 20, 2020 at 2:04

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It mainly has to do with curvature. The electric field is more concentrated at places that have higher curvature. A sphere has the same curvature everywhere, so there's no preferred place for a streamer or arc to start. A torus has the greatest curvature at its outer rim, so streamers/arcs are more likely to start there. If you make the minor diameter of the torus smaller than the diameter of the secondary coil, it helps make sure that the arcs come from there and not directly from the coil itself, which could damage its insulation.

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    \$\begingroup\$ Mathematically speaking the curvature of a torus is actually maximum on its inner diameter, so although your explanation makes some sense, and probably predict what's actually observed, I suspect there is more detail to why the streamers emanate from the outer, rather than inner diameter of the torus. Diverging vs converging field potential? For the sphere potential lines diverge equally over the entire surface. \$\endgroup\$
    – docscience
    Commented May 5, 2015 at 13:55
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    \$\begingroup\$ @docscience: Yes, that's "a trap for young players", as Dave Jones would say. The curvature inside the hole of a torus is negative, which makes it mathematically less than the curvature of the outermost surface, which is positive. \$\endgroup\$
    – Dave Tweed
    Commented May 5, 2015 at 14:53
  • \$\begingroup\$ As I finished that post it also occurred to me the 'magnitude' of curvature is greater - but yes indeed negative. So then with a toroid atop your secondary, what's the best way to terminate the winding? \$\endgroup\$
    – docscience
    Commented May 5, 2015 at 15:00
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Dave tweed's answer is correct, though only partial. It does not explain why a toroid is used instead of a sphere. This reason is very simple--To help prevent the arcs from striking the primary or secondary coil. I have seen Tesla coils with spherical toploads where the streamers constantly strike near the primary, which could eventually damage your driver circuitry and the secondary itself. The toroid helps direct the streamers away from the coils, while maintaining enough surface area to act as a capacitor between the topload and ground.

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  • \$\begingroup\$ Thanks. Yes the streamers on my coil are not only coming from the sphere, but the upper part of the secondary coil as well. So far so good though, no damage for years of use. My primary is protected by an guard ring that draws any streamers in that direction to the ground plabe rather than the coil. \$\endgroup\$
    – docscience
    Commented May 5, 2015 at 13:58
  • \$\begingroup\$ The shape of the top electrode has very little to do with where the arcs end up. That's more of a function of how well the low-voltage end of the Tesla coil is bonded to an actual earth ground. If it isn't bonded, then the bottom of the coil is really the only place they can go. And beware of using a "guard ring" -- it could create a "shorted turn" that seriously weakens your coil's power by wasting most of the primary energy. \$\endgroup\$
    – Dave Tweed
    Commented May 5, 2015 at 15:05
  • \$\begingroup\$ @DaveTweed I beg to differ. The shape of the topload (the Toroid, in this case) has a LOT to do with where the arc ends up, mainly because it helps determine where it begins. The sharper curve on the outer edge of the torus is usually where the streamer "wants" to arc from, for the same reason that breakout points work--The arc wants to start at the sharpest point. A sphere has no "sharpest point" to our eyes, so it will start at seemingly random points. Much of the time, this allows it to strike the primary (or guard rail). The torus "encourages" the streamer to reach away from the coil \$\endgroup\$
    – DerStrom8
    Commented May 5, 2015 at 15:22
  • \$\begingroup\$ As for the guard rail, if made properly it will not be a shorted turn. It's simple enough to avoid--Just leave a 3/4"-1.5" gap between the ends and you should be all set =) \$\endgroup\$
    – DerStrom8
    Commented May 5, 2015 at 15:23
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actually nickoli was trying to transmit wireless energy ,not make lightning. a toroid ring is to suppress high voltage corona. on a spark gap tesla coil it is good so that you can build up to a high voltage before it arcs. on solid state, you want your streamers to make up most of the capactiance, but you still want the mininum toroid ring to suppress upper secondary arching.

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