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1.1.1 The international system of units (SI)
In the second half of the nineteenth century the centimetre, gram and second were in fairly general use as base units for scientific work even in such countries as the UK and the USA where the foot and the pound were employed for commerce and engineering. As a result, the units required by the rapidly emerging science of electricity were based on the centimetre, gram and second, with which they formed a coherent system known as the CGS electromagnetic system. A system of units is said to be coherent when derived units are formed from the base units without the insertion of factors of proportionality other than unity. There was also the CGS electrostatic system, but the only quantities frequently expressed in electrostatic units were electric charge, electric potential, and capacitance.
The young but fast-growing electrical industry soon found that many CGS electromagnetic units were of an extremely inconvenient size for its needs. Accordingly, in 1881, international agreement was reached to fix the practical unit of potential, to be called the volt, at 108 CGS units (which is approximately equal to the e.m.f. of a primary cell), and the unit of resistance, the ohm, at 109 CGS units (which is approximately the resistance of a column of mercury 1 m long and 1 mm2 in cross-section). The unit of electric current, the ampere, was made a tenth of the CGS unit. A coherent system of practical electric units was thus secured which, however, was not coherent with the mechanical units based on the centimetre and gram. The practical electric units suited the needs of telegraphy, which was then the main electrical industry, and they also happen to be convenient for heavy electrical engineering and for electronics.
The magnetic units, however, were left at their CGS values, presumably because the CGS unit of magnetic flux density, subsequently called ‘gauss’, is of the order of the flux density of the Earth’s field, and, as it was suitable for geomagnetism, there seemed no point in changing it for a unit 104 times larger. Coherence was thereby lost to electromagnetism as it had already been lost to the system embracing the mechanical units and the practical electric units.
Whereas the electric units, by the agreement of 1881, were chosen to be of suitable magnitude for everyday use, and whereas the centimetre and the second have acceptable sizes, the gram is too small for the practical needs of man, which are better served by a unit nearer the size of the pound or the kilogram. Moreover, the CGS unit of force, the dyne, and the unit of energy, the erg, are much too small. On the other hand, the unit of energy provided by the practical electric units, the volt-ampere-second, called the joule—which equals 107 ergs—is of a satisfactory size.
These considerations—the advantages of coherence and the fortuitous circumstance that a mechanical system based on the metre and the kilogram has precisely the same unit of energy as is provided by the practical electric units—led G. Giorgi in 1902 to propose a system based on the metre, the kilogram, the second, and one of the practical electric units. He pointed out that if magnetic field strength were expressed as amperes per metre instead of 4π times amperes per metre, which is the definition corresponding to that of the CGS unit, the number π would disappear from most electric and magnetic formulae involving rectilinear geometry, but would appear, as is to be expected, in those involving cylinders or spheres.
The International Electrotechnical Commission eventually chose the ampere as the fourth base unit of the MKSA or ‘Giorgi’ system, and in 1948 the 9th General Conference of Weights and Measures† recommended it for science and technology, as well as for commerce and industry. This system admirably covers mechanics and electromagnetism, but it does not provide for other branches of science such as heat. In 1960, in the hope of securing world-wide uniformity in the units employed in natural science, the 11th CGPM added to the units metre, kilogram, second and ampere, the kelvin for thermodynamic temperature, the candela for luminous intensity, and the radian and steradian for plane and solid angle. The first two joined the original four in being called ‘base’ units, and the last two were called ‘supplementary’ units. Any unit formed from two or more base units is called ‘derived’. The radian and steradian are regarded as derived units. The MKSA system thus broadened is called the International System of Units, often abbreviated to SI, and is the most satisfactory system of units we have had so far, in that it caters for the commercial and industrial activities of man as well as for the needs of science. In 1971, the 14th CGPM added the mole, the unit of amount of substance used in chemistry, to the list of base units, thus making them seven in all.
† The General Conference of Weights and Measures (CGPM) is the authority set up by the Metre Convention of 1875 to promote and improve the metric system, and to secure international uniformity in metric units and standards of measurement.
Definitions of some SI units
The seven base quantities, each with its unit and unit symbol, are listed below.
SI base quantities and units
Derived units. The table below lists some of the more common SI derived quantities, each with its unit and unit symbol. Some of these derived units have special names in the SI.
*Wave numbers in the infra-red are still often expressed in cm−1
Prefixes. Prefixes may be used, instead of powers of 10, to express certain decimal multiples of the units. Their names and symbols are listed below.
An exponent attached to a symbol containing a prefix indicates that the multiple of the unit is raised to the power expressed by the exponent.
Example: 1 cm3 = 10−6 m3; 1 cm−1 = 102 m−1.
Compound prefixes should not be used, e.g. use p not μμ. Names of multiples of the unit of mass are formed by attaching prefixes to the word ‘gram’.
(1) The International System of Units, 8th edition
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