Editing Electrometer (section)

=== Solid-state electrometers ===
The most modern electrometers consist of a [[solid state (electronics)|solid state]] [[amplifier]] using one or more [[field-effect transistor]]s, connections for external measurement devices, and usually a display and/or data-logging connections. The amplifier amplifies 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 [[diode]]s or [[ionization chamber]]s for [[ionising radiation]] measurement. The display or data-logging connections allow the user to see the data or record it for later analysis. Electrometers designed for use with ionization chambers may include a high-voltage power supply, which is used to bias the ionization chamber.

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 10<sup>14</sup> Ω). A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure currents as small as a few femtoamperes. Combined with an internal voltage source, the current measuring mode can be adapted to measure very high [[Electrical resistance|resistances]], of the order of 10<sup>17</sup> Ω. Finally, by calculation from the known [[capacitance]] of the electrometer's input terminal, the instrument can measure very small [[electric charge]]s, down to a small fraction of a picocoulomb.