The metre, kilogram, second system of units, also known more briefly as MKS units or the MKS system, is a physical system of measurement based on the

metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...

, and

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

(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 defined as kilogram times metres per second squared. The modern

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

(SI, from the French name ) 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.

History

By the mid-19th century, there was a demand by

scientists A scientist is a person who researches to advance knowledge in an area of the natural sciences. In classical antiquity, there was no real ancient analog of a modern scientist. Instead, philosophers engaged in the philosophical study of nature ...

to define a coherent system of units. 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 United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that ...

are an example of a non-coherent set of units. In 1874, the

British Association for the Advancement of Science The British Science Association (BSA) is a Charitable organization, charity and learned society founded in 1831 to aid in the promotion and development of science. Until 2009 it was known as the British Association for the Advancement of Scienc ...

(BAAS) introduced the CGS system, a coherent system based on the

centimetre upright=1.35, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is in-between 1 meter to 1 millimeter. A centimetre (International spelling) or centimeter (American ...

gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram. Originally defined in 1795 as "the absolute Mass versus weight, weight of a volume ...

and second. These units were inconvenient for electromagnetic applications, since

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

units derived from these did not correspond to the commonly used ''practical units'', such as the

volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...

ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...

and

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (1 ...

. After the

Metre Convention 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, German Empire, Ge ...

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 The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...

(CGPM) in 1889, thus formalizing the MKS system by using the kilogram and metre as base units. In 1901,

Giovanni Giorgi Giovanni Giorgi (November 27, 1871 – August 19, 1950) was an Italian physicist and electrical engineer who proposed the ''Giorgi system'' of measurement, the precursor to the International System of Units (SI). Early life Giovanni Giorgi was bo ...

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 In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

, such as the volt, ohm or ampere, be used to create a coherent system using practical units. This system was strongly promoted by electrical engineer George A. Campbell. 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. The

International Electrotechnical Commission The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronics, electronic and related technologies. IEC standards cover a va ...

(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. 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 The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

and

candela The candela (symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radi ...

as base units in 1960, thus forming the

. The mole was added as a seventh base unit in 1971.

Derived units

Mechanical units

Electromagnetic units

References

External links