This new system slowly gained acceptance, and, on 20 May 1875, 17 nations signed the ‘Metre Convention’ with the aim of promoting the metric system, creating standards for the metre and the kilogram, and setting up an organisation to maintain and coordinate comparisons. Another important step was the incorporation of the ‘astronomical second’ as the unit of time.
At the 11th meeting of Metre Convention signatories in 1960 (otherwise known as the General Conference on Weights and Measures), the International System of Units (Système International d'Unités) - or SI - was introduced. Three further units were ratified for use in the system; the ampere (electrical current), the kelvin (thermodynamic temperature) and the candela (luminous intensity). Eleven years later, the mole was added as well, the unit for the amount of a substance, completing the set of SI base units that are in use today.

