Themetre, kilogram, second system of units, also known more briefly asMKS units or theMKS system,[1][2][3] is a physical system of measurement based on themetre,kilogram, andsecond (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 thenewton unit of force which is defined as kilogram times metres per second squared.
The modernInternational System of Units (SI, from the French nameSystème international d'unités) was originally created as a formalization of the MKS system. The SI has been redefined several times since then and is nowbased 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 byscientists to define acoherent 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. TheUnited States customary units are an example of a non-coherent set of units.[5] In 1874, theBritish Association for the Advancement of Science (BAAS) introduced theCGS system, a coherent system based on thecentimetre,gram and second. These units were inconvenient for electromagnetic applications, sinceelectromagnetic units derived from these did not correspond to the commonly usedpractical units, such as thevolt,ampere andohm.[4][6] After theMetre Convention of 1875, work started on international prototypes for the kilogram and the metre, which were formally sanctioned by theGeneral 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 theAssociazione elettrotecnica italiana (AEI) that the MKS system, extended with a fourth unit to be taken from the practical units ofelectromagnetism, 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 engineerGeorge 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] TheInternational Electrotechnical Commission (IEC) adopted Giorgi's proposal as theM.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.
Thermks system (rationalized metre–kilogram–second) combines MKS withrationalization of electromagnetic equations.
The MKS units with the ampere as a fourth base unit is sometimes referred to as theMKSA system. This system was extended by adding thekelvin andcandela as base units in 1960, thus forming theInternational System of Units. Themole 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 |
