Metric System - Relating SI To The Real World

Relating SI To The Real World

Although SI, as published by the CGPM, should, in theory, meet all the requirements of commerce, science and technology, certain units of measure have acquired such a position within the world community that it is likely they will be used for many years to come. In order that such units be used consistently around the world, the SI Brochure, the CGPM catalogued such units in the SI brochure. The 8th edition of the brochure catalogued the following categories:

• Non-SI units accepted for use with the International System of Units (Table 6). This list includes the hour and minute, the angular measures (degree, minute and second of arc) and the historic metric units, the litre, tonne and hectare (originally agreed by the CGPM in 1879)
• Non-SI units whose values in SI units must be obtained experimentally (Table 7). This list includes various units of measure used in atomic and nuclear physics and in astronomy such as the dalton, the electron mass, the electron volt, the astronomical unit and a number of other units of measure that are well-established, but dependent on experimentally-determined physical quantities.
• Other non-SI units (Table 8). This list catalogues a number of units of measure that have been used internationally in certain well-defined spheres including the bar for pressure, the ångström for atomic physics, the nautical mile and the knot in navigation.
• Non-SI units associated with the CGS and the CGS-Gaussian system of units (Table 9). This table catalogues a number of units of measure based on the cgs system and dating from the nineteenth century. They appear frequently in the literature, but their continued use is discouraged by the CGPM.