SI Units of Measurement and Their Realisation at NPL
| Quantity | Unit (Symbol) | Definition | Realisation |
| Time | second (s) | 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. | The second is realised by primary caesium frequency standards to about 2 parts in 1015. The majority are traditional caesium-beam designs, but the latest use lasers to control and detect the atoms. |
| Length | metre (m) | The metre is the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. | At NPL, the metre is currently realised through the wavelength of the 633 nm radiation from an iodine-stabilised helium–neon laser, with an uncertainty of about 3 parts in 1011. |
| Mass | kilogram (kg) | The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. | Kilogram masses and sub-multiples of 1 kg, made from similar materials, may be compared on the NPL precision balance to 1 μg. |
| Electric current | ampere (A) | The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10−7 N per metre of their length. | The ampere is realised, via the watt, to about 0.08 μA using NPL’s current weighing and induced-emf method. The ohm is realised at NPL via a Thomson-Lambert calculable capacitor to about 0.05 μΩ and maintained via the quantised Hall resistance to about 0.01 μΩ. The volt is maintained to 0.01 μV using the Josephson effects of superconductivity. |
| Thermodynamic temperature | kelvin (K) | The kelvin is the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water. | Triple point of water cells are used at NPL to realise the triple point temperature with a reproducibility of 0.1 mK via the International Temperature Scale in terms of which platinum resistance and other thermometers are calibrated within the range of 0.6–3000 K. |
| Amount of substance | mole (mol) | The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12. | Measurements of amount of substance do not require the mole to be realised directly from its definition. They are made using primary methods that give results expressed in moles by combining measurements made in other SI units. The number of entities in one mole is known to be 1 part in 107. |
| Luminous intensity | candela (cd) | The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hz and that has a radiant intensity in that direction of 1/683 W·sr−1. | The candela has been realised at NPL with an uncertainty of 0.02%, using a cryogenic radiometer that equates the heating effect of optical radiation with that of electric power. A solid-state photometer has been developed to evaluate light of other frequencies according to the spectral luminous efficiency curve of the human eye with an uncertainty of 0.1%. |
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