Static Electricity / Negative Ion Detektor

This circuit relies upon the extra high input impedance of a FET, and also demonstrates
the gate terminals sensitivity to changes in voltage.  The gate terminal here is left open circuit,  connected only to the "probe" this being just a few inches of bare copper wire. With no fixed DC biasing, the  gate terminal  will respond to micro  changes in voltage or  "field  strength".  It is  important not to make this circuit on veroboard or PCB  material as this will reduce the effective gate impedance. Instead  use an "open" construction  technique soldering  each component together. The probe should not be touched directly and is best insulated in a plastic pen sleeve.
As static  electricity can   have either a  positive or negative charge, the meter used should be a centre zero type. Full scale deflection can be 1mA or 250uA for greater sensitivity. Remove the meter and use a multimeter to measure the voltage between FET drain and the preset  resistor. Adjust  the preset for 0 volts  and then replace  the meter. This  will  avoid "bending" the needle.

If  placed in a  room the  meter  will detect  changes in static  charge, positive  charge deflecting  the needle  one way and negative  the other way.  You can test  the circuit by placing the unit say 5 feet away from a TV set. When switched on, the meter needle should jump to full scale deflection and then drop down again. If you have a room ioniser, its output can be monitored by moving the probe in front of it. As the detector responds to  changes in charge, you may need to move the detector around to see the effect, but it will prove the output from such an ioniser.