Once upon a time there were many measures of physical quantities such as mass and length. Until about 1800 workers in various countries all used their own systems of units. The English used inches and feet to measure length and continental scientists used cm and metres. Various international committees of scientists have met over many years to produce a single system of units upon which all are agreed. In 1960 the General Conference of Weights and Measures recommended that everyone use a metric system of measurement called the International System of Unite – SI – derived from the earlier MKS system, so called because the basic units were the metre, Kilogram and second.
Physical laboratories around the world work to improve the standards of physical quantities, by changing the ways they are measured or to increase the precision of measurements.
Originally in the SI system the metre was defined as the distance between two lines on a platinum iridium bar kept at the International Office of Weights and Measures in Paris. Standard of this kind undergo minute change in length over the years, so the trend now is to define physical quantities in terms of fundamental properties of matter. In 1983, the General Conference of Weights and Measures redefined the metre as the length travelled by light in a vacuum in
of a second.
The Kilogram is still defined in the 'old' way as the mass of a certain cylindrical piece of platinum iridium alloy kept at the International Office of Weights and Measures in Paris. It was originally defined as the mass of
of pure water at the temperature of maximum density, 4 degrees Celsius. An error at the time of this measurement meant the volume was actually
Debate is taking place about a more fundamental definition.
Time has taken the same unit, the second, for most of recorded history. It was defined in terms of physical constants in 1967, as the time interval occupied by 9192631770 cycles of a specified energy change in the Caesium atom.
Together with the SI unit of temperature, Kelvin K, these units constitute the basis of the system of units in use today. Other units are defined in terms of these base units, and the system has the advantage of being self consistent so that no conversion factors are required when the same quantities appear in different equations.