The Kilogram Gets a Redefinition

The kilogram, along with some other units in the metric system, has been given a revamp this month. The new definition of this standard of mass is now based on physics fundamentals rather than the mass of a metal stored in France.


Standard Units Redefined

For the last 130 years, a kilogram has been defined as the weight of a metal cylinder stored safely in Paris. This definition has become outdated, and scientists have now moved to something more standard. The new unit is now based on a fundamental constant of nature—the Planck constant. This constant is not affected by time and space, which makes it a suitable reference for a worldwide standard. By contrast, the metal cylinder previously used had to be stored under very sensitive and secure conditions to prevent any damage or corrosion from the elements or from its handling as well as to eliminate the risk of theft. The changes came into effect on May 20, 2019.

Several other units on the International System of Units (SI system) have also been updated. The work that went into all the changes required a team of scientists working with the very best and most accurate instruments to explore precise and accurate definitions. The quantities looked at included weight, electric current, and temperature.

The 26th General Conference on Weights and Measure, which took place in Versailles, France in November 2018, also agreed on the redefinition of a few other standard units. The Kelvin for temperature, the ampere for electric current, and the mole for an amount of substance have each been redefined.

The next unit due for an update is the unit of time—the second. This update could see the definition of a second move from using atomic clocks made of cesium atoms to the newer optical atomic clocks, which are known for far higher precision.

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