The Weights and Measures Act 1985 (Amendment) and Units of Measurement Regulations 1986 (Amendment) Regulations 2019
Citation and commencement1.
These Regulations—
(a)
may be cited as the Weights and Measures Act 1985 (Amendment) and Units of Measurement Regulations 1986 (Amendment) Regulations 2019; and
(b)
come into force on 13th June 2020.
Amendment of the Weights and Measures Act 19852.
(a)
“for which the symbol “A” is used, is the SI unit of electric current, defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10-19 when expressed in the unit C, which is equal to A s, where the second is defined by taking the fixed numerical value of the caesium frequency ΔvCs, 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.”;
(b)
in the definition given for “WATT”, after “joule”, insert “, where the second has the meaning given in the definition of “AMPERE””.
Amendment of the Units of Measurement Regulations 19863.
“SI base units.
Quantity
Unit
Name
Symbol
Time
second
s
Length
metre
m
Mass
kilogram
kg
Electric current
ampere
A
Thermodynamic temperature
kelvin
K
Amount of substance
mole
mol
Luminous intensity
candela
cd
Definitions of SI base units
Unit of time
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔvCs, 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.
Unit of length
The metre, symbol m, is the SI unit of length. It is 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 in terms of ΔvCs.
Unit of mass
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s-1, where the metre and the second are defined in terms of c and ΔvCs.
Unit of electric current
The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10-19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ΔvCs.
Unit of thermodynamic temperature
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10-23 when expressed in the unit J K-1, which is equal to kg m2 s-2 K-1, where the kilogram, metre and second are defined in terms of h, c and ΔvCs.
Unit of amount of substance
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number.
The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
Unit of luminous intensity
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W-1, which is equal to cd sr W-1, or cd sr kg-1 m-2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔvCs.
Special name and symbol of the SI derived unit of temperature for expressing Celsius temperature
Quantity
Unit
Name
Symbol
Celsius temperature
degree Celsius
°C
Celsius temperature t is defined as the difference t = T – T0 between the two thermodynamic temperatures T and T0 where T0 = 273.15 K. An interval or difference of temperature may be expressed either in kelvins or in degrees Celsius. The unit “degree Celsius” is equal to the unit “kelvin”.”.
These Regulations amend the Weights and Measures Act 1985 and the Units of Measurement Regulations 1986, both of which make provision for legal units of measurement for use in the United Kingdom. These Regulations partly implement Commission Directive 2019/1258 amending, for the purpose of its adaptation to technical progress, the Annex to Council Directive 80/181/EEC as regards the definitions of SI base units (OJ L 196, 24.7.2019, p. 6), which relates to the definition of units of measurement. The amendments in the Commission Directive update European law to reflect seven new definitions of expressions of measurement and indications of quantity of the International System of Units (SI) adopted by the General Conference on Weights and Measures at its 26th meeting which took place from 13th to 16th November 2018 (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). The amendments do not alter the value of those units of measurement, but substitute new definitions expressed in terms of natural constants.
Regulation 2 amends the definition of the ampere in Part 7 of Schedule 1 to the Weights and Measures Act 1985, and amends the definition, in the same Part, of the watt in relation to the reference to the second, to ensure consistency of interpretation.
Regulation 3 amends the Units of Measurement Regulations 1986 by substituting a new paragraph 1 in Schedule 1 to those Regulations to reflect the substitution made by the Commission Directive of section 1.1, Chapter 1 of the Annex to Council Directive 80/181/EEC. New definitions are provided for the second, metre, kilogram, ampere, kelvin, mole and the candela. A small amendment is made in the representation of the decimal point in the definition for the degree Celsius.
An impact assessment has not been produced for this instrument as no, or no significant, impact on the private, voluntary or public sector is foreseen.