MKS units
Coherent measurement system that evolved into the International System of Units.
The MKS system (metre, kilogram, second) is a coherent system of measurement, meaning that all derived units are directly formed from the base units without requiring any conversion factors. Distances are expressed in metres, mass in kilograms, and time in seconds; derived units, such as velocity, are defined as metres per second, while some, like the newton (kilogram times metres per second squared), have their own names. The system was formalized following the Metre Convention of 1875, with international prototypes for the metre and kilogram sanctioned by the General Conference on Weights and Measures (CGPM) in 1889. This addressed a demand from scientists for a coherent system, as earlier systems like the CGS (centimetre, gram, second) introduced in 1874 were inconvenient for electromagnetic applications because their derived units did not match practical units like the volt, ampere, and ohm. In 1901, Giovanni Giorgi proposed extending the MKS system with a fourth base unit from practical electromagnetism, a proposal adopted by the International Electrotechnical Commission (IEC) in 1935. The Consultative Committee for Electricity (CCE) recommended the ampere as the fourth base unit in 1939, which the CGPM approved in 1954. The resulting MKSA system was later expanded by adding the kelvin and candela in 1960, forming the International System of Units (SI), with the mole added in 1971. Although the SI has since been redefined based on fundamental physical constants, it still closely approximates the original MKS units for most practical purposes. The MKS system was widely used in commerce and engineering throughout the 20th century, alongside the CGS system.
- field
- Physical measurement systems
- known_for
- Base units of metre, kilogram, and second; precursor to the International System of Units (SI)
Lore & Background
The MKS system is a coherent physical measurement system built upon three base units: the metre for distance, the kilogram for mass, and the second for time. All other units are derived directly from these three without requiring conversion factors; for example, velocity is expressed as metres per second, and the unit of force, the newton, is defined as kilogram times metres per second squared. This system was formalized after the Metre Convention of 1875, with international prototypes for the kilogram and metre sanctioned in 1889. It was developed partly to address the inconvenience of the earlier CGS system, whose electromagnetic units did not match the practical units like the volt and ampere used in commerce and engineering. In the early 20th century, Giovanni Giorgi proposed extending MKS with a fourth unit from practical electromagnetism to create a fully coherent system for that field. The International Electrotechnical Commission adopted this proposal in 1935, and by 1939 the ampere was recommended as the fourth base unit. This extended system, sometimes called MKSA, later formed the foundation of the modern International System of Units (SI) when the kelvin and candela were added as base units in 1960, and the mole in 1971. The MKS system remains closely approximated by the current SI, which is now defined by fundamental physical constants.
Reader's Guide
The MKS system is significant as the direct precursor to the International System of Units (SI), which is now the global standard for measurement. Its coherence—where derived units like the newton (kg·m/s²) are defined without conversion factors—made it practical for commerce, engineering, and science. Although SI has been redefined based on fundamental physical constants, it still closely approximates the original MKS units for most practical purposes. The MKS system's legacy lies in providing a stable, coherent framework that enabled international standardization and the eventual creation of the modern metric system.
Did You Know?
- The MKS system with the ampere as a fourth base unit is sometimes called the MKSA system.
- The International System of Units (SI) was originally created as a formalization of the MKS system.
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