International System of Units - SI (Basic Units)

# International System of Units - SI

## BASIC UNITS

• Unit of length : metre ( m )
The metre is the length of 1 650 763,73 wavelength in a vacuum of the radiation corresponding to the transition between the levels 2P10 and 5d5 of the atom of krypton 86.
• Unit of mass: kilogramme ( kg )
The kilogramme is the mass of the lump of platinum indium kept at the "International Bureau of Weights and Measures" (BIPM) at Sevres.
• Unit of time : second (s)
The second is the term of 9 192 770 periods of the radiation corresponding to the transition between the two levels of the ground state of the atom of caesium 133.
• Unit of intensity of electric current : ampere (A)
The ampere is the intensity of a constant electric current which, by passing in two rectilinear parallel wires, of infinite length, stretched out one metre apart in a vacuum, would create a force of 2x10-7 newtons ( 0,000 0002 N ) per each metre of length.
• Unit of thermodynamic temperature : kelvin ( K)
The kelvin is the fraction 1/273,15 of the thermodynamic temperature of the triple point of water.
• Unit of quantity of substance : mole ( mol)
The mole is the quantity of substance of a system which contains as many elementary entities as the atoms which are in 0,012 kilogramme of carbon 12. When the mole is used, the elementary entities must be specified; they can be atoms, molecules, ions, electrons, other particles or specified groups of such particles.
• Unit of luminous intensity: candela (cd )
The candela is the intensity of the radiation, in perpendicular direction, given off by 1/600 000 of square metre of a rough, completely black surface at the melting point of plati¬num under the pressure of 101 325 newtons per square metre.