# Re: More: 120 bps vs. 240 bps comparison tests

```My two cents for what it's worth:

I thought a while ago that there was a relationship
between the frequency of commutation of the tank circuit and
the length of the spark if all else was equal.
My reasoning was that the sparks we see from these
things aren't single strokes but several happening together in
a short space of time.  Take a close look at a time exposure of
a tesla coil discharge and you'll notice a banding effect like
someone had drawn wavy lines parallel to each other.
They come from the action of the discharge.  It is
a series of pulses not a single dancing stroke.
So the discharges strobe.
Same thing happens in a lightning stroke.  Lightning
isn't a d.c. pulse either folks.
Ok...
Each time a breakdown occurs it leaves behind an
ionized path that starts to dissipate.  The next time the gap
commutates the energy will tend to propagate along the
pre-ionized trail left from the previous breakdown.
It's as though you were trying to shatter a piece of
glass that was self healing.  You hit it once and create a crack
and as you draw back to hit it again it starts to close.  If you
hit it fast enough you can build on the remnant of the first
crack.  If you don't you just make the same size crack each
time.
A aquaintance of mine has a floor standing coil
that procduced free air streamers of a bit over five feet in
length.  He was running a salient pole synchronous motor
on his gap with eight poles at eighteen hundred rpm.
When he went to a thirty-six hundred rpm
synchronous motor his arcs grew out to six feet and some.
He didn't change anything else that I know of.
I suspect that Tesla Coil spark length is the result
of a combination of potential at the terminal capacitance,
available current at the terminal capacitance, frequency
of commutation and average power through put of the
system.
In simpler terms it is the power per pulse and
how often you deliver that power that governs the
true length of the streamer.  Not raw potential alone.

Just a thought...

John W.

>Original Poster: FutureT-at-aol-dot-com
>
>
><<                     cap                            spark
>                        watts    watt     amp     length   power     charge
>> BPS   Joules   calc    meter   meter   inches   factor %    eff. %
>
>> 120     3.85      463      550     2.6        42         88           84
>> 240     5.41      650      800     3.8        42         88           81
>  >>
>
>Malcolm, Barry, all,
>
>I should mention that in the table above from my previous posting,
>that the 240 bps joule figure is based on two bangs, so the figure
>is really joules per ac half cycle.
>
>But I did some more tests:
>
>First, I adjusted the gap phase by a few degrees to better equalize the
>bang sizes, this had no noticeable effect on the TC operation or
>efficiency.
>
>I started thinking some more about the whole comparison, and I
>started to wonder if I might be seeing the effect of a sort of "sweet
>spot" in the coil, which could be skewing the results.  As a cross-
>check, I did a new test, running the coil at 240 bps with the same
>bang sizes as previously used at 120 bps.  This of course doubled
>the input power, and the spark increased by 23%.  Next I ran the
>coil at 120 bps, but used the smaller bang size equal to that used
>previously at 240 bps, and of course the spark was shorter.  Here's
>a new table showing the new results:
>
>                       cap
>                      watts    watt     amp      spark      length
>BPS   Joules    calc    meter   meter    length     increase
>
>120      2.7        325      400      1.8         34"
>240      5.41      650       800     3.8         42          23%
>120      3.85      463       550     2.6         42
>240      7.7        926      1100                  51          21%
>
>These results suggest that doubling the power input by a doubling
>of the break rate (keeping bang size the same), gives about 1/2 the
>spark length benefit as a doubling in cap size instead.  A doubling
>of the cap size (bang size) gives about a 41% spark length increase
>(shown by other experiments).  So these new tests continue to
>indicate that longest sparks for a given wallplug input power can be
>achieved with low break rates with larger caps.  Again the 240 bps
>joule figures in the table above are for 2 bangs (1/2 ac cycle).
>
>These tests also show that charging efficiency remains good at the
>higher break break rate.  The inefficiency of high break rates must
>be occuring due to the physics of spark growth in the air.  The
>sparks are brighter and fuller though at the high break rate, so it
>would seem that at high break rates, the power is going partially into
>creating fuller, brighter sparks, and partially into making them longer.
>
>Yet, Greg Leyh's sparks seemed to grow tremendously as he raised
>his break rate in his excellent DC powered TC (he has equal bang
>size at all break rates).  I forget the exact figures though, i.e. for a
>doubling of the break rate, how much longer did the sparks grow?
>
>John Freau

```