Appendix C SI base units

Chapter contents

C.1 Mass 347

C.2 Length 347

C.3 Time 347

C.4 Electric current 347

C.5 Temperature 347

C.6 Luminous intensity 347

C.7 Amount of substance 347

SI base units were discussed in Chapter 4. Below are the precise definitions of these units.

C.1 Mass

The unit of mass is the kilogram. The mass of one kilogram is equal to the mass of the international platinum–iridium prototype of the kilogram.

C.2 Length

The unit of length is the metre. The metre is the length equal to 1 650 763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the levels 2p₁₀ and 5d₅ of the krypton-86 atom.

C.3 Time

The unit of time is the second. 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 caesium-133 atom.

C.4 Electric current

The unit of electric current is the ampere. The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newton per metre of length.

C.5 Temperature

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

C.6 Luminous intensity

The unit of luminous intensity is the candela. The candela is the luminous intensity, in the perpendicular direction, of the surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre.

C.7 Amount of substance

The unit of amount of substance is the mole. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12.