Measurement
The International System of Units (SI)

The following is taken from Contemporary College Physics - 2nd Edition, Jones and Childers, Addison-Wesley Publishing Company, Appendix B: The International System of Units, p. 921

Definitions of the SI Base Units

Meter - unit of length

• - The 17th General Conference on Weights and Measures (CGPM, 1983) abolished the former definition of the meter and adopted a new definition which reads:
  • The meter is the length of the path traveled by light in vacuum during a time interval of 1/229 792 458 of a second.

  The old prototype of the meter, which was legalized by the 1st CGPM in 1889, is still kept at the International Bureau of Weights and Measures (BIPM).
  
  

  Kilogram - unit of mass

  - The 1st CGPM (1889) legalized the international prototype of the kilogram and declared:

  • This prototype shall henceforth be considered to be the unit of mass.

  This international prototype made of platinum-iridium is kept at the BIPM under conditions specified by the 1st CGPM in 1889.
  
  

  Second - unit of time

  - The second was defined originally as 1/86 400 of the mean solar day. Because a more precise definition was needed, the 13th CGPM (1967) replaced the astronomical definition of the second by the following:

  • 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 cesium-133 atom.

  
  
  

  Ampere - unit of electric current

  - The 9th CGPM (1948) adopted the ampere for the unit of electric current, with the following 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 2x10-7 newton per meter of length.

  
  
  

  Kelvin - unit of thermodynamic temperature

  - The 13th CGPM (1967) adopted the name kelvin (symbol K) and defined the unit as follows:

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

  In addition to the thermodynamic temperature (symbol T), expressed in kelvins, use is also made of Celsius temperature (symbol t) defined by the equation t = T - To, where t0 - 273.15 K by definition.
  
  

  Mole - unit of amount of substance

  - The following definition of the mole, adopted by the 14th CGPM (1971):

  • The mole is the amount of substance of a system that contains as many elemantary 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.

  
  
  

  Candela - unit of luminous intensity

  - The unit based on flame or incandescent filament standards before 1948 was replaeced initially by a unit based on a Planckian radiator (a black body) at the temperature of freezing platinum. Because of the difficulties and new experimental techniques, the 16th CGPM (1979) adopted the following definition:

  • The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

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last update August 23, 2004 by Jerry Stanbrough