# SI Base Units

## Second (s)

Quantity: Time Unit: Second (s) Definition: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

## Meter (m)

Quantity: Length Unit: Μeter (m) Definition: The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. The definition of meter is directly linked to the definition of the unit of time and determines exactly the value of the speed of light in vacuum, c=299 792 458 m/s. The realization of the unit of length is achieved by frequency stabilized lasers and interferential methods which are used for the calibration of gauge blocks.

## Kilogram (kg)

Quantity: Mass Unit: Kilogram (kg) Definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. Note: This international prototype is manufactured from platinum and iridium and is kept in the international office of weights and measures, (BIPM), Paris, France. The international kilogram prototype was manufactured in 1889, and is still used until today as the ultimate reference standard. All efforts of metrologists for a new definition of the kg based on a natural constant are giving less precise results until now. International institutions of metrology have their copies, which are named “national prototypes”. Each one of them is compared regularly with his international equivalent. Nevertheless, the various national prototypes present more and more divergence. Consequently, scientists search intensively for a method by which the unit of mass can also be based on one fundamental constant.

## Ampere (A)

Quantity: Electric current Unit: ampere (A) Definition: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2·10-7 Newton per meter of length. It is very difficult to realize the ampere unit with high precision in the laboratory. For this reason metrology laboratories around the world instead of realizing αmpere they realize the unit of electric potential (V) and electrical resistance (Ω).

## kelvin (K)

Quantity: Thermodynamic temperature Unit: kelvin (K) Definition: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The triple point of water is the unique temperature at which the three phases of water (water vapour, liquid water and ice) coexist at equilibrium. The relation between degrees Celsius and degrees kelvin is: t/°C = T/K-273.15. Thus, it follows from the definition of kelvin that the temperature of the triple point of water is 273.16 K or 0.01 °C exactly.

## Candela (cd)

Quantity: Luminous intensity. Unit: Candela (cd). Definition: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540·1012 Hertz and that has a radiant intensity in that direction of 1/683 Watt per steradian.

## Mole (mol)

Quantity: Amount of substance. Unit: Mole (mol). Definition: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. The number of elementary particles contained in one mole of any substance is called Avogadro constant and it is equal to 6,02214179(30)×1023.