International System of Units - History - Towards SI

Towards SI

In the nineteenth century attempts to produce a coherent set of electrical units was beset with difficulties.

At the close of the nineteenth century three different systems of units of measure existed for electrical measurements – a CGS-based system for electrostatic units (also known as the Gaussian system), a CGS-based system for electromechanical units and an MKS-based system (the "International system") for electrical distribution systems. In 1900 Giovanni Giorgi published a paper in which he advocated using a fourth base unit alongside the existing three base units. The fourth unit could be either electric current or voltage or electrical resistance.

In the late nineteenth and early twentieth centuries a number of non-coherent units of measure were developed such as the Pferdestärke or "metric horsepower" for power, the darcy for permeability and the use of "millimetres of mercury" for the measurement of both barometric and blood pressure. All these units incorporate standard gravity in their definitions.

At the end of Second World War, a number of different systems of measurement were in use throughout the world. Some of these systems were metric system variations, whereas others were based on customary systems of measure. It was recognised that additional steps were needed to promote a worldwide measurement system. After representations by the International Union of Pure and Applied Physics (IUPAP) and by the French Government, the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name, Le Système international d'unités. The BIPM has also described SI as "the modern metric system". The seventh base unit, the mole, was added in 1971 by the 14th CGPM.