Electrometer

An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid state technology can be used to measure extremely small currents down to 1 femtoampere.

Simple charge indicating device

The gold-leaf electroscope was one of the first instruments to indicate electric charge. It is still used for science demonstrations but has been superseded in most applications by electronic measuring instruments.

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The instrument consists of two thin leaves of foil suspended from an electrode. When the electrode is charged by induction or by contact, the leaves acquire similar electric charges and repel each other due to the Coulomb force. Their separation is a direct indication of the net charge stored on them.

Related Topics:
Electrode - Induction - Coulomb force

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The leaves may be enclosed in a glass envelope to protect them from draughts, and the envelope may be evacuated to minimize charge leakage. A further cause of charge leakage is ionizing radiation, so to prevent this, the electrometer must be surrounded by lead shielding.

Related Topics:
Evacuated - Radiation - Lead

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It should be noted that this type of electroscope purely acts as an indicator and not a measuring device. The Braun electroscope replaced the gold-leaf electroscope for more accurate measurements.

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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.

Related Topics:
Tantalum - Platinum - Monofilament

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The most common radiation measurement device, and widely used in the nuclear industry, the Quartz Fibre Electrometer (or QFE) personal dosimeter, is actually a ruggedized, calibrated electroscope. This uses the leakage effect mentioned above to detect ionising radiation.

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