Editing Electrometer (section)

== Modern electrometers ==
A modern electrometer is a highly sensitive electronic [[voltmeter]] whose [[input impedance]] is so high that the current flowing into it can be considered, for most practical purposes, to be zero. The actual value of input resistance for modern electronic electrometers is around 10<sup>14</sup>Ω, compared to around 10<sup>10</sup>Ω for nanovoltmeters.<ref>Keithley, Making precision low current and high resistance measurements, "A greater measure of confidence" brochure, 2011, page 8</ref> Owing to the extremely high input impedance, special design considerations (such as [[driven shield]]s and special insulation materials) must be applied to avoid leakage current.

Among other applications, electrometers are used in [[nuclear physics]] experiments as they are able to measure the tiny charges left in matter by the passage of [[ionizing radiation]]. The most common use for modern electrometers is the measurement of radiation with ionization chambers, in instruments such as [[geiger counter]]s.<ref>{{Cite journal |last=Glasser |first=Otto |title=THE ROLE OF THE GEIGER COUNTER IN A MEDICAL INSTITUTION |url=https://www.ccjm.org/content/ccjom/18/3/207.full.pdf |journal=Cleveland Journal of Medicine |pages=207|pmid=14848971 }}</ref>

=== Vibrating reed electrometers ===
[[Reed (music)|Vibrating reed]] electrometers use a variable [[capacitor]] formed between a moving electrode (in the form of a vibrating reed) and a fixed input electrode. As the distance between the two electrodes varies, the capacitance also varies and electric charge is forced in and out of the capacitor. The [[alternating current]] signal produced by the flow of this charge is amplified and used as an analogue for the [[Direct current|DC]] voltage applied to the capacitor. The DC input resistance of the electrometer is determined solely by the leakage resistance of the capacitor, and is typically extremely high, (although its AC input [[Electrical impedance|impedance]] is lower).

For convenience of use, the vibrating reed assembly is often attached by a cable to the rest of the electrometer. This allows for a relatively small unit to be located near the charge to be measured while the much larger reed-driver and amplifier unit can be located wherever it is convenient for the operator.<ref>[https://frank.pocnet.net/sheets/009/x/XL7900.pdf XL7900 Vibrating capacitor data sheet]</ref>

=== Valve electrometers ===
Valve electrometers use a specialized [[vacuum tube]] (thermionic valve) with a very high [[Gain (electronics)|gain]] ([[transconductance]]) 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 electrode|plate]]) circuit. Valves designed for electrometer use have leakage currents as low as a few femtoamperes (10<sup>−15</sup> amperes). Such valves must be handled with gloved hands as the salts left on the glass envelope can provide leakage paths for these tiny currents.<ref>{{Cite web |url=http://www.mif.pg.gda.pl/homepages/frank/sheets/138/5/5889.pdf |title=CK5889 Electrometer pentode data sheet |access-date=2016-08-29 |archive-date=2016-09-20 |archive-url=https://web.archive.org/web/20160920182129/http://www.mif.pg.gda.pl/homepages/frank/sheets/138/5/5889.pdf |url-status=dead }}</ref>

In a specialized circuit called ''inverted triode'', the roles of anode and grid are reversed. This places the control electrode at a maximum distance from the space-charge region surrounding the filament, minimizing the number of electrons collected by the control electrode, and thus minimizing the input current.<ref>{{cite book |title=Inverted Triode for Industrial Measurements, Manual of Industrial Circuits Electronics |publisher=[[Electronics (magazine)|Electronics]] |date=December 1944 |page=176}}</ref>

=== 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.