Science Fair Project Encyclopedia
An electrometer also known as an electroscope measures electric charge.
The standard electrometer is two thin leaves of gold foil suspended from an electrode. When the electrode is charged, the leaves repel each other and move apart. Their separation is a direct measure of the charge on the electrode. Another way to construct an electrometer is to suspend two aluminum or copper foil pieces on wire (tantalum or platinum is best), or (far less expensively) nylon monofilament. Charged electrometers are discharged by ionizing radiation. The most common radiation measurement device, the dosimeter, is actually a ruggedized, calibrated electrometer.
The term is also used to refer to a special type of thermionic valve. This particular type does not have a negative bias on its control grid as is more normal. Instead a tiny current is permitted to flow into the grid, and this is vastly amplified in the anode (plate) circuit. The best examples of these valves can detect currents as low as a few femtoamps 10-15 Amps. This type of valve can be ruined by handling with ungloved hands as the salts left on the glass envelope can provide an alternate path for these tiny currents.
They are of use in nuclear physics as they are able to amplify the tiny 'photo' currents created by radiation. They are, however, being supplanted by semiconductor devices.
- US1716700 -- Electroscope -- Richard D. Kleeman -- 1929
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