Editing Physical constant (section)

=== Numerical values ===
Whereas the [[physical quantity]] indicated by a physical constant does not depend on the unit system used to express the quantity, the numerical values of dimensional physical constants do depend on choice of unit system. The term "physical constant" refers to the physical quantity, and not to the numerical value within any given system of units. For example, the speed of light is defined as having the numerical value of {{val|299792458}} when expressed in the [[SI unit]] metres per second, and as having the numerical value of 1 when expressed in the [[natural units]] [[Planck units|Planck length]] per Planck time. While its numerical value can be defined at will by the choice of units, the speed of light itself is a single physical constant.
[[File:SI Illustration Base Units and Constants Colour Full.svg|thumb|Illustration of the SI system of units, with base units and defining constants used to define them: '''s''' – the [[Hyperfine structure#Use in defining the SI second and meter|hyperfine structure transition frequency]] in the caesium atom for the '''[[second]]''', '''kg''' – Planck's constant for the '''[[kilogram]]''', '''mol''' – the [[Avogadro constant]] for the '''[[Mole (unit)|mole]]''', '''cd''' – the [[luminous efficacy]] of monochromatic radiation of frequency {{val|540|u=THz}} for the '''[[candela]]''', '''K''' – the [[Boltzmann constant]] for the '''[[kelvin]]''', '''A''' – [[elementary charge]] for the '''[[ampere]]''', '''m''' – the [[speed of light]] for the '''[[metre]]'''.]]