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RE: 12 MV 100 years ago
Original poster: "John H. Couture by way of Terry Fritz <teslalist-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>
Malcolm -
I believe you are referring to the Voltage vs Spark Length graph shown in my
Tesla Coil Construction Guide. To my knowledge this is the only graph of
it's type ever published for Tesla coils. Don't confuse this graph with John
Freau's Watts vs Spark Length equation. You appear to be confusing equations
for Voltage vs Energy with equations for Voltage vs Spark Length. There are
no theoretical equations for Tesla coil Voltage vs Spark Length, only
empirical equations based on tests under specific conditions.
I agree the single sparks from a Marx generator are not the same as multiple
sparks from the Tesla coil. However, I do not agree that the voltage for a
one foot spark can be the same as the voltage for a 5 foot spark. Tests from
high voltage research labs indicate spark length definitely increases with
voltage.
As I mentioned in my 1998 post my graph is based only on test information
available and estimates of the voltage by coilers at the time the graphs
were made. As there are no theoretical equations available for this type of
graph I mentioned that the graph is speculative and hopefully more accurate
graphs would be made in the future when more accurate test info becomes
available. It appears we are all waiting for this new graph to appear. Until
that graph shows up the one in the TCC Guide will have to do.
It would help if you would make a Voltage vs Spark Length graph with your
up-to-date info so coilers would have at least two graphs to compare.
John Couture
----------------------------------
-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, May 21, 2003 6:47 PM
To: tesla-at-pupman-dot-com
Subject: Re: 12 MV 100 years ago
Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<m.j.watts-at-massey.ac.nz>
Hi Peter,
On 21 May 2003, at 7:53, Tesla list wrote:
> Original poster: "Peter Terren by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <pterren-at-iinet-dot-net.au>
<snip>
> My simple point was that spark length is not a linear function of
voltage
> and that 12 MV may give huge sparks of length well beyond any manmade
sparks
> before. The exact relationships are the product of many variables and
> uncertainties. I have provided 3 references for this to give estimates,
all
> from very different points of view, including those of respected coilers
> such as John Couture who has influenced Bill Wysock's thinking on the
matter
> and reported in the list archives in threads in 1998.
> http://www.pupman-dot-com/listarchives/1998/january/msg00458.html
> You have been a bit harsh in your criticism only looking at one of the
> references without providing any clear ones of your own or explaining
> exactly what 'has been known about for some time'. It is also not clear
from
> your statement how energy conservation invalidates a voltage vs spark
length
> relationship.
The first point to make is that maximum theoretical output voltage
for a disruptive coil is well-defined. This is all old territory but
I'll repeat it once again. The conservation of energy argument is
based on the assumption (read on before jumping ;) that all energy
which is contained in the primary capacitor ends up in the secondary
(it doesn't of course as there are losses occurring in the process of
transfer from one to the other but if losses are left out of the
picture, that is the best you can do). The effective capacitance of
the secondary (i.e. its capacitance which causes it to resonate at
its fundamental) is approximately defined by Medhurst and more
closely defined as Ces by more exact modelling (refer to the TSSP
website). The two are close enough for the purposes of this
exposition.
For a given primary energy Ep ending up in the secondary, the
secondary peak voltage is defined as Vs = SQRT(2Ep/Cs) where Cs is
the sum of all secondary capacitances (coil plus terminal). The
derivation is simple:
Ep = Es where Es is the energy ending up in the secondary
and Ep = 0.5Cp.Vp^2
hence Ep = 0.5Cs.Vs^2
which when re-arranged gives the result above
You can plug the figures in for any coil knowing Cp, Vp (the voltage
the gap is set to fire at) and Cs. That deals with Vs.
The second part is the correspondence between sparklength and Vs.
The recent Marx generator posts should give you some idea what spark
distances are like vs voltage for single shots. But a normally
operating TC is not used as a single shot device. The air is hammered
by successive shots a few mS apart and streamers grow visibly with
time. You can see it all happening in slow motion on video. To take
an example that I will repeat for the second time in a fortnight, I
have a coil whose theoretical Vout is about 370kV. In reality it is
less because of losses. I expect it to be closer to 300kV and the
fact that I can get connected sparks approximately a foot long when I
operate it in single shot mode is indicative. However, when I bump
the breakrate up to 100 - 200 BPS, the sparks will stretch and
connect with objects over 5 feet from the terminal. The output
voltage has not changed - the gap still fires at about the same
voltage being a fixed static type, but the sparklength has increased
fivefold.
The point is that there is a strong correspondence between power
fed in and sparklength but only a weak one at best between voltage
ans sparklength in normal repetitive operation.
> I am a newbie with 20 years tesla experience and am keen to increase my
> knowledge and accept the collective scientific wisdom and experience of
> others. The purpose of this forum is to do that.
Agreed, but I can't really be expected to apologize for being asked
to repeat again what must be referred to in the list archives dozens
of times. My intention is to be as objective as possible. I see one
of the aims of research as being to sweep old myths aside where they
conflict with evidence.
Malcolm