The heart of the electroscope is the two junction FETs Ql and Q2 connected in a balanced-bridge circuit. The gate input of Ql is connected to the wire pick-up antenna, while Q2`s gate is tied to the circuit`s common ground through R2. That type of bridge circuit offers excellent temperature stability; therefore, Ql is allowed to operate in an open-gate configuration.
Potentiometer R7 is used to balance the bridge circuit, and R6 sets the maximum meter swing. Capacitors Cl and C2 help to reduce the 60-Hz pickup and add to the short-term stability of the circuit.