[Prev][Next][Index][Thread]

Re: Displacement Current Revisited



On 03/30/99 17:43:44 you wrote:
>
>Original Poster: "John H. Couture" <COUTUREJH-at-worldnet.att-dot-net> 
>
>
>  Richard W. -
>
>  Thank you for doing the compass test. To my knowledge no one has ever made
>this test before now. As Ed  points out it is a known fact that there is a
>DC bias in the operating Tesla coil. The question is whether this bias is
>due to currents in the secondary or due to rectification in the discharge of
>a spark. Is this bias due to an unsymmetrical waveform?

I haven't read much about the TC having a known DC bias as a "known fact".  I 
suspect the retrospectoscope is being employed here with 20-20 accuracy. %^)
I've noticed most coilers have a built in aversion to considering the TC as a 
hybrid machine with both AC and DC characteristics.  However, more and more
are slowly coming to the viewpoint that TCs are incredibly complex in their 
function and there is much to learn from and about them.
 
I point out that the above rectification would also produce a net DC current 
in the secondary.  Unsymmetrical wave form?  These secondary waves are damped 
sinusoidal waveforms.  Terry has some beautiful examples of these waveforms 
from his coils.  I guess someone could intregrate the areas of the wave form 
above and below RF ground looking for asymmetry.  I'm not a math wizard 
though, so I'll pass.
 
>
>  It is my belief that the instanstaneous currents in the secondary winding
>are sufficient to affect a compass. The next question is whether the
>polarity at the secondary terminal is positive or negative. Tests made to
>date have been contraditory.
>
>  John Couture

What is the source of the instantaneous currents in the secondary winding 
and how would they affect the compass?  The compass has a lot of mechanical 
inertia and low frequency response.  It will not respond to an instantaneous
current.  Only a net DC current and magnetic field.

Ah! the old polarity of the secondary terminal issue.  I read your post last 
year about how to measure the polarity of a TC.  If recollection serves me 
correctly you used about two or three stages in in your instrumentation to 
judge polarity.  Too much room for error in my opinion.  I suggest you use
your published fet electrometer to evaluate the electrostatic fields around
the secondary.  It's much simpler, easier and accurate. 

My experiments indicate that ES fields around a disruptive TC are not 
necessarily symmetrically arranged around the terminal of a TC.  There 
is demarkation around a breakout point with negative polarity on one side
and positive polarity on the other side.  The polarity around the spark is 
the opposite of that around the rest of the terminal.  Spark has a huge 
bearing on polarity.  If polarity around a terminal were uniform there would 
probably be just an even brush discharge rather than active individual sparks. 


So terminal polarity, like so many tesla coil characteristics, is a very 
complex problem.  There are many variables such as polarity reference, 
geometry and presence of discharge sparks or lack thereof.  Instrumentation 
is also crucual.  My polarity experiments were carried out with a 
Keithley 621R a Keithley electrostatic head.


RWW