[volt-nuts] Bohnenberger electrometer

[ed breya]

There is another kind of static electric field meter that was commonly 
used over the past few decades for monitoring charges/voltages in work 
areas dealing with sensitive semiconductors. It has a small motor 
spinning a hollow brass cylinder that has a radial hole or slot that 
alternately shields and exposes a center cylinder inside, which is the 
pickup electrode. This action causes a small AC signal on the electrode, 
that can be amplified up to represent the electric field strength from 
any nearby object. The signal is then rectified and trips a comparator 
and LED indicator if the level exceeds a certain amount.

I have a couple of these units, but have never experimented with them 
yet. They don't show any kind of readout or provide a measuring signal - 
just the LED warning of excessive (unknown trip point) static charge 
nearby. I figured someday I would modify one up and add a signal output 
port and a sync output from the motor, allowing a lock-in analyzer to 
read the result over a wide range, and maybe even be fairly accurate or 
calibrate-able.

Ed