Only true boffins will be able to grasp Blighty's new legal definitions of the humble metre and kilogram

How units derive from first principles enshrined in Great British law


As much of the world's capitals erupt in protests and millions look fearfully at news of job losses and economic contraction, Great Britain has taken the legal step of redefining the metre and kilogram in law.

Instead of merely stating the definitions of the two main metric measurements, Parliament has now gone a step further and enshrined in law exactly how to derive them from first principles.

It's not quite within the reach of Average Joe and Josephine, sadly, but after the EU made a directive last year to more precisely define the metre and kilogram in accordance with SI units, Britain – still a de facto member of the bloc, if not in name – has followed suit.

New amendments to the Weights and Measures Act 1985 now explain exactly how you measure and define the m and kg. For the metre, the legal definition in Great Britain is updated to this:

...for which the symbol "m" is used, is the SI 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 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.

Previously, Schedule 1 of the Weights and Measures Act merely stated the speed of light in a vacuum rather than relating that to vibrating caesium atoms.

As for the kilogram, in November 2018, precision-seeking clever-clogses redefined it with reference to caesium atoms and Planck's constant, as now defined in UK law:

...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 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.

Since late 2018, all seven SI base units – the kilogram, the kelvin, the ampere, the mole, the candela, the metre, and the second – are now defined by their relationship to constants.

Curiously, in 1995 official definitions of miles, gallons and yards were deleted from the Weights and Measures Act, which hasn't stopped everyone from carrying on using them in the decades since.

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Famously, in the early 2000s when metric measures became compulsory, a group of market traders tried and failed to mount a legal challenge against laws making it a crime to sell bananas solely by the pound instead of by the lb and the 454g. The authorities' heavy-handed response (and later climbdown) set back the everyday use of SI units in Britain by decades.

The maximum velocity of a sheep in a vacuum, as defined, is unaffected by the recent changes to the law. ®


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