Finally a Field Mill!

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

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