Diagram left:
LED = Light Emitting Diode
Power = 9 Volt Battery
Switch = Rocker switch
FET = Field Effect Transistor
Resistor = 100 MOhms
Antenna = Extendable

We came across this circuit diagram which We felt may help on investigations particularly indoor ones.  The idea of this project originally was to make a negative ion detector which proved very complicated to build. However we did in the end come up with the electrometer. Whilst it can detect negative ions , we must emphasise it is not a precision piece of equipment relying on an LED (Light Emitting DIODE) to determine any reading’s. This is all down to the FET (Field Effect Transistor) , a type of transistor that  is able to detect electric fields.

Power runs from the battery when the switch is turned on to connect the circuit
The LED turns on when a positive charge approaches the antenna, and turns off when negative charge approaches. This is perhaps a very simplistic description of how the electrometer operates so in effect this circuit can detect e-fields (electric fields).