units. It actually consists of two related systems: the U.S. customary units and the
British Imperial System. The background of the units of measurement is historically
rich and includes modern familiar terms, such as foot, inch, mile, and pound, as well
as less well-known units, such as span, cubit, and rod. The official policy of the United
States government is to designate the metric system as the preferred system for trade
and commerce, but customary units are still widely used on consumer products and
in industrial manufacturing.
In order to link all systems of weights and measures, both metric and non-metric,
there is a network of international agreements supporting what is known as the Inter-
national System of Units. It is abbreviated as SI (but notS.I.), in reference to the first
two initials of its French name, Système International d’Unités. It was developed from
an agreement signed in Paris on May 20, 1875, known as the Treaty of the Meter (Con-
vention du Mètre). To date, 48 nations have signed the treaty. The SI is maintained by
a small agency in Paris, the International Bureau of Weights and Measures (BIPM, or
Bureau International des Poids et Mesures). Because there is a need to change or
update the precision of measurements over time, the SI is updated every few years by
the international General Conference on Weights and Measures (or CGPM, or Con-
férence Générale des Poids et Mesures), the two most recent meetings being in 2003
and 2007. SI is also referred to as the metric system, which is based on the meter. The
word can also be used in mathematics (for example, metric space) or even computing
(fontmetric file). It is often referred to incorrectly as “metrical.” (See below for more
about the metric system.)What are the base SI units?
There are several base units at the heart of the International System (SI). The follow-
50 ing lists the seven base units:
Who was Adrien-Marie Legendre?
A
drien-Marie Legendre (1752–1833) was a brilliant French mathematician
and physicist. He is known for his studies of ellipsoids (leading to what we
now call the Legendre functions) and celestial mechanics, and he worked on the
orbits of comets. In 1787 he helped measure Earth using a triangulation survey
between the Paris and Greenwich observatories. In 1794 Legendre published
Eléments de géométrie,an elementary text on geometry that would essentially
replace Euclid’s Elementsand would remain the leading text on the topic for
close to a century. Finally, Legendre also had an important connection to mea-
surement: In 1791 he was appointed to the committee of the Académie des Sci-
ences, which was assigned the task of standardizing weights and measures.