Electrometer
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An electrometer also known as an electroscope measures electric charge.
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Simple Charge Measurement Device
Electroscope.png
The gold-leaf electrometer is two thin leaves of gold foil suspended from an electrode in a vacuum. 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. The vacuum is desirable in order to minimize charge leakage. 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.
Valves
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 conventionally used in valves. 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 have, however, been supplanted by semiconductor devices in modern electrometers.
Modern electrometers
Modern electrometers consist of an amplifier circuit, connections for external measurement devices, and also possibly a display, data-logging connections, and/or a high voltage supply. The solid-state amplifier circuit does the job of the vacuum tube era valve discussed above, amplifying small currents so that they are more easily measured. The external connections are usually of a co-axial or tri-axial design, and allow attachment of diodes or ionization chambers for radiation measurement. The display or data-logging connections allow the user to see the data or record it for later analysis. The high voltage supply is an integral power supply which is used to supply a voltage to a ionization chamber or diode.
The most common use for modern electrometers is the measurement of radiation with ionization chambers.
See also
External links
- Template:US patent -- Electroscope -- Richard D. Kleeman -- 1929
- Build this FET electrometer (http://www.amasci.com/emotor/chargdet.html) - A very simple circuit - 2 components
- Simple FET electrometer (http://www.vk2zay.net/article.php/9) - A simple bridged circuit
- An opamp electrometer (http://amasci.com/electrom/sas51p1.html#Electro)de:Elektroskop