Standards - Changes in the Definition of Metric Units (3)

6. Mole (Number of Molecules) Original Definition: The actual number of atoms in 12g of Carbon. Current Definition: Redefined using Avogadro's constant. Reason for Change: Varies depending on isotope ratio. 7. Candela (Luminosity) Original Definition: The brightness of a candle as a standard. Secondary Definition: The brightness of a black body of a reference area at the melting point of platinum. Current Definition: Redefined using frequency and radiant intensity (light is green light with a wavelength of 555nm). Reason for Change: A candle's brightness varies with combustion conditions. Difficulty in implementing a high-temperature black body.