Weights And Measures Citation and commencement 1 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 1985 2 In the third column of the table in Part 7 of Schedule 1 to the Weights and Measures Act 1985 (definitions of units of measurement of electricity)— a for the definition given for “AMPERE” substitute— 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 Δ v 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 . ; 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 1986 3 For paragraph 1 of Schedule 1 to the Units of Measurement Regulations 1986 (definitions of SI base units ) substitute— SI base units. Quantity Unit Name Symbol 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 Δ v 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 . 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 Δ v Cs . 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 m 2 s -1 , where the metre and the second are defined in terms of c and Δ v Cs . 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 Δ v Cs . 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 m 2 s -2 K -1 , where the kilogram, metre and second are defined in terms of h , c and Δ v Cs . 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 × 10 23 elementary entities. This number is the fixed numerical value of the Avogadro constant, N A , 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 × 10 12 Hz, K cd , 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 s 3 , where the kilogram, metre and second are defined in terms of h , c and Δ v Cs. 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 Special name and symbol of the SI derived unit of temperature for expressing Celsius temperature Quantity Unit Name Symbol Celsius temperature t is defined as the difference t = T – T 0 between the two thermodynamic temperatures T and T 0 where T 0 = 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”. Celsius temperature degree Celsius °C . Kelly Tolhurst Parliamentary Under Secretary of State Department for Business, Energy and Industrial Strategy 3rd September 2019