Definitions of base units of the metric system.
Section 4
SEC. 4. Definitions of base units of the metric system.— For purposes of this Act: a. The base unit of length shall be the metre, which is the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom. b. The base unit of mass shall be the kilogram, which is equal to the mass of the international prototype of the kilogram, made of platinum-iridium, the standard of which is kept at the Bureau of International Weights and Measures at Sevres, France. c. The base unit of time shall be the second, which 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. d. The base unit of electric current shall be the ampere, which is that constant current which, if maintained in two parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length. e. The base unit of thermodynamic temperature shall be the. kelvin, which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. f. The base unit of luminous intensity shall be the candela, which is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a blackbody at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre. g. The base unit of amount of substance shall be the mole, which is the amount of substance of a system which contains as many elementary 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.