Re: 12 MV 100 years ago

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 5/21/03 10:54:30 PM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

>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.


John C,

Your graph ignores the large effect of the break-rate upon the voltage
vs. spark length issue.


>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.


Most tests at high voltage research labs use single shot, or slow repetition
type measurements.  Such sparks never get the chance to grow as Tesla
coil sparks do.  The fact that sparks grow longer as the break-rate
increases (within certain limits) for a given bang size, is well-known.
Tests from typical high voltage research labs cannot be used to judge
the growth characteristics of Tesla coil sparks.  Very little research has
been done on fast repeating RF sparks, which is why TC spark growth has
not been well quantified yet.


>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.


I believe more accurate information has been available for some time.


>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.


Such a chart would need to take the break-rate into account, along
with other factors such as the toroid size.

I fully support Malcolm's comments.

John F.


>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>
>
>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
>