# Centimetre–gram–second system of units

CGScgs unitsCGS unitCGS systemcentimetre–gram–secondCGS system of unitscentimetre gram second system of unitscentimeter gram second system of unitscentimeter-gram-secondcentimeter–gram–second
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.wikipedia
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### Gaussian units

Gaussiancgs-Gaussian unitsGaussian-cgs units
Furthermore, within CGS, there are several plausible choices of electromagnetic units, leading to different unit "sub-systems", including Gaussian units, "ESU", "EMU", and Lorentz–Heaviside units.
This system is the most common of the several electromagnetic unit systems based on cgs (centimetre–gram–second) units.

### Dyne

dyndyne-centimeters
For example, the CGS unit of force is the dyne which is defined as 1 g⋅cm/s 2, so the SI unit of force, the newton (1 kg⋅m/s 2), is equal to 100,000 dynes.
The dyne (symbol dyn, from Greek δύναμις, dynamis, meaning power, force) is a derived unit of force specified in the centimetre–gram–second (CGS) system of units, a predecessor of the modern SI.

### International System of Units

SISI unitsSI unit
The CGS system has been largely supplanted by the MKS system based on the metre, kilogram, and second, which was in turn extended and replaced by the International System of Units (SI).
The motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them.

### Pressure

water pressurenegative pressurefluid pressure
In measurements of purely mechanical systems (involving units of length, mass, force, energy, pressure, and so on), the differences between CGS and SI are straightforward and rather trivial; the unit-conversion factors are all powers of 10 as 100 cm = 1 m and 1000 g = 1 kg.
The CGS unit of pressure is the barye (Ba), equal to 1 dyn·cm −2, or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre (g/cm 2 or kg/cm 2 ) and the like without properly identifying the force units.

### MKS system of units

MKSMKS systemmetre–kilogram–second system
The CGS system has been largely supplanted by the MKS system based on the metre, kilogram, and second, which was in turn extended and replaced by the International System of Units (SI).
Adopted in 1889, use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering.

### Energy

energy transferenergiestotal energy
In measurements of purely mechanical systems (involving units of length, mass, force, energy, pressure, and so on), the differences between CGS and SI are straightforward and rather trivial; the unit-conversion factors are all powers of 10 as 100 cm = 1 m and 1000 g = 1 kg.
The CGS energy unit is the erg and the imperial and US customary unit is the foot pound.

### Gram

ggramsgr
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.
The gram was the fundamental unit of mass in the 19th-century centimetre–gram–second system of units (CGS).

### Metric system

metricmetric unitsmetric unit
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.
The CGS system had two units of energy, the erg that was related to mechanics and the calorie that was related to thermal energy; so only one of them (the erg) could bear a coherent relationship to the base units.

### Barye

dyn/cm²
Thus, for example, the CGS unit of pressure, barye, is related to the CGS base units of length, mass, and time in the same way as the SI unit of pressure, pascal, is related to the SI base units of length, mass, and time:
The barye (symbol: Ba), or sometimes barad, barrie, bary, baryd, baryed, or barie, is the centimetre–gram–second (CGS) unit of pressure.

### Erg

dune seaergs
It originated in the centimetre–gram–second (CGS) system of units.

### Electromagnetism

electromagneticelectrodynamicselectromagnetic force
All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism.
The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.

### Poise (unit)

poisecentipoisecP
The poise (symbol P; ) is the unit of dynamic viscosity (absolute viscosity) in the centimetre–gram–second system of units.

### Second

ssecmegasecond
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.
BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units.

### Gal (unit)

galmGalmilligal
The gal is a derived unit, defined in terms of the centimeter–gram–second (CGS) base unit of length, the centimeter, and the second, which is the base unit of time in both the CGS and the modern SI system.

### Ampere

AmAamp
(This approach was eventually used to define the SI unit of ampere as well).
The earlier [[Centimetre gram second system of units#Derivation of CGS units in electromagnetism|CGS measurement system]] had two different definitions of current, one essentially the same as the SI's and the other using electric charge as the base unit, with the unit of charge defined by measuring the force between two charged metal plates.

### Joule

JkJMJ
The cgs system had been declared official in 1881, at the first International Electrical Congress.

### Watt

kWMWmegawatt
Noting that units in the practical system of units were named after leading physicists, Siemens proposed that Watt might be an appropriate name for a unit of power.

### Conversion of units

conversion factorunit conversionConversions
In measurements of purely mechanical systems (involving units of length, mass, force, energy, pressure, and so on), the differences between CGS and SI are straightforward and rather trivial; the unit-conversion factors are all powers of 10 as 100 cm = 1 m and 1000 g = 1 kg.

### Kilogram

kgmgmilligram
The CGS system has been largely supplanted by the MKS system based on the metre, kilogram, and second, which was in turn extended and replaced by the International System of Units (SI).
During the 19th century, the standard system of metric units was the centimetre–gram–second system of units, treating the gram as the fundamental unit of mass and the kilogram as a derived unit.

### Wavenumber

wave numberangular wavenumbercm −1
In spectroscopy it is usual to give wavenumbers in cgs unit (i.e., reciprocal centimeters; cm −1 ); in this context, the wavenumber was formerly called the kayser, after Heinrich Kayser (some older scientific papers used this unit, abbreviated as K, where 1K = 1cm −1 ).

### Lorentz–Heaviside units

Heaviside–Lorentz unitsLorentz-Heaviside unitsHeaviside–Lorentz
Furthermore, within CGS, there are several plausible choices of electromagnetic units, leading to different unit "sub-systems", including Gaussian units, "ESU", "EMU", and Lorentz–Heaviside units.
Lorentz–Heaviside units (or Heaviside–Lorentz units) constitute a system of units (particularly electromagnetic units) within CGS, named from Hendrik Antoon Lorentz and Oliver Heaviside.

### Vacuum permeability

magnetic constantpermeability of free spacepermeability of vacuum
Furthermore, if we wish to describe the electric displacement field D and the magnetic field H in a medium other than vacuum, we need to also define the constants ε 0 and μ 0, which are the vacuum permittivity and permeability, respectively.
In the old "electromagnetic (emu)" system of equations defined in the late 19th century, k m was chosen to be a pure number, 2, distance was measured in centimetres, force was measured in the cgs unit dyne, and the currents defined by this equation were measured in the "electromagnetic unit (emu) of current" (also called the "abampere").

### Oersted

megagauss oerstedsOeOERSTEDS
For instance electric field strength is in units of volts per centimetre, magnetic field strength is in oersteds and resistivity is in ohm-cm.
The oersted (symbol Oe) is the unit of the auxiliary magnetic field H in the centimetre–gram–second system of units (CGS).  It is equivalent to 1 dyne per maxwell.

### Viscosity

viscouskinematic viscositydynamic viscosity
The CGS unit (g·cm −1 ·s −1 = 0.1 Pa·s) is called the poise (P), named after Jean Léonard Marie Poiseuille.

### Carl Friedrich Gauss

GaussCarl GaussCarl Friedrich Gauß
The CGS system goes back to a proposal in 1832 by the German mathematician Carl Friedrich Gauss to base a system of absolute units on the three fundamental units of length, mass and time.