"The Sl is not static but evolves to match the world's increasingly demanding requirements."
International Bureau of Weights and Measures
Scientists and engineers constantly need to measure distances, masses, times, temperatures, densities, velocities, electrical currents, and so on. All these quantities are then expressed as a number, and this means that units are absolutely vital.
The idea of implementing common bases for all the units began with the British Association for the Advancement of Science in 1874. They suggested the centimeter for length, the gram for weight, and the second for time, which was known as the C.G.S. sytem. Prefixes such as mega- and micro- could then be used to indicate decimal multiples and submultiples.
Unfortunately, this C.G.S. system was rather inconvenient in the field of magnetism and electricity. In 1889 the Conference Generaledes Poidset Mesures decided that the meter, kilogram, and second (m.k.s.) might be more appropriate. In 1946 the ampere, a unit of electrical current—named for Andre-Marie Ampere—was added.
By 1971 this system had grown, and at the fourteenth Conference Generate des Poids et Mesures, these four base units were joined by the Kelvin (for temperature), the candela (a unit of luminous intensity), and the mole (a unit quantifying the amount of substance, or atoms and molecules).
Le Systeme International d'Unites—the SI unit—was thus established. The international system of units has replaced many traditional measurement systems around the world. Today, most countries employ the SI, although the United States, Liberia, and Myanmar have not officially adopted the SI unit as their primary system of measurement.
