Editing System of units of measurement (section)

==Natural units==
[[Natural units]] are [[units of measurement]] defined in terms of universal [[physical constants]] in such a manner that selected physical constants take on the numerical value of one when expressed in terms of those units.  Natural units are so named because their definition relies on only properties of [[nature]] and not on any human construct. Varying systems of natural units are possible, depending on the choice of constants used.

Some examples are as follows:
* [[Geometrized unit system]]s are useful in [[Theory of relativity|relativistic physics]]. In these systems, [[speed of light]] and the [[gravitational constant]] are among the constants chosen.
* [[Planck units]] is system of geometrized units in which the [[reduced Planck constant]] is included in the list of defining constants. It is based on only properties of [[free space]] rather than of any object or particle.
* [[Stoney units]] is a system of geometrized units in which the [[Coulomb constant]] and the [[elementary charge]] are included.
* [[Atomic units]] are a system of units used in [[atomic physics]], particularly for describing the properties of [[electron]]s. The atomic units have been chosen to use several constants relating to the electron: the [[electron mass]], the [[elementary charge]], the [[Coulomb constant]] and the [[reduced Planck constant]]. The unit of [[energy]] in this system is the total energy of the [[electron]] in the [[Bohr atom]] and called the [[Hartree energy]]. The unit of length is the [[Bohr radius]].