Basics of Coordinate Metrology
Learn module 1:  Units - Si Units

Digression
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Base quantity, Formula symbol

Base unit, (Symbol of the unit), Definition

Length l

The meter (m) is the length of the path traveled by light in vacuum during 1/299,792,458th of a second.

Mass m

The kilogram (kg) is the unit of mass. It is equal to the mass of the international prototype of the kilogram.

Time t

One second (s) is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between the two hyper fine levels of the ground state of the ¹³³Cs (cesium 133) atoms.

Electric current I

One Ampere (A) is the constant current which, when flowing through two parallel, straight conductors of infinite length in vacuum maintained at a distance of one meter of negligible cross-section, would produce between these conductors a force of 2 * 10^-7 Newton per meter of conducting length.

Thermodynamic temperature T

One Kelvin (K), the unit of the thermodynamic temperature, is the 273.16th part of the thermodynamic temperature of the triple point of water.

Amount of substance n

One mole (mol) is the amount of substance of a system containing as many elementary particles as there are atoms in 0.012 kilogram of ¹²C carbon. When mole is used as the amount of substance, the elementary particles must be specified and can be atoms, molecules, ions, electrons and other particles or groups of particles of precisely defined composition.

Luminous intensity I_v

One candela (cd) is the luminous intensity in a given direction of a source of radiation that emits monochromatic radiation of the frequency 540 * 10 ¹² Hertz and whose radiant intensity in this direction is (1/683) of watts per steradian.

Quantity, Formula symbol

Base unit, (Unit symbol), Definition

Plane angle

One radian (rad) is the angle between two circle radii that cut an arc from the circle perimeter whose length is equal to the radius.

Solid angle

One steradian (sr) is the solid angle whose vertex is in the center of a sphere and which cuts an area from the surface of this sphere that is equal to that of a square whose side has the same length as the radius of the sphere.

Quantity of information h

One bit corresponds to the smallest unit of information that defines a decision between two possible states.