# Re: John's formula, spark length

```Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 9/3/02 2:17:12 PM Eastern Daylight Time, tesla-at-pupman-dot-com
writes:

>
> Haven't looked at John's page for a while.  Does the formula somehow take
> into
> consideration the size of the coil?  My 6.0" diameter coil which I run at
> 6kva should produce about 132" sparks according to the formula.  The best I
> have achieved is 104".  I do believe that a 10" or 12" diameter coil with
> this
> much power input could certainly produce 132" sparks.  For a given size coil,
> there is a maximum level of power that can be input to the system.  By just
> using the formula, one could expect 132" sparks with 6,000 watts input to a
> 3.0" diameter secondary - which I don't think is feasible.
>
> Sometimes I think I am on the verge of destroying my secondary at 6 kva.
>
> Ed Sonderman
>

Hi Ed, All,
A universal problem with empirical formulas is that they do well in
interpolation but the uncertainty increases exponentially when they are used to
extrapolate. I don't think there is any data for, nor can we get data for
John's equation for a 3" 6KVA coil because it will catch fire and burn before
that power level is reached. Such equations should only be used over the data
range from which they were extracted. Beyond that is a guess and the
mathematical results may not be meaningful. Another example of this would be
the formulas to determine the capacitance of a toroid. Some of these allow
toroids with a negative inner radius which are very difficult to build. ;-)

Matt D.

```