Electroscope

The electroscope pictured in Fig. 1 can be used to display static energy charges from sources such as TV sets, electrostatic generators, carpet cruising, and hair combing. The electroscope is the sophisticated cousin of the static-electricity detector shown in Fig. 1-9. The electroscope would make an excellent science-fair project or addition to your electronics bench.
The heart of the electroscope circuit is the two FETs, Ql and Q2, connected in a balanced bridge configuration. The gate of Ql is connected to the wire pick-up antenna via a 1.5-Ω resistor, and the gate of Q2 is tied to the circuit's common ground through the other 1.5-Ω resistor. This type of bridge circuit offers excellent temperature stability. Ql operates in an open-gate configuration. The 500-Ω potentiometer balances the null bridge circuit. The 5-kΩ potentiometer and capacitors Cl and C2 help reduce stray 60-Hz pickup and increase the stability of the circuit. The 1-mA meter connected between the drain pins of Ql and Q2 indicates an electrostatic field. The electroscope requires little current consumption, and therefore, it can be operated from a 9-V tran-sistor-radio battery.

Electroscope parts list
Quantity Part Description
2 R1,R7 1.5-Ω, ¼-W resistor
2 R2,R5 2.2-kΩ, ¼-W resistor
1 R3 5-kΩ, ¼-12-Watt potentiometer
1 R4 680-Ω, ¼-W resistor
1 R6 500-Ω, ¼-Watt potentiometer
2 Cl,C3 39-pF, 25-V capacitor
1 C2 100-μF, 25-V electrolytic capacitor
2 Q1,Q2 2N4342 FET
1 M 0-1-mA panel meter
1 SW-1 SPST toggle switch
1 ANT Telescoping whip antenna
1 BATT 9-V transistor-radio battery

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