The Weights and Measures Act 1985 (Definitions of “Metre” and “Kilogram”) (Amendment) Order 2020

Amendment of the Weights and Measures Act 1985

2.—(1) Schedule 1 to the Weights and Measures Act 1985 (definitions of units of measurement) is amended as follows.

(2) In Part 1 (measurement of length), in the second column of the table headed “metric units”, for the definition of “METRE” substitute—

“for which the symbol “m” is used, is the SI(1) unit of length, defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined by taking the fixed numerical value of the caesium frequency Δv_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^-1.”.

(3) In Part 5 (measurement of mass or weight), in the second column of the table headed “metric units”, for the definition of “KILOGRAM” substitute—

“for which the symbol “kg” is used, is the SI unit of mass, defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10^-34 when expressed in the unit J s, which is equal to kg m² s^-1, where the second is defined by taking the fixed numerical value of the caesium frequency Δv_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^-1.”.

(1)

The term “SI” refers to the International System of Units (“the SI”) adopted by the eleventh meeting of the General Conference on Weights and Measures in 1960 which is updated and amended from time to time. See The International System of Units (9th edition (2019)) edited by the International Bureau of Weights and Measures, Pavillon de Breteuil, F-92312 Sèvres Cedex France, ISBN 978-92-822-2272-0.