The seven: second, metre, kilogram, ampere, kelvin, mole and candela
Everything else in the system is derived from these.
Science · Physics
The SI base units
System established 1960; redefined 20 May 2019
Physics — metrology
The International System of Units rests on seven base units: the second, metre, kilogram, ampere, kelvin, mole and candela, with symbols s, m, kg, A, K, mol and cd. Every other unit — the newton, the volt, the watt — is built from them. For most of their history these units were tied to objects and recipes: a platinum bar in a Paris vault, the triple point of water, twelve grams of carbon-12. On 20 May 2019 the system was rebuilt so that all seven follow instead from fixed numerical values of seven constants of nature, among them the speed of light, the Planck constant, the elementary charge and the Boltzmann constant. With that change the last physical artefact in the system, the platinum-iridium kilogram prototype kept at Sèvres since 1889, was retired.
How to spot them
- ▸Seven of them, and one is about how bright something looks to a human eye
- ▸Since 2019 every one is defined by fixing a constant of nature, not by an object
- ▸The lump of platinum-iridium in a Paris vault that they finally made redundant
- ▸Their symbols are s, m, kg, A, K, mol and cd
- ▸The change took effect on World Metrology Day, 20 May 2019
Key facts
Rebuilt on 20 May 2019 so that fixed values of nature's constants define them all (2019)
Agreed by the 26th General Conference on Weights and Measures in November 2018.
Their last physical artefact was a platinum-iridium cylinder nicknamed Le Grand K (1889)
Kept under three glass bell jars at Sèvres, it had drifted about 50 micrograms against its copies.
One of them is defined by 9,192,631,770 oscillations of a caesium-133 atom (1967)
That number fixes the second, and through it the metre.
One of them alone is defined by human physiology, using 540 terahertz green light
The candela is scaled to the eye's peak daytime sensitivity.
Seven of them, and since 2019 not one is defined by a physical object (2019)
It is now simply a fixed number of entities: 6.02214076 × 10²³.
Key ideas
- Defining a unit by fixing a constant means the unit can be realised anywhere, by anyone with good enough apparatus, and never drifts — unlike an object that can be scratched, cleaned or lost.
- The kilogram is now realised with a Kibble balance, which weighs a mass against an electromagnetic force measured in quantum electrical units, or by counting atoms in a near-perfect silicon-28 sphere.
- The metre was redefined in 1983 by fixing the speed of light, which is why c is exactly 299,792,458 m/s and not a measured quantity.
Quick-fire facts
- •The seven base units are the second (s), metre (m), kilogram (kg), ampere (A), kelvin (K), mole (mol) and candela (cd)
- •The 2019 redefinition took effect on 20 May 2019, World Metrology Day and the anniversary of the 1875 Metre Convention
- •The defining constants include c = 299,792,458 m/s, h = 6.62607015 × 10⁻³⁴ J s, e = 1.602176634 × 10⁻¹⁹ C, k = 1.380649 × 10⁻²³ J/K and N_A = 6.02214076 × 10²³ mol⁻¹
- •The second is fixed by the caesium-133 hyperfine transition frequency of exactly 9,192,631,770 Hz, a definition dating from 1967
- •The International Prototype of the Kilogram, 'Le Grand K', is a cylinder of 90% platinum and 10% iridium about 39 mm tall and wide, made in 1879 and sanctioned in 1889
- •It is kept at the International Bureau of Weights and Measures at the Pavillon de Breteuil in Sèvres, near Paris, under three nested bell jars; opening its safe needs three separately held keys
- •The kilogram is the only base unit whose name still carries an SI prefix
- •The kelvin was previously defined by the triple point of water, exactly 273.16 K
- •Four new prefixes — ronna, quetta, ronto and quecto — were added in 2022, extending the range to 10³⁰
Drill it — 10 cards
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Sources
- Bureau International des Poids et Mesures (BIPM) — SI base units
- National Institute of Standards and Technology (NIST) — SI Redefinition