APPENDIX B
UNITS, MEASUREMENTS AND CON-
STANTS
Measurements are comparisons. The standard used for the comparison is called a unit.
any different systems of units have been used throughout the world. Unit systems
are standards; they always confer a lot of power to the organisation in charge of them, as
can be seen most clearly in the computer industry; in the past the same applied to measure-
ment units. To avoid misuse by authoritarian institutions, to eliminate at the same time all
problems with differing, changing and irreproducible standards, and – this is not a joke –
to simplify tax collection, already in the 18th century scientists, politicians and economists
have agreed on a set of units. It is called the Système International d’Unités, abbreviated
SI, and is defined by an international treaty, the ‘Convention du Mètre’. The units are main-
tained by an international organisation, the ‘Conférence Générale des Poids et Mesures’,
and its daughter organisations, the ‘Commission Internationale des Poids et Mesures’ and
the ‘Bureau International des Poids et Mesures’, which all originated in the times just before
the French revolution.
Ref. 975
All SI units are built from seven base units whose official definitions, translated from
French into English, are the following, together with the date of their formulation:
‘The second is the duration of 9 192 631 770 periods of the radiation corresponding to
the transition between the two hyperfine levels of the ground state of the caesium 133 atom.’
(1967)∗
‘The metre is the length of the path travelled by light in vacuum during a time interval
of 1/299 792 458 of a second.’ (1983)
‘The kilogram is the unit of mass; it is equal to the mass of the international prototype
of the kilogram.’ (1901) ∗
‘The ampere is that constant current which, if maintained in two straight parallel con-
ductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in
vacuum, would produce between these conductors a force equal to 2 ·10−7 newton per metr