At 03:25 04/06/96 -0600, you wrote:
>From MALCOLM-at-directorate.wnp.ac.nzMon Jun 3 22:26:54 1996
>Date: Tue, 4 Jun 1996 13:26:02 +1200
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>Subject: Re:Humid atmosphere
> On your variable visitor tuning.....
>> I did some checking ... I could not find any resonant frequency
>> within the range of my signal generator, until the visitor went
>Can you post the specs on your coil please? I'd be very interested
>in things like dimensions, resonant frequency, terminal size etc.
>The low capacitance of these systems does make them susceptible to
>the proximity of nearby objects, but 15' is rather a long way away.
>I think you may have a bad grounding problem.
The coil is wound on a 5ft 8" high polyetheleyne sewer pipe of 2ft diameter.
Primary is a flat 2mm thick 3inch wide copper strip wound ten times at
1/2inch spacing. The output terminal is a tin cooking pot. The round bottom
Indian type pots used for catering purposes which has a diameter of
22inches. (It was the biggest they make and only cost $15 US.) The spark gap
is a variable speed rotary gap. Capacitor is 0.1mfd.
>The secondary resonant frequency, measured by connecting the frequency
generator through a 1k resistor to the base of the secondary and measuring
from the base to ground with the oscilloscope, was around 85Khz when I was
alone. I did not understand the changes on the oscilloscope when the second
body approached. First the trace goes to the frequency of the signal
generator, then as the body gets nearer a second trace appears, inbetween
the first as if the frequency has doubled. (Is this possibly a higher
harmonic frequency of the original?) It is not stable and varies in
frequency but not amplitude. After that there are many traces visable at
about 15 ft and I could not even tell which was the original or if it was
even still there. The scope can go up to 3Mhz but still read multiple criss
cross lines no matter what frequency you fed in from the signal generator.
As the body moves away the reverse happens and the resonance returns to the
original frequency. The coil was standing on a 4ft high wooden stage about
20ft square and a 10ft high grounded wire fence was around the outer edge of
the stage to protect the coil from the visitors. The body was outside the
grounded wire fence.
After reading what I have just written it is clear I have missed something
as none of it makes any sense. I assumed it was the high conductivity of the
air causing the problem but it may be something as simple as the damp wooden
stage conducting from the base of the secondary to the cage. I never did
find the problem and I have tried the baby coil (14") in the jungle
conditions but still get greatly reduced arcs, (4 inches) which increase in
length in the towns air conditioned trade centre.
This may not be linked but at the National Science Centre here we had some
Australians fit all the exhibits with capacitance type touch switches. That
was 360 switches. The type that are glued to the back of the exhibit panel
and work through the wood or plastic when you touch the outside of the
panel. These switches were all installed and worked perfectly when adjusted
at 6pm. Next morning ALL 360 switches were not working and all had to be
readjusted. This was finished by ten staff by 10am. By 1.00pm after lunch
they were all not working again and all had to be readjusted. By 6.00pm they
were off again. This happened every day and ALL 360 switches were removed
but the Australians could find no fault with them. Maybe whatever changes
the capacitance of the switches is effecting the Tesla coil?
PAUL (Why does nothing work round here) MILLOTT.