In 2019 the kilogram stopped being a metal cylinder
#0024 Science & nature
Fact rank
S
Raised from the expected A to S: the change of definition, the size of the drift and the history of Japan's prototype were all confirmed in primary sources (BIPM and AIST)
Wow rank
S
Surprising history: the metal cylinder that was "the kilogram itself" was evacuated in wartime and became a cultural property
Usefulness
—
A fact worth knowing for its own sake
Verification: Supported by evidence
Rule check: an article whose "why" cannot be written with sources is capped at fact rank B.
AI verification reports the evidence it found; it does not guarantee that a claim is true.
Until May 2019 the kilogram was the mass of the International Prototype Kilogram kept near Paris, but comparisons with national copies showed they drifted apart by tens of micrograms over about a century, so the definition was changed to one based on the Planck constant. Japan's prototype (No. 6), kept by AIST, was evacuated to the Kakioka geomagnetic observatory in Ibaraki in 1944 during the war, and became an Important Cultural Property in 2022.
Why
(No sourced explanation yet.)Evidence
Primary BIPM, "History of the kilogram". The 1946 verification showed the masses of the copies diverging from the international prototype; in 1989–91 the median difference was about 25 micrograms link
Primary BIPM announcement. From 20 May 2019 the International System of Units (SI) underwent one of its most significant changes (the kilogram and other units defined by physical constants) link
Primary AIST document "About Japan's national prototype kilogram" (2018). Number 6. Evacuated in 1944 to the Kakioka geomagnetic observatory of the Central Meteorological Observatory in Ibaraki. After the redefinition it is planned to serve as a very good weight for maintaining the mass standard link
Primary AIST announcement (2022). Japan's prototype kilogram and related standards were added on 22 March 2022 to the Important Cultural Property "standards relating to the Metre Convention and the Weights and Measures Act" link
Notes and limits
The drift is relative: one can say the international prototype got lighter or the copies got heavier, and which one changed cannot be determined. How Japan's prototype is used now (since 2019) could not be confirmed; AIST's 2018 document only says it is "planned". The redefinition and the cultural-property designation are also in the Japanese Wikipedia article on the prototype kilogram.
Derivations
Derived from: 体重計の数字は緯度が高いほど大きく出る
Derived facts: —