1.2 Two classes of SI units
SI units are divided into two classes:
�base units;
�derived units.
From the scientific point of view, the division of SI units into these two classes
is to a certain extent arbitrary, because it is not essential to the physics of
the subject. Nevertheless, the CGPM, considering the advantages of a single,
practical, world-wide system of units for international relations, for teaching,
and for scientific work, decided to base the International System on a choice of
seven well-defined units which by convention are regarded as dimensionally
independent: the meter, the kilogram, the second, the ampere, the kelvin, the
mole, and the candela (see 2.1, p. 5). These SI units are called base units.
The radian and steradian, units of plane and solid angle, were admitted to the SI as a separate class of units, called
supplementary units, by the 11th CGPM (1960, Resolution 12; CR, 87). The 20th CGPM (1995, Resolution 8; CR, 223
and Metrologia, 1996, 33, 83) eliminated the supplementary units as a separate class within the SI and included the
radian and steradian in the class of derived units.
Recommendations of the CIPM are recorded in the Proce`s-Verbaux des Se�ances du Comite� International des Poids et
Mesures and are here identified by the letters PV.
The second class of SI units is that of derived units . These are units that are
formed as products of powers of the base units according to the algebraic
relations linking the quantities concerned. The names and symbols of some
units thus formed in terms of base units may be replaced by special names and
symbols which can themselves be used to form expressions and symbols for
other derived units (see 2.2, p. 9).
The SI units of these two classes form a coherent set of units, where coherent is
used in the specialist sense of a system whose units are mutually related by rules
of multiplication and division with no numerical factor other than 1. Following
CIPM Recommendation 1 (1969; PV, 37, 30-31 and Metrologia , 1970, 6, 66),
the units of this coherent set of units are designated by the name SI units.
It is important to emphasize that each physical quantity has only one SI unit,
even if this unit can be expressed in different forms. The inverse, however, is not
true; in some cases the same SI unit can be used to express the values of several
different quantities.
At 08:52 PM 10/23/02 -0400, Joseph B. Reid wrote:
Terry Simpson asked in USMA 22908:
So I understand that you mean that SI is a subset of metric. Where do
you get this definition from? What makes something metric and
non-metric?
If a unit is defined as some combination of SI base units and powers of ten it is a metric unit. If no powers of ten are involved it is an SI derived unit.
Joseph B. Reid
17 Glebe Road West
Toronto M5P 1C8Telephone 416-486-6071
