Japan's metre, kilogram prototypes shown to press

The National Institute of Advanced Industrial Science and Technology, or AIST, showed Japan's metre and kilogram prototypes to the press on Monday, ahead of the 150th anniversary later this month of the conclusion of the Metre Convention in 1875.

Near the end of the 18th century, 1 metre was defined as one-10 millionth of the distance from the equator to the North Pole along a meridian through Paris and 1 kilogram as the mass of a litre of water.

International metre and kilogram prototypes made of platinum-iridium alloy were created after the convention that standardised the units of measurement was concluded. Copies of them were delivered to Japan in 1890.With the advances of technologies making it impossible to ignore the metal prototype's deterioration over time as a margin of error, the metre standard was changed from the metal prototype to one employing the wavelength of light in 1960. A method using the distance light travels in a certain amount of time was adopted to define 1 metre in 1983.

The kilogram standard was updated to one using the Planck constant, a minimum unit of light energy, in 2019.The original kilogram prototype has been kept in a temperature- and humidity-controlled steel safe at AIST, in the city of Tsukuba, Ibaraki Prefecture, eastern Japan. It showed the smallest change in mass in the past 100 years among the prototypes provided to countries across the world.

(2025/05/13-00:00)