Derived units are defined using the appropriate combinations, such as velocity in metres per second.
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] The International Electrotechnical Commission (IEC) adopted Giorgi's proposal as the M.K.S.
[10] In 1939, the Consultative Committee for Electricity (CCE) recommended the adoption of Giorgi's proposal, using the ampere as the fourth base unit.