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An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges.


Older electrometers and charge indicating devices

Gold-leaf electroscopes

The gold-leaf electroscope was one of the first sensitive instruments used to indicate electric charge. It is still used for science demonstrations but has been superseded in most applications by electronic measuring instruments. The instrument consists of two thin leaves of gold 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. 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. This type of electroscope usually acts as an indicator and not a measuring device, although it can be calibrated. The Braun electroscope replaced the gold-leaf electroscope for more accurate measurements.

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. It uses the leakage effect mentioned above to detect ionizing radiation.

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