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.
Reading
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.
Measurement |
Units |
Explanation |
---|---|---|
Unit of length |
Metre |
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 |
Kilogram |
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. |
Unit of time |
Second |
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 |
Ampere |
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 |
Kelvin |
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 |
Mole |
|
Unit of luminous intensity |
Candela |
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. |
Source: http://physics.nist.gov/cuu/Units/current.html
To maintain constant standards for some base units, prototypes are held for comparison. Figure 1.1 shows you a prototype for the kilogram.
Source: http://www.itc.gov.hk/ch/quality/scl/mm/mm2.htm
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.
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