Re: Finally a Field Mill!

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Richard Hull (I believe.. it might have been Steve Roys) published a design
for an electrostatic field probe of comparable sensitivity using some very
very low leakage op-amps inside an appropriate shielded can, etc.

The problem with field mills and op-amp field probes is their frequency
response.   A TC is at 100 kHz, so I don't know that you could measure very
well.

However, you might be able to use a field mill type probe with a known DC
voltage on the topload to generate a calibration curve for another sort of
probe.  Somehow, you'd need to account for the field due to the secondary,
though, and for DC you won't get the nice linear curve of voltage you
want... (Maybe, run a wire up the center of the secondary to draw some
current through the secondary (milliamps)...


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, June 17, 2003 11:18 AM
Subject: Finally a Field Mill!


 > Original poster: "Charles Brush by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <cfbrush-at-interport-dot-net>
 >
 > Hi everyone,
 >
 > One of the topics that naturally comes up on this list from time to time
is
 > the measurement of electric fields.  A couple of years ago there was a
very
 > interesting thread about using x-ray tubes for voltage measurements of a
 > running Tesla coil (I think Dr. Resonance or someone tried this....in a
 > ditch for shielding!).  One of the devices which naturally comes up in any
 > measurement discussion is the field mill or "generating voltmeter", and
 > these are devices I have always been interested in.  For those unfamiliar
 > with this device, in a nutshell it uses a series of pie-shaped sensor
 > plates that are alternately shielded and exposed by a grounded rotor.  The
 > idea is that as the plates are exposed to an electric field, induction
will
 > charge them to a certain value, current will flow from the plate to an
 > amplifier (on its way to ground), and then as the plate is shielded the
 > current will cease providing an AC signal that is directly proportional to
 > the electric field the unit is exposed to. Most mills also have a second
 > set of vanes that interrupt a beam of light so that the unit can tell the
 > orientation of the rotor and determine a negative or positive field value.
 >
 > For years now I have been thinking of building one of these to investigate
 > some of the residual electrostatic fields created by Tesla coils, but like
 > many things I have never gotten around to it. I have also never been able
 > to find one surplus despite much searching......that is until a couple of
 > weeks ago when I came across a really interesting unit in excellent
working
 > condition, for about $60 which was several thousand below list.  It is
 > basically a hand held field mill designed for industrial applications,
with
 > a clockwork mechanism that powers the rotor, and a rechargeable battery
for
 > the amplifier circuit.  The clockwork mechanism is used instead of a motor
 > because the unit is designed for use in a potentially explosive
 > atmosphere.  The best thing is that it has selectable scales of 10kV/m,
 > 100kV/m, and 1,000kV/m, though I sure wouldn't want to be anywhere near a
 > 1mV/m field in an explosive atmosphere!
 >
 > Anyway I thought this was a really  fascinating device and that people
 > might like to see what it looks like, so I posted some images to a
 > temporary directory on my web site:
 >
 > http://www.voltnet-dot-com/temp/fieldmill/fieldmill_4.JPG
 >
 > http://www.voltnet-dot-com/temp/fieldmill/fieldmill_1.JPG
 >
 > http://www.voltnet-dot-com/temp/fieldmill/fieldmill_2.JPG
 >
 > http://www.voltnet-dot-com/temp/fieldmill/fieldmill_3.JPG
 >
 > Once I get my big coil set up for the summer it should be very interesting
 > trying out this mill.  It would probably be good to shield the back of the
 > case to avoid damage when the coil is running however.  The mill itself is
 > set in a  sort of virtual ground plane that is connected to the user
 > through the handle, with the user's body basically providing a
 > counterpoise.  Continuing the ground around the back (and actually
 > grounding the unit) presumably should not alter measurements.  I also have
 > an electrostatic voltmeter, and it will be interesting to see how its
 > results compare to those of the field mill.    Anyway I'd certainly be
 > interested in any comments!  I know of some good field mill plans and
other
 > info online if anyone else is interested in this stuff.
 >
 >
 > Zap!
 >
 >
 > Charles Brush
 > http://www.VoltNet-dot-com
 >
 >
 >