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APPENDIX

International System of Units

The International System of Units (abbreviated SI from French:
Système international d'unités) is the modern form of the metric system.
It comprises a system of units of measurement devised around seven

base units and the convenience of the number ten. The SI was
established in 1960, based on the meter-kilogram-second system, rather
than the centimeter-gram-second system, which, in turn, had several

variants. The SI has been declared to be an evolving system; thus
prefixes and units are created and unit definitions are modified through
international agreement as the technology of measurement progresses,
and as the precision of measurements improves.

SI is the world's most widely used system of measurement, used in both
everyday commerce and science. The system has been nearly globally
adopted with the United States being the only industrialised nation that

does not mainly use the metric system in its commercial and standards
[6]
activities. The United Kingdom has officially adopted a partial
metrication policy, with no intention of replacing imperial units entirely.

Canada has adopted it for many purposes but Imperial units, which are
used in the United States, are still legally permitted and remain in
common use throughout many sectors of Canadian society, particularly

in the retail food, buildings trades, and railways sectors.

Table I. Basic Units

Quantity Unit Symbol Definition

The path travelled by light in
vacuum during a time interval

Length meter m of 1/299792458 seconds. This
fixes the speed of light to
exactly 299792458 m/s.

Mass of the platinum-

Mass kilogram kg iridium prototype at BIPM in
Sevres.

One second equals

Time second s 9192631770 periods of the
radiation due to the transition

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