The Seven Base SI Units

- The seven base SI or System International units that form the basis of the Système International d'Unités or International System of Units.

The International System of Units - "Système International d'Unités" or SI units are the basic system that ahs been used for measurements and units for many years. The SI was established in 1960 by the 11th General Conference on Weights and Measures (CGPM). Many of the older systems were not as easy to handle or use, and none were standardized across the world.

As a result the "System International" or SI was developed and introduced. Within SI there are seven base units upon which all others are based. The base units include: mass, length, time, temperature, amount of substance, electric current, and luminous intensity.

Table of the SI base units

Physical quantity Dimension Symbol Unit name Unit Symbol
Mass M Kilogram kg
Length L Metre m
Time T Second s
Temperature ° Kelvin k
Amount of substance N Mole mol
Current I Ampere A
Luminous intensity J Candela cd

SI unit definitions

In order that each of the SI units and quantities can be standardised across the globe, it is necessary to have exact definitions of each of them. While it is unlikely that these definitions of the SI units will be used in anything but a standards laboratory, they are often useful to see and know.

  • Metre:   The metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second.
  • Kilogram:   The kilogram is the unit of mass equal to the mass of the international prototype of kilogram.
  • Second:   The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levers (F=4, mF=0 to F=3, mF=0) of the ground state of the cesium 133 atom.
  • Ampere:   The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2x10-7 Newton per meter of length
  • Kelvin:   The Kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
  • Mole:   The mole is the mount of substance of a system which contains as many elementary entities as there are atoms in .012 kg of carbon 12 (about 6.022x1023 atoms). 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.
  • Candela:   The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Supplementary SI Units

Physical quantity Dimension Symbol Unit name Unit Symbol
Plane angle α Radian rad
Solid Angle ω Steradian sr

Reason for seven SI base units

The SI or System International has the set of seven base units. These have been chosen to fulfil the requirements for science and technology measurements. The selection of seven base units is the responsibility of the International Committee of Weights and Measures (CIPM) which ahs defined and now maintains the SI.

The base units are to a certain extent an arbitrary choice. When the metric system was set up a three dimensional mechanical system was chosen with the metre kilogram and second as base units. This was later extended to a four dimensional system to include first the ampere, then the kelvin, candela and finally the mole were added. These additions brought the number of base SI units to seven.

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