The following circuit is an electronic approximation of the ‘gold leaf electroscope’, except that this electrometer indicates polarity as well as electric field magnitude. It is incredibly sensitive! It will detect a television or an electrostatically charged comb from the other side of a room. It can even ‘see’ people moving about!
The neon bulb serves two purposes. It provides leakage at the FET gate and also helps to defend the FET against an electrostatic discharge. Do not omit the neon bulb – the FET won’t get the proper biasing without it. Ideally, the neon bulb should be in darkness, though the circuit seems to work okay with the neon bulb illuminated. The only effect is that the meter ‘autozero’ has a much shorter time constant when the neon bulb is illuminated.
The meter can be of the ‘VU’ variety though meters in the range 100 μA to 1 mA can be made to work. You may need to change the value of the adjustable resistor if a meter of widely different FSD to that specified is employed. A source of small, cheap meter movements is those cheap battery testers – their meters tend to be in the range 0.5 mA to 1 mA.
You won’t be able to walk around with this electrometer – the needle will just kick between its end stops if you try. Instead, it should be operated on a firm surface and left to settle for a minute or two. Adjustment of the ‘METER CENTRE’ adjustment is inevitably a trial and error affair, as bringing your hand near the electrometer will probably alter its reading.
The probe can be just a few inches of wire. The impressive looking probe on my meter is nothing more than a redundant adapter from an emergency mobile phone battery! I provided an earth connection through the body of the switch in anticipation that I might need to use my hand to remove excessive charge from the probe, but I haven’t actually needed to do that yet.
