Electrometer

Electrometer

Overview
An electrometer is an electrical
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 instrument for measuring electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 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
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 or solid-state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere.
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Encyclopedia
An electrometer is an electrical
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 instrument for measuring electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 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
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 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
Electroscope
An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert...

, 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
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

. When the electrode is charged by induction
Electrostatic induction
Electrostatic induction is a redistribution of electrical charge in an object, caused by the influence of nearby charges. Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generators, such as the Wimshurst machine, the...

 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
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 to minimize charge leakage. A further cause of charge leakage is ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

, so to prevent this, the electrometer must be surrounded by lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

 shielding. This type of electroscope usually acts as an indicator and not a measuring device, although it can be calibrated. The Braun
Braun (company)
' , formerly Braun AG, is a German consumer products company based in Kronberg, Germany.From 1984 until 2005, Braun was a wholly owned subsidiary of The Gillette Company, which had purchased a controlling interest in the company in 1967...

 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
Dosimeter
Dosimeters measure an individual's or an object'sexposure to something in the environment — particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime...

, is actually a ruggedized, calibrated electroscope. It uses the leakage effect mentioned above to detect ionizing radiation.

The Kearny Fallout Meter
Kearny Fallout Meter
The Kearny Fallout Meter or KFM is an expedient radiation meter designed to be able to be constructed immediately before or during a nuclear attack by someone with a normal mechanical ability and from common household items. It also is a science project demonstrating the effect of ionizing...

 works the same way, though it uses aluminum foil rather than gold.

The instrument was developed in the 18th century by several researchers, among them Abraham Bennet
Abraham Bennet
Abraham Bennet FRS was an English clergyman and physicist, the inventor of the gold-leaf electroscope and developer of an improved magnetometer...

 and Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

.

Repulsion electrometers


Based on the principle of gold-leaf electroscope, those are more sensitive, indicating devices using a form of torsion balance. The most known designs are named after the inventors: Dellmann, Peltier
Peltier
Peltier is a French surname and may refer to:* Jean Charles Athanase Peltier , French physicist, documented the Peltier effect* Lee Peltier , English football player...

, Bohenberger and others. The Peltier electrometer, developed by the Jean Charles Peltier, is using a form of magnetic compass to measure the deflection by balancing the electrostatic force and a magnetic needle.

Attraction electrometers


Also known as Attracted-Disk Electrometers, are sensitive balances measuring the attraction between charged disks. Snow Harris is credited with the invention of this instrument and it was improved by Lord Kelvin.

Quadrant electrometers


Developed by Lord Kelvin, this is the most sensitive and accurate of all the mechanical electrometers. The original uses a light aluminum sector suspended inside a drum cut into four segments. The segments are insulated and connected diagonally in pairs. The charged aluminum sector is attracted to one pair of segments and repelled from the other. The deflection is observed by a beam of light reflected from a small mirror attached to the sector, just as in a galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...

. The engraving on the right shows a slightly different form of this electrometer using four flat plates rather than closed sectors. The plates can be connected externally in the conventional diagonal way (as shown), or in a different order for specific applications.

A more sensitive form of quadrant electrometer was developed by Frederick Lindemann. It employs a metal-coated quartz fiber instead of the aluminum sector. The deflection is measured by observing the movement of the fiber through a microscope. Initially used for measuring star light, it was employed for the infrared detection of airplanes in the early stages of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

.

Some of the electrometers were housed inside a cage often referred to as “bird cage”. This is a form of Faraday Cage
Faraday cage
A Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks out external static and non-static electric fields...

 that protected the instrument from external electrostatic charges.

Modern electrometers


In modern parlance, an electrometer is a highly sensitive electronic voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...

 whose input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...

 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 1014Ω, for instance compared to around 1010Ω for nanovoltmeters.

Among other applications, they are of use in nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

 as they are able to measure the tiny charges left in matter by the passage of ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

. The most common use for modern electrometers is probably the measurement of radiation with ionization chambers, in instruments such as geiger counter
Geiger counter
A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a...

s.

Vibrating reed electrometers


Vibrating reed electrometers use a variable capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 formed between a moving electrode (in the form of a vibrating reed) and a fixed input electrode. As the distance between the two electrodes varied, the capacitance also varies and electric charge is forced in and out of the capacitor. The ac
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 signal produced by the flow of this charge is amplified and used as an analogue for the dc
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 voltage applied to the capacitor. The input resistance of the electrometer is determined solely by the leakage resistance of the capacitor (although its ac input impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

 is clearly less than infinity).

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.

Valve electrometers


Valve type electrometers use an unbiased vacuum tube with a very high gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 (transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...

) 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
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 (plate
Plate electrode
A plate is a type of electrode that formed part of a vacuum tube. The plate is impressed with a positive charge so that it may capture and flow electrons within a circuit....

) circuit. The best examples of these valves have leakage 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.

Solid-state electrometers


The most modern electrometers consist of a solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

 circuit using FET
Fet
Fet is a municipality in Akershus county, Norway. It is part of the Romerike traditional region. The administrative centre of the municipality is the village of Fetsund.Fet was established as a municipality on 1 January 1838...

s, 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 diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

s or ionization chamber
Ionization chamber
The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...

s for radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 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.

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 resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

s, of the order of 1017 ohms. Finally, by calculation from the known capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 of the electrometer's input terminal, the instrument can measure very small electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

s, down to a small fraction of a picocoulomb, on the terminal. http://www.keithley.com/data?asset=11894

See also

  • Electrical measurements
    Electrical measurements
    Electrical measurements are the methods, devices and calculations used to measure electrical quantities. Measurement of electrical quantities may be done to measure electrical parameters of a system...

  • Electron
    Electron
    The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

  • Radiation
    Radiation
    In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

  • Faraday cup electrometer
    Faraday cup electrometer
    The Faraday cup electrometer is the simplest form of an electrical aerosol instrument used in aerosol studies. It consists of an electrometer and a filter inside a Faraday cage...


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