Definition of kilogram, amp, and kelvin set to change forever


A shift from physical artefact to natural constant marks a huge shake-up in how the world defines units of measurement. Andrew Masterson reports.


Last days of the kilogram: the silicon sphere held at the at the Physikalisch-Technische Bundesanstalt that currently defines the unit.

Photo by Julian Stratenschulte/picture alliance via Getty Images

From Monday, 20 May, the official definitions for several units – including the kilogram, the ampere and the kelvin – will change forever in what is being hailed as a fundamental revamp of the way humanity measures things.

The date of the change, which will see the formal adoption by almost 100 countries of a new International System of Units (SI), is not coincidental. It marks World Metrology Day – wherein “metrology” denotes the science of measurement.

In one sense, the shift in the way units of measure are defined represents incremental, rather than radical, change.

After several years of research, an international collaboration of institutes, led by Germany’s Physikalisch-Technische Bundesanstalt (PTB), has concluded that units of weight, temperature, substance, and electric current are best defined in terms of the strict values of naturally occurring constants rather than by physical artefacts.

The process began half a century ago, when the official definition of a “second” was redrafted to align with the unvarying transition between two energy levels of an atom, moving away from the previous measurement, which involved the division of 24 hours by 86,400.

The same principle was applied to the formal definition of a metre. In the nineteenth century the universal metre was a length of metal comprising 90% platinum and 10% iridium. The measure was changed in 1960, and finally cemented in 1983.

From that year, a metre was forever enshrined as “the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second”.

The reason for this is clear. A lump of metal, no matter how well crafted, will change (albeit on a molecular scale) in response to external forces. The speed of light won’t – it’s the speed of light, one of the unwavering constants in the universe.

From 20 May, therefore, other units of measurements will also be based on natural events. To do this, researchers at the PTB and colleagues have succeeded in establishing, or confirming, fixed values in seven constants.

These include the speed of light in a vacuum, which remains at the previous estimation for the definition of length, and the “frequency of the hyperfine structure transition of the ground state in the caesium-133 atom” as the marker for a second.

Planck’s constant, which correlates photon energy with electromagnetic wave frequency, will be the fundamental measure for the kilogram.

The elementary charge – the amount of electricity carried by a single proton – will be the basis of the ampere.

The Boltzmann constant – the measure of the relative kinetic energy of particles in a gas – will underpin the absolute definition of the base unit of temperature, the kelvin.

The Avogadro constant will be used to formalise the measure of the basic unit of substance, known as the mole. The constant is used to calculate the number of atoms, molecules or ions in a single unit.

And finally, photometric spectral luminous efficacy, which describes the number of luminous units, known as lumens, produced per watt of power, will be drafted in to nail down the definition of the universal measure of light, the candela.

The candela, by the way, until the changeover, is defined as the luminosity of a source that emits monochromatic radiation of frequency 5.4×1014 hertz and is an approximation of the light given off by a candle fashioned from whale blubber.

In the brave new world of constant-derived metrology, at least, no cetaceans will be harmed during the process of calculation.

  1. https://www.nist.gov/pml/weights-and-measures/metric-si/si-units
  2. https://physics.nist.gov/cuu/Units/second.html
  3. https://physics.nist.gov/cuu/Units/meter.html
  4. https://www.nist.gov/si-redefinition/kilogram-mass-and-plancks-constant
  5. https://www.nist.gov/si-redefinition/kelvin-boltzmann-constant
  6. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Atomic_Theory/The_Mole_and_Avogadro
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