How was the gram established?

Answer 1

Concepts similar to those behind the metric system had been discussed in the 16th and 17th centuries. Simon Stevin had published his ideas for a decimal notation and John Wilkins had published a proposal for a decimal system of measurement based on natural units. The first practical realisation of the metric system came in 1799, during the French Revolution, when the existing system of measure, which had fallen into disrepute, was temporarily replaced by a decimal system based on the kilogram and the metre. The work of reforming the old system of weights and measures was sponsored by the revolutionary government, including the approval of Louis XVI before his fall from power. The metric system was to be, in the words of philosopher and mathematician Condorcet, "for all people for all time". In the era of humanism, the basic units were taken from the natural world: the unit of length, the metre, was based on the dimensions of the Earth, and the unit of mass, the kilogram, was based on the mass of water having a volume of one litre or one thousandth of a cubic metre. Reference copies for both units were manufactured and placed in the custody of the French Academy of Sciences. By 1812, due to the unpopularity of the new metric system, France had reverted to units similar to those of their old system.

In 1837 the metric system was re-adopted by France, and also during the first half of the 19th century was adopted by the scientific community. In the middle of the century, James Clerk Maxwell put forward the concept of a coherent system where a small number of units of measure were defined as base units, and all other units of measure, called derived units, were defined in terms of the base units. Maxwell proposed three base units: length, mass and time. This concept worked well with mechanics, but attempts to describe electromagnetic forcesin terms of these units encountered difficulties. By the end of the 19th century, four principal variants of the metric system were in use for the measurement of electromagnetic phenomena: three based on the centimetre-gram-second system of units (CGS system), and one on the metre-kilogram-second system of units (MKS system). This impasse was resolved by Giovanni Giorgi, who in 1901 proved that a coherent system that incorporated electromagnetic units had to have an electromagnetic unit as a fourth base unit.

Until 1875, the French government owned the prototype metre and kilogram, but in that year the Convention of the metrewas signed, and control of the standards relating to mass and length passed to a trio of inter-governmental organisations, the senior of which was the General Conference on Weights and Measures (in French the Conférence générale des poids et mesures or CGPM). During the first half of the 20th century, the CGPM cooperated with a number of other organisations, and by 1960 it had responsibility for defining temporal, electrical, thermal, molecular and luminar measurements, while other international organisations continued their roles in how these units of measurement were used.

In 1960, the CGPM launched the International System of Units (in French the Système international d'unités or SI) which had six "base units": the metre, kilogram, second, ampere, degree Kelvin (subsequently renamed the "kelvin") and candela; as well as 22 further units derived from the base units. The mole was added as a seventh base unit in 1971. During this period, the metre was redefined in terms of the wavelength of the waves from a particular light source, and the second was defined in terms of the frequency of radiation from another light source. Since the end of the 20th century, an effort has been undertaken to redefine the ampere, kilogram, mole and kelvin in terms of the basic constants of physics.