<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> <P> It has long been an objective in metrology to define the kilogram in terms of a fundamental constant, in the same way that the metre is now defined in terms of the speed of light . The 21st General Conference on Weights and Measures (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram ." Two possibilities have attracted particular attention: the Planck constant and the Avogadro constant . </P>

Which of the following in not a si base unit