Is there a general rule of thumb for ac spark length?
Are you asking something like "what's the breakdown voltage of a given space in air?"
There is a relationship between the voltage and the length of the spark, it is also effected by the surrounding gas (air) pressure.
Here is a site that gives some mathematical equations, statistics and graphs showing the relationships.
I build a small tesla coil that produces about 12" sparks. I dont have a way to measure the voltage so I am looking for an approximate spark length to voltage conversion.
I think the rule of thumb varies with several factors: the number of windings in your coils, the transformer, wire thickness, primary spark gap settings, etc. But I read somewhere that for a medium sized unit the rule of thumb is something like 8.5 Kv per inch, this sounds possible for yours, 8.5 x 12 would put you at about 102,000 volts, a little low for a tesla coil, but possible I think.
There are several internet sites that deal with calculating this voltage based on the various factors directly associated with your Tesla Coil.
Here is one:
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