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Circuit 140: Electrometer with moving-coil instrument
Setup information
Install the two transistors, the ceramic capacitor and two resistors as shown in 2.140. The seven jumper
wires come next followed by the jumper, which serves as a switch (shown dashed). For your guidance, we
labelled the "switch" with charge and discharge. Last but not least, connect the moving-coil instrument.
Check the placement on the board again and only then connect the battery.
Experiment
If everything is wired correctly, switch to charge. The moving-coil instrument needle is not moving. In no
time the ceramic capacitor is fully charged. This means that the capacitor's electrodes have accumulated
as many electrons as possible. In our case, we are talking about one hundred billion of electrons This is a
sheer inconceivable number, and yet, all these electrons can flow through your skin without causing you
any harm. You won't feel anything at all. The electrical energy contained in these electrons would just be
enough to lift a mass of one thousandth of a gram or one thousandth of a millimetre. That is not much
power, and yet our circuit clearly shows this charging amount. Shift the switch to discharge. The needle of
the moving-coil meter moves a tiny bit.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.140.
166
Components required:
2 x 2N3904 transistors,
1 x 1 kΩ resistor,
1 x 4.7 kΩ resistor,
1 x 10 nF ceramic capaci-
tor, 1 x moving-coil instru-
ment, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.140: Electrometer with
moving-coil instrument
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