Hi Joe,12" is ok, but depends on power your giving the coil. Grounding may or may not help (you will need to retune after inserting an rf ground). Regardless of helping sparks or not, you should have a dedicated RF ground for the high frequency currents. I have no problem with the NST case and secondary at the same potential, but both should go to an RF ground. I like to run my small coils and even medium sized coils in the garage. So, I drilled a 1/2" hole in the cement and pounded an 8 foot rod down into that hole (yes, I stood on a ladder for the first couple feet of pounding). I left 3" of rod above the cement. I attached an electrical fitting to the rod which accepts up to 4 connections. This may appear extreme but this RF grounding point in the garage has come in real handy for me over the years (and it was very easy to do).
If power is low and the toroid is corrugated, you will certainly get little sparks all around the toroid. Making the toroid smooth can help some, but if power is low, even smooth toroids can emit those small little sparks (in which case, you then try putting a grounded object near the toroid and once in a while, the spark will leap out and hit it). The toroid size may be too large for the power. You might want to experiment with smaller top load sizes for whatever power your putting to the coil. A better option in my opinion is to pick up an identical NST and connect it in parallel to double up on the current and power. Rather than mess with the top load, upgrade the power where the corrugated tubing will be forced to emit constant strikes rather than the small brushy sparks.
As far as measuring spark length, there are two types of spark lengths common. 1) Free air sparks (measured by the coilers ability to decipher "about" the length of the sparks shooting out into no mans land). 2) Use a grounded target (Aluminum ladder, little test fixture, or whatever). The grounded target will usually find the longest length.
Here's a hint. Set up a piece of bare wire (brass wire at Home Depot works well) on a rod that extends from your toroid to the floor and ground the wire to RF ground. The wire makes an excellent "target" over an area that is from the top of the toroid to the floor.
A well tuned coil will do better than a detuned coil, but "power" is the ultimate spark length extender. As you upgrade power, the next step is to keep an eye on how well your spark gap is processing that power.
Take care, Bart jocatch wrote:
Hello. I have built my first coil using standard parts and dimensions and have fired it up and so far I am producing sparks but only about 12" long. I am using a 12KV nst with .0125uf mmf, 13 turns of copper tubing as primary and 20" of #28 magnet wire on a 4" form with aluminum cloths dryer ducting for the toroidMy questions concerning grounding. I have read alot about discharge spark length but not much regarding how to measure it. Above I said 12" discharge spark length, that was to a key ring held at the end of a boom stick handle. Should the metal object that I am drawing the spark to be grounded? To earth ground? Right now nothing is grounded to earth or electrical ground. The bottom side of the secondary coil is connected to the metal case of the nst. Should the secondary coil and nst be grounded to earth ground? Lastly, the aluminum toroid is discharging all around its circumference. I know the fewer discharge points on the top load, the bigger the discharge length will be. If I tape the toroid with aluminum tape to produce a smoother surface, will it help?I have a web page showing photos of my first tesla coil before I recently updated it to copper tubing and new toriod. The web page shows the old primary wire coil and old topload. I will post new pictures of the update soon.http://www.joecool.org/joe_s_tesla_coil.htm Thanks for your help. joe _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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