The metre, kilogram, second system of units, also known more briefly as MKS units or the MKS system, [1] [2] [3] is a physical system of measurement based on the metre, kilogram, and second (MKS) as base units. Distances are described in terms of metres, mass in terms of kilograms and time in seconds. Derived units are defined using the appropriate combinations, such as velocity in metres per second. Some units have their own names, such as the newton unit of force which is the combination kilogram metre per second squared.
The modern International System of Units (SI), from the French Système international d'unités, was originally created as a formalization of the MKS system. The SI has been redefined several times since then and is now based entirely on fundamental physical constants, but still closely approximates the original MKS units for most practical purposes.
By the mid-19th century, there was a demand by scientists to define a coherent system of units. [4] A coherent system of units is one where all units are directly derived from a set of base units, without the need of any conversion factors. The United States customary units are an example of a non-coherent set of units. [5] In 1874, the British Association for the Advancement of Science (BAAS) introduced the CGS system, a coherent system based on the centimetre, gram and second. These units were inconvenient for electromagnetic applications, since electromagnetic units derived from these did not correspond to the commonly used practical units, such as the volt, ampere and ohm. [4] [6] After the Metre Convention of 1875, work started on international prototypes for the kilogram and the metre, which were formally sanctioned by the General Conference on Weights and Measures (CGPM) in 1889, thus formalizing the MKS system by using the kilogram and metre as base units. [7]
In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that the MKS system, extended with a fourth unit to be taken from the practical units of electromagnetism, such as the volt, ohm or ampere, be used to create a coherent system using practical units. [8] [6] This system was strongly promoted by electrical engineer George A. Campbell. [9] The CGS and MKS systems were both widely used in the 20th century, with the MKS system being primarily used in practical areas, such as commerce and engineering. [4] The International Electrotechnical Commission (IEC) adopted Giorgi's proposal as the M.K.S. System of Giorgi in 1935 without specifying which electromagnetic unit would be the fourth base unit. [10] In 1939, the Consultative Committee for Electricity (CCE) recommended the adoption of Giorgi's proposal, using the ampere as the fourth base unit. This was subsequently approved by the CGPM in 1954.
The rmks system (rationalized metre–kilogram–second) combines MKS with rationalization of electromagnetic equations.
The MKS units with the ampere as a fourth base unit is sometimes referred to as the MKSA system. This system was extended by adding the kelvin and candela as base units in 1960, thus forming the International System of Units. The mole was added as a seventh base unit in 1971. [6] [7]
| Quantity | Quantity symbol | Unit | Unit symbol | MKS equivalent |
|---|---|---|---|---|
| frequency | f | hertz | Hz | s−1 |
| force | F | newton | N | kg⋅m⋅s−2 |
| pressure | p | pascal | Pa | kg⋅m−1⋅s−2 |
| energy | E | joule | J | kg⋅m2⋅s−2 |
| power | P | watt | W | kg⋅m2⋅s−3 |
| Quantity | Quantity symbol | Unit | Unit symbol | MKSA equivalent |
|---|---|---|---|---|
| electric charge | Q | coulomb | C | s⋅A |
| voltage | U | volt | V | kg⋅m2⋅s−3⋅A−1 |
| electric capacitance | C | farad | F | kg−1⋅m−2⋅s4⋅A2 |
| electric resistance | R | ohm | Ω | kg⋅m2⋅s−3⋅A−2 |
| electric conductance | G | siemens | S | kg−1⋅m−2⋅s3⋅A2 |
| magnetic flux | ΦB | weber | Wb | kg⋅m2⋅s−2⋅A−1 |
| magnetic flux density | B | tesla | T | kg⋅s−2⋅A−1 |
| electric inductance | L | henry | H | kg⋅m2⋅s−2⋅A−2 |
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 coulomb (C) moving past a point per second. It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.
The centimetre–gram–second system of units 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. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.
The joule is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of one newton displaces a mass through a distance of one metre in the direction of that force. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).
The Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela.
The International System of Units, internationally known by the abbreviation SI, is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures it is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.
The volt is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI).
The metric system is a decimal-based system of measurement. The current international standard for the metric system is the International System of Units, in which all units can be expressed in terms of seven base units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd). These can be made into larger or smaller units with the use of metric prefixes.
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is equal to the electric charge delivered by a 1 ampere current in 1 second and is defined in terms of the elementary charge e, at about 6.241509×1018 e.
The oersted is the coherent derived unit of the auxiliary magnetic field H in the centimetre–gram–second system of units (CGS). It is equivalent to 1 dyne per maxwell.
The maxwell is the CGS (centimetre–gram–second) unit of magnetic flux.
In physics, the weber is the unit of magnetic flux in the International System of Units (SI). The unit is derived from the relationship 1 Wb = 1 V⋅s (volt-second). A magnetic flux density of 1 Wb/m2 is one tesla.
Giovanni Giorgi was an Italian physicist and electrical engineer who proposed the Giorgi system of measurement, the precursor to the International System of Units (SI).
Vacuum permittivity, commonly denoted ε0, is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric constant, or the distributed capacitance of the vacuum. It is an ideal (baseline) physical constant. Its CODATA value is:
The ohm is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861.
The International Electrical Congress was a series of international meetings, from 1881 to 1904, in the then new field of applied electricity. The first meeting was initiated by the French government, including official national representatives, leading scientists, and others. Subsequent meetings also included official representatives, leading scientists, and others. Primary aims were to develop reliable standards, both in relation to electrical units and electrical apparatus.
The International System of Electrical and Magnetic Units is an obsolete system of units used for measuring electrical and magnetic quantities. It was proposed as a system of practical international units by unanimous recommendation at the International Electrical Congress, discussed at other Congresses, and finally adopted at the International Conference on Electric Units and Standards in London in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the International System of Units (SI) at the 9th General Conference on Weights and Measures in 1948.
The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.
The following outline is provided as an overview of and topical guide to the metric system:
A coherent system of units is a system of units of measurement used to express physical quantities that are defined in such a way that the equations relating the numerical values expressed in the units of the system have exactly the same form, including numerical factors, as the corresponding equations directly relating the quantities. It is a system in which every quantity has a unique unit, or one that does not use conversion factors.
