How much does a kilogram weigh? Scientists have explored this seemingly simple problem for hundreds of years.
In 1795, France promulgated a law that stipulated “gram” as “the absolute weight of water in a cube whose volume is equal to one hundredth of a meter at the temperature when the ice melts (that is, 0°C).” In 1799, scientists discovered that the volume of water is the most stable when the density of water is the highest at 4°C, so the definition of the kilogram has changed to “the mass of 1 cubic decimeter of pure water at 4°C”. This produced a pure platinum original kilogram, the kilogram is defined as equal to its mass, which is called the archives kilogram.
This archival kilogram has been used as a benchmark for 90 years. In 1889, the First International Conference on Metrology approved a platinum-iridium alloy replica closest to the archival kilogram as the international original kilogram. The weight of “kilogram” is defined by a platinum-iridium alloy (90% platinum, 10% iridium) cylinder, which is approximately 39 mm in height and diameter, and is currently stored in a basement on the outskirts of Paris.
International original kilogram
Since the Age of Enlightenment, the surveying community has been committed to establishing a universal survey system. Although it is a feasible way to use the physical object as the measurement benchmark, because the physical object is easily damaged by man-made or environmental factors, the stability will be affected, and the measurement community has always wanted to abandon this method as soon as possible.
After the kilogram adopts the international original kilogram definition, there is a question that metrologists are very concerned about: how stable is this definition? Will it drift over time?
It should be said that this question was raised at the beginning of the definition of the mass unit kilogram. For example, when the kilogram was defined in 1889, the International Bureau of Weights and Measures produced 7 platinum-iridium alloy kilogram weights, one of which is the International The original kilogram is used to define the mass unit kilogram, and the other 6 weights made of the same material and the same process are used as secondary benchmarks to check whether there is drift over time between each other.
At the same time, with the development of high-precision technology, we also need more stable and accurate measurements. Therefore, a plan to redefine the international basic unit with physical constants was proposed. Using constants to define measurement units means that these definitions will meet the needs of the next generation of scientific discoveries.
According to the official data of the International Bureau of Weights and Measures, in the 100 years from 1889 to 2014, the quality consistency of other original kilograms and the international original kilogram changed by about 50 micrograms. This shows that there is a problem with the stability of the physical benchmark of the quality unit. Although the change of 50 micrograms sounds small, it has a great impact on some high-end industries.
If the basic physical constants are used to replace the kilogram physical benchmark, the stability of the mass unit will not be affected by space and time. Therefore, in 2005, the International Committee for Weights and Measures drafted a framework for the use of basic physical constants to define some basic units of the International System of Units. It is recommended that the Planck constant be used to define the mass unit kilogram, and competent national-level laboratories are encouraged to carry out related scientific research work.
Therefore, at the 2018 International Conference on Metrology, scientists voted to officially decommission the international prototype kilogram, and changed the Planck constant (symbol h) as the new standard to redefine the “kg”.
Post time: Mar-05-2021