Rapid advances in science and technology in the 19th and 20th centuries fostered the development of several overlapping systems of units of measurements as scientists improvised to meet the practical needs of their disciplines. The early international system devised to rectify this situation was called the metre-kilogram-second (MKS) system. The General Conference on Weights and Measures added three new units (among others) in 1948: a unit of force (the newton), defined as that force which gives to a mass of one kilogram an acceleration of one metre per second per second; a unit of energy (the joule), defined as the work done when the point of application of a newton is displaced one metre in the direction of the force; and a unit of power (the watt), which is the power that in one second gives rise to energy of one joule. All three units are named for eminent scientists.
The 1960 International System
builds upon the MKS system. Its seven basic units, from
which other units are derived, are currently defined as
follows: for length, the metre, defined as
the distance traveled by light in a vacuum in 1/299,792,458 second; for
mass, the kilogram (about
2.2 pounds avoirdupois), which equals 1,000 grams as defined by the international
prototype kilogram of platinum-iridium in the keeping of the International
Bureau of Weights and Measures in Sèvres, France; for time, the second,
the duration of 9,192,631,770 periods of radiation associated with a
specified transition of the cesium-133 atom; for electric current, the ampere,
which is the current that, if maintained in two wires placed one metre
apart in a vacuum, would produce a force of 2 ´
10-7 newton per metre of length;
for luminous intensity, the candela, defined
as the intensity in a given direction of a source emitting radiation of
frequency 540 ´ 1012 hertz
and that has a radiant intensity in that direction of 1/683 watt per
steradian; for amount of substance, the mole,
defined as containing as many elementary entities of a substance as
there are atoms in 0.012 kilogram of carbon-12; and for thermodynamic
temperature, the kelvin (see absolute zero).