Centimetre gram second system of units
The centimetre-gram-second system (abbreviated CGS
or cgs) is a metric system of physical units based on cen-
timetre as the unit of length, gram as a unit of mass, and
second as a unit of time. All CGS mechanical units are
unambiguously derived from these three base units, but
there are several different ways of extending the CGS
system to cover electromagnetism.
The CGS system has been largely supplanted by the
MKS system, based on metre, kilogram, and second. MKS
was in turn extended and replaced by the International
System of Units (SI). The latter adopts the three base
units of MKS, plus the ampere, mole, candela and kelvin.
In many fields of science and engineering, SI is the only
extant system of units. However, there remain certain
subfields where CGS is prevalent.
In measurements of purely mechanical systems (in-
volving units of length, mass, force, energy, pressure,
etc.), the differences between CGS and SI are straightfor-
ward and rather trivial; the unit-conversion factors are
all powers of 10 arising from the relations 100 cm = 1 m
and 1000 g = 1 kg. For example, the CGS derived unit of
force is the dyne, equal to 1 g·cm/s2, while the SI derived
unit of force is the Newton, 1 kg·m/s2. Thus it is straight-
forward to show that 1 dyne=10-5 newton.
On the other hand, in measurements of electromag-
netic phenomena (involving units of charge, electric and
magnetic fields, voltage, etc.), converting between CGS
and SI is much more subtle and involved. In fact, formu-
las for physical laws of electromagnetism (such as Max-
well’s equations) need to be adjusted depending on what
system of units one uses. This is because there is no one-
to-one correspondence between electromagnetic units
in SI and those in CGS, as there is for mechanical units.
Furthermore, within CGS, there are several plausible
choices of electromagnetic units, leading to different
unit "sub-systems", including Gaussian, "ESU", "EMU",
and Heaviside-Lorentz. Among these choices, Gaussian
units are th