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Siemens | |
---|---|

Unit system | SI derived unit |

Unit of | Electric conductance |

Symbol | S (= Ω^{−1}) |

Named after | Ernst Werner von Siemens |

In SI base units: | kg ^{−1}⋅ m ^{−2}⋅ s ^{3}⋅ A ^{2} |

The **siemens** (symbol: **S**) is the derived unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is redundantly equal to the reciprocal of one ohm, and is also referred to as the * mho *. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.^{ [1] }

The unit is named after Ernst Werner von Siemens. In English, the same form *siemens* is used both for the singular and plural.^{ [2] }

For a conducting element, electrical resistance *R* and electrical conductance *G* are defined as

where *I* is the electric current through the object and *V* is the voltage (electrical potential difference) across the object.

The unit **siemens** for the conductance *G* is defined by

where Ω is the ohm, A is the ampere, and V is the volt.

For a device with a conductance of one siemens, the electric current through the device will increase by one ampere for every increase of one volt of electric potential difference across the device.

The conductance of a resistor with a resistance of five ohms, for example, is (5 Ω)^{−1}, which is equal to 200 mS.

A name that is used as an alternative to the *siemens* is the * mho* /moʊ/ , the reciprocal of one ohm. It is derived from spelling

NIST's *Guide for the Use of the International System of Units (SI)* refers to the mho as an "unaccepted special name for an SI unit", and indicates that it should be strictly avoided.^{ [5] }

The SI term *siemens* is used universally in science and often in electrical applications, while *mho* is still used in some electronic contexts. The inverted capital omega symbol, while not an official SI abbreviation, is less likely to be confused with a variable than the letter S when doing algebraic calculations by hand, where the usual typographical distinctions (such as italic for variables and Roman for unit names) are difficult to maintain. Likewise, it is difficult to distinguish the symbol *S* from the lower-case *s* where *second* is meant, potentially causing confusion.^{ [6] } So, for example, a pentode’s transconductance of 2.2 mS might alternatively be written as 2.2 or 2200 (most common in the 1930s) or 2.2 mA/V. A handwritten "S" can also be misread as the frequency space variable "s", commonly used in transfer functions.

- ↑ Minutes of the 14. General Conference on Weights and Measures, 1971, page 78
- ↑
*NIST Guide to the SI, Chapter 9: Rules and Style Conventions for Spelling Unit Names*, National Institute of Standards and Technology, 2008, retrieved 2017-12-22 - ↑ Maver, William: American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation. 1903.
- ↑ "Siemens (Unit of Electrical Conductance)".
*www.tech-faq.com*. - ↑
*NIST Guide to the SI, Chapter 5: Units Outside the SI*, National Institute of Standards and Technology, 2008, retrieved 2017-12-22 - ↑ Eugene R. Weiner,
*Applications of Environmental Aquatic Chemistry: A Practical Guide*, p. 109, CRC Press, 2013 ISBN 1439853320

The **ampere**, often shortened to "amp", is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

In physics, **power** is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt (W) in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower ; 1 horsepower equals about 745.7 watts. Being the rate of work, the equation for power can be written as:

The **volt** is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. It is named after the Italian physicist Alessandro Volta (1745–1827).

The **coulomb** is the International System of Units (SI) unit of electric charge. It is the charge transported by a constant current of one ampere in one second:

**Ohm's law** states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

The **electrical resistance** of an object is a measure of its opposition to the flow of electric current. The inverse quantity is **electrical conductance**, and is the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).

The **henry** is the SI derived unit of electrical inductance. If a current of 1 ampere flowing through the coil produces flux linkage of 1 weber turn, the coil has a self inductance of 1 henry. The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

Components of an electrical circuit or electronic circuit can be connected in series, parallel, or series-parallel. The two simplest of these are called **series** and **parallel** and occur frequently. Components connected in series are connected along a single conductive path, so the same current flows through all of the components but voltage is dropped (lost) across each of the resistances. In a series circuit, the sum of the voltages consumed by each individual resistance is equal to the source voltage. Components connected in parallel are connected along multiple paths so that the current can split up; the same voltage is applied to each component.

In electrical engineering, **admittance** is a measure of how easily a circuit or device will allow a current to flow. It is defined as the reciprocal of impedance. The SI unit of admittance is the siemens ; the older, synonymous unit is mho, and its symbol is ℧. Oliver Heaviside coined the term *admittance* in December 1887.

In electrical engineering, **susceptance** (*B*) is the imaginary part of admittance, where the real part is conductance. The inverse of admittance is impedance, where the imaginary part is reactance and the real part is resistance. In SI units, susceptance is measured in siemens. Oliver Heaviside first defined this property in June 1887.

**Acoustic impedance** and **specific acoustic impedance** are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal second per cubic metre or the rayl per square metre, while that of specific acoustic impedance is the pascal second per metre or the rayl. In this article the symbol rayl denotes the MKS rayl. There is a close analogy with electrical impedance, which measures the opposition that a system presents to the electrical flow resulting from an electrical voltage applied to the system.

In physics, the **weber** is the SI derived unit of magnetic flux. A *flux density* of one Wb/m^{2} is one tesla.

A **magnetic circuit** is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path. Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays, lifting electromagnets, SQUIDs, galvanometers, and magnetic recording heads.

The **ohm** is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect.

The **impedance of free space**, *Z*_{0}, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, *Z*_{0} = |**E**|/|**H**|, where |**E**| is the electric field strength and |**H**| is the magnetic field strength. Its presently accepted value is

A **conventional electrical unit** is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as Δ*ν*_{Cs} used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol *V*_{90} – as they came into international use on 1 January 1990.

The **watt** is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by .

The **gyrator–capacitor model** - sometimes also the capacitor-permeance model - is a lumped-element model for magnetic fields, similar to magnetic circuits, but based on using elements analogous to capacitors rather than elements analogous to resistors to represent the magnetic flux path. Windings are represented as gyrators, interfacing between the electrical circuit and the magnetic model.

The **motor size constant** and **motor velocity constant** are values used to describe characteristics of electrical motors.

Performance modelling is the abstraction of a real system into a simplified representation to enable the prediction of performance. The creation of a model can provide insight into how a proposed or actual system will or does work. This can, however, point towards different things to people belonging to different fields of work.

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