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Hull Electrometer Evaluation

Subject:  Hull Electrometer Evaluation
  Date:   Sat, 10 May 1997 14:14:17 -0500 (CDT)
  From:  rwall-at-ix-dot-netcom-dot-com (Richard Wayne Wall)
    To:  tesla-at-pupman-dot-com


5/10/97


I recently built and tested the Hull Electrometer designed by R. Hull.

In a word, this simple little instrument is very sensitive, rugged, 
accurate and inexpensive to build.  This little instrument will open up 
a great deal of experimentation not only in electrostatics in general, 
but also in discovery of TC electrostatic characteristics.

Basically, the heart of the eletrometer is a LMC-6081 IC.  This is a 
precision CMOS op amp.  It is the most expensive part and costs about 
$6-8.  There is an additional LM-324N quad op amp IC for two successive 
stages and a low pass 2nd order Butterworth filter.  The unit is 
powered by two 9 V batteries.  

The collection terminal is builders choice as well as the measuring or 
monitoring device.  I used a 50-0-50 uA analog null meter that I had 
laying around.  One of those cheap $5 digital VMs would work well also. 
I believe RH uses a scope.  I use a 10" Al ball terminal supported on 
an 18" long by 1" diameter plexiglas 1/8" walled cylinder.  This whole 
thing is mounted on the top of a small metal housing containing the 
meter and circuit.  Actually, the batteries and circuit can be mounted 
inside the ball terminal with only a shielded lead out according to RH. 
I have also used a 6" Al ball terminal.  I used soldered pad perf board 
construction and I think RH originally used wire wrap.  The circuit is 
very easy to construct.

The precision CMOS op amp is an electrostatic impedance translator.  It 
converts electrostatic potential in realtime to EM which is filtered to 
exclude 60Hz noise and then the signal is measured by your choice of 
measurement device.  The instrument is calibrated on a volt for 
measured volt basis and is unity gain.  This is very nice feature in 
that an electrostatic potential and charge can be quantitated.

Just how sensitive is it?  VERY!!!  This thing is a winner.
Even in this humid environment on the St Johns River, I can detect a 
Teflon wiggle wand over twenty feet away.  Of course, the triboelectric 
charge imparted to Teflon is negative and the meter is right on.  I'm 
using the little 50-0-50 uA analog null meter and I can hardly see the 
meter needle at 20 feet!  But, the needle moves.   I have tested it 
with a 6" ball rather than the 10" and the sensitivity is decreased by 
over half, but I can see the meter easier at 10 feet. This morning 
while moving about the room I noticed the needle moving and when I 
stopped the needle held.  I slid my shoe a little on the carpet and the 
needle responded.  It was picking up my body capacitance about 12 feet 
away!  I also have install ed a switch and BNC connector for more 
remote monitoring.  I have not tested it remotely with a scope yet. 
Charles Yost, I'm told, has detected an electrostatic signal with this 
electrometer through a wall some 16 feet aw
ay!

A lot of us have requested a cheap, simple, accurate instrument to 
measure electrostatics in our experiments and thanks to Richard, here 
it is.  At one hundreth the price, it's not a Kiethly, but it sure gets 
the job done.  In my opinion RH has an excellent design here.  

RWW