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Base units

19 January, 2016 - 17:44

The SI unit system distinguishes physical units into two classes:

  • base units; and
  • derived units.

These two categories cover all the units that affect our daily lives.

In the following reading, the National Institute of Standards and Technology (NIST) in the USA provides a good introduction to the SI units.


National Institute of Standards and Technology, US, 'SI units'.
Follow the links under the heading 'Essentials of the SI'.

If you would like to read more, you have the option of going to the website for the international standards organization called the Bureau International des Poids et Mesures (BIPM) for its information on the SI units.

Each measurement unit has a base quantity that has been adopted by convention. In each coherent system of units, there is only one base unit for each base quantity. Each of these base units can be further decomposed. There are seven base units, each representing, by convention, different kinds of physical quantities. Learn more about these base units in Table 1.1.

Table 1.1 Definitions of the SI base units




Unit of length


The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.

Unit of mass


The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Unit of time


The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Unit of electric current


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 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.

Unit of thermodynamic  temperature


The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Unit of amount of substance


  1. 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; its symbol is 'mol.'
  2. 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.

Unit of luminous intensity


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.



To maintain constant standards for some base units, prototypes are held for comparison. Figure 1.1 shows you a prototype for the kilogram.

Figure 1.1 The standard for mass


The Society of Construction Law (HK) keeps this primary mass standard in its laboratories located in Wan Chai. SCL uses this working mass as the basis for comparisons with samples submitted for calibration.