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Ed, Searched my files and this is the only info I hit on... Not RQ, but maybe it will help.



This question led to a number of experiments and the discovery of some unrecognized (at the time) instrumentation artifacts from Keithley 600 series electrometers. The final answer depends on whether the Tesla coil is actually generating corona or streamer discharges, terminal or receiver breakout configuration, and in some cases, unique driver/coil characteristics.

In 1996, Richard Hull did a number of measurements using an isolated sphere, a HV cap or a Keithley series 610 electrometer, a medium spark gap coil, and a low power (15W) Tesla Coil. The latter system used a primary circuit that was switched by a hydrogen thyratron. This setup generated a single unidirectional (half-sinusoid) current pulse of fixed amplitude through the primary. This effectively "shock excited" the secondary, causing it to ring with a relatively small, fixed amount of energy - very repeatable. Classical primary ringdown and secondary ringup was prematurely terminated since the thyratron only fired for the first positive 1/2 cycle (at the primary frequency), then blocked conduction in the reverse direction and thereafter until the grid was triggered again.

Even though the energy transfered to the secondary was much too low to cause breakout, Richard was able to consistently measure significant charge transfer and negative DC voltages on an isolated sphere placed a short distance from the TC when using the electrometer. Because the TC power was so low, breakout (or even corona) did not develop, so corona rectification was eliminated as a possible cause, and no other potential mechanisms could be found for the phenomenon at the time.

In a medium power sparking coil, one can connect an isolated "receiving" sphere to one end of a HV capacitor, the other end of the cap connected to ground. When the TC is operated, the capacitor will (usually) tend to accumulate a negative DC voltage. Corona rectification is the generally accepted mechanism for this case, especially when one terminal preferentially breaks out (non-uniform E-field). Corona occurs at a lower voltage when the topload is negatively polarized, but streamers tend to grow quicker when the topload is positively polarized. This effect can be used to create a "corona rectifier". It is also possible (by using a sharp point on the receiving sphere, or via unique coil and driver configurations) to develop a positive voltage on the isolated terminal/HV capacitor.

Some of these effects were initially reported by Richard Hull in July, 1996 in the TCML, followed by additional discussions in August, 1996. Here are some of the more relevant postings:


The explanation for isolated sphere charging from the non-sparking low power thyratron coil was finally resolved thanks to a number of careful experiments conducted by Scott Fusare and Richard Hull several years later.

The apparent charging measured by the Keithley electrometer was actually found to be caused by instrumentation artifacts from the solid state front-end of the Keithley 600 series electrometers. Rectification could occur in the presence of high dv/dt signals, thereby introducing a DC offset that was not actually there.

By using other measurement techniques, it was determined that a "non-sparking" TC does NOT induce a DC potential on an isolated terminal. The complete report and conclusion can be found in Issue 35 of the Electric Spacecraft Journal (December 16th, 2002). However, a TC that causes air discharges (on either the topload or receiving terminal) WILL tend to develop a DC potential on the receiving terminal. See:


Bottom line:
Non-sparking coils should not induce a charge on an isolated receiving terminal. And, although most sparking TC's will cause a negative voltage to develop on the isolated terminal, the polarity can be reversed by exchanging topload and receiver breakout roles, or by using certain coil/driver techniques that preferentially promote positive topload "burst corona" breakout. The latter effect has been demonstrated on pancake coils during a number of experiments (not on TCML) privately communicated to me by Jeff Behary. For most spark gap Tesla coils using a topload with a breakout point, the voltage developed on the isolated terminal is _usually_ of negative polarity.

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futuret@xxxxxxx wrote:
I seem to remember that there was some controversy about the
polarity, and the original results were over-turned a couple of times
as various experimenters found differing results. An article was
published in Electric Spacecraft magazine, and then there were some
follow-up articles.  I don't remember offhand what the final outcome
was.  Maybe Bert Hickman or others have something to say about this
issue also?  I think the question was eventually resolved.


-----Original Message----- From: Bert Pool <micro_wave@xxxxxxxxxxx> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> Sent: Sun, Jan 31,
2010 9:49 am Subject: Re: [TCML] Ambient Potential Polarity of TC  -
John F or Dave S has answer?

As I recall, Richard Hull did some measurements with a Keithly
electrometer, and determined that there was a steady state field. We
know that positive and negative electrical discharges do NOT have
identical characteristics, and *might* lead to a net charge of one
polarity or the other. John Freau or Dave Sharpe, both who have
visited Hull's labs back in the day,  might be able to confirm this
with more detail.  Guys?

Bert Pool

stork3264@xxxxxxxxxxx wrote:

Assuming a simple quarter wave Tesla coil referenced to ground is
there a measurable electrostatic field  potential of the ambient
space surrounding the top load?  Without doubt, potential is
varying between positive and negative with respect to ground.  The
question is there a measurable ES field of a single polarity?  Is
there favoring of positive or negative plasma discharge in air?  Is
there an inherent rectification of the plasma?  For safety of our
equipment measurements are usually AC.  Has anyone performed  a DC
measurement of the ambient potential field referenced to ground?


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