<P> 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 ." 14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to ." (Added by CIPM in 1980) </P> <Tr> <Td> candela </Td> <Td> cd </Td> <Td> luminous intensity </Td> <Td> "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000 ♠ 540 × 10 hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian ." 16th CGPM (1979, Resolution 3; CR, 100) </Td> <Td> The candlepower, which is based on the light emitted from a burning candle of standard properties . </Td> <Td> J </Td> </Tr> <Tr> <Th> Name </Th> <Th> Symbol </Th> <Th> Measure </Th> <Th> Current (2005) formal definition </Th> <Th> Historical origin / justification </Th> <Th> Dimension symbol </Th> </Tr> <P> The definitions of the base units have been modified several times since the Metre Convention in 1875, and new additions of base units have occurred . Since the redefinition of the metre in 1960, the kilogram is the only unit that is directly defined in terms of a physical artifact, rather than a property of nature . However, the mole, the ampere, and the candela are linked through their definitions to the mass of the platinum--iridium cylinder stored in a vault near Paris . </P>

Basic unit of volume in the si system