Today, based on what we know about our physical world, we need sevenfundamentalor
base dimensionsto correctly express what we know of the natural world. They arelength,
mass,time,temperature,electric current,amount of substance, andluminous intensity.
With the help of these base dimensions we can derive all other necessary physical quantities that
describe how nature works.6 6..2 2 SSyysstteemmss ooff UUnniittss
Throughout the world, there are several systems of units in use today. The most common sys-
tems of units are International System (SI), British Gravitational (BG), and the U.S. Custom-
ary units, which we will discuss next.International System (SI) of Units
We begin our discussion of systems of units with the International System (SI) of units, because
SI is the most common system of units used in the world. The origin of the present day Inter-
national System of units can be traced back to 1799 with meter and kilogram as the first two
base units. By promoting the use of the second as a base unit of time in 1832, Carl Friedrich
Gauss (1777–1855), an important figure in mathematics and physics, including magnetism
and astronomy, had a great impact in many areas of science and engineering. It was not until
1946 that the proposal for the ampere as a base unit for electric current was approved by the
General Conference on Weights and Measures (CGPM). In 1954, CGPM included ampere,
kelvin, and candela as base units. The mole was added as a base unit by the 14th CGPM in- A list of SI base (fundamental) units is given in Table 6.1.
132 Chapter 6 Fundamental Dimensions and Units
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