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2.65 Electrometer
Circuit 139: Electrometer with LED
Setup information
Install the two transistors, the diode, the ceramic capacitor and two resistors as shown in Fig. 2.139 The six
jumper wires come next followed by the jumper, which serves as a switch (shown dashed). For your guid-
ance, we labelled the "switch" with charge and discharge. Check the placement on the board again and
only then connect the battery.
Experiment
If everything is wired correctly, switch to charge. The red LED is dark. The ceramic capacitor is charged in
no time. This means that the capacitor's electrodes have accumulated as many electrons as possible. In our
case, we are talking about six 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 0.05 millimetres. That is not much power, and yet our circuit clearly shows this amount of charge.
Shift the switch to discharge. The red LED flashes only once, but effectively.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.139.
Components required:
2 x 2N3904 transistors,
1 x red LED, 2 x 1 kΩ resis-
tors, 1 x 10 nF ceramic
capacitor, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.139: Electrometer with
LED
165
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