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.

Modern electrometers

In modern parlance, an electrometer is a highly sensitive electronic voltmeter whose input impedance is so high that the current flowing into it can be considered, for practical purposes, to be zero. They are of use in nuclear physics as they are able to amplify the tiny 'photo' currents created by radiation. The most common use for modern electrometers is probably the measurement of radiation with ionization chambers

Related Topics:
Voltmeter - Input impedance - Nuclear physics

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Valve electrometers

Valve type electrometers use an unbiased vacuum tube with a very high gain (mutual conductance) and input resistance. The input current is allowed to flow into the high impedance grid, and the voltage so generated is vastly amplified in the anode (plate) circuit. The best examples of these valves can detect currents as low as a few femtoamperes (10-15 amperes). This type of valve can be ruined by handling with ungloved hands as the salts left on the glass envelope can provide leakage paths for these tiny currents.

Related Topics:
Gain - Mutual conductance - Anode - Plate

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Solid-state electrometers

The most modern electrometers consist of a solid-state amplifier circuit using FETs, connections for external measurement devices, and also possibly a display, data-logging connections, and/or a high-voltage supply. The amplifier magnifies 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 the ionization chamber or diode.

Related Topics:
Solid-state - Amplifier - FET - Diode - Ionization chamber - Radiation

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Solid-state electrometers are often multipurpose devices that can measure voltage, charge, resistance and current. They measure voltage by means of "voltage balancing", in which the input voltage is compared with an internal reference voltage source using an electronic circuit with a very high input impedance (of the order of 1014 ohms). A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure tiny currents of the order of one femtoampere. Combined with an internal voltage source, the current-measuring mode can be adapted to measure very high resistances, of the order of 1017 ohms. Finally, by calculation from the known capacitance of the electrometer's input terminal, the instrument can measure very small electric charges, down to a small fraction of a picocoulomb, on the terminal. http://www.keithley.com/data?asset=11894

Related Topics:
Resistance - Capacitance - Electric charge

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