The "original kilogram" has served its day

General Conference for Measure and Weight approves new system of units

Now it is official: The General Conference for Measures and Weights in Paris has adopted the new International System of Units. © A. Nicolaus / PTB
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Turning point in physics: In Paris, the revision of the International System of Units was adopted on 16 November. From May 20, 2019, all SI units - meters, kilograms, seconds, moles, candela, kelvin and amperes - will be converted to natural constants as reference values. With this, the reference kilogram, a cylinder made of a platinum alloy, has finally become obsolete. The advantage: The new reference units can be measured more accurately and anywhere on the globe.

For a unit to be of benefit, it must be clearly defined, reliably measurable and precisely calibrated to a benchmark. Ideally, it goes back to a natural constant, such as the second on electron jumps in the cesium atom or the meter on the duration of light in a vacuum. But that was not the case for all units. For Kelvin, amperes, moles and kilograms, the official conversion to natural constants was still pending.

New era for the base units

But now it's done: at the 26th General Conference on Weights and Measures in Paris, the states of the so-called Meter Convention finally adopted the revision of the International System of Units on 16 November. You have now also confirmed the redefinition of the four remaining units and the permissible measurement methods. This means that all seven basic SI units can now be measured and calibrated using physical basic constants.

"The redefinition of SI units is a milestone in scientific advances, " said Martin Milton, director of the International Bureau of Weights and Measures (BIPM). "By using the fundamental constants of nature as a basis for important concepts such as mass and time, we have a solid foundation on which to deepen our scientific understanding, develop new technologies, and address some of society's greatest challenges."

The SI units and their physical basis in the form of natural constants © BIPM

New for kilograms, amps, Kelvin and moles

Specifically, this means that the original kilogram in Paris, which has been the measure of all things for 130 years, has now become obsolete. Until now, this platinum cylinder, which has been treasured, served as a reference for all calibration weights worldwide. Now two physical measuring methods take their place. One uses a so-called Watt scale and thus electromagnetic forces as a measuring aid, the other the number of particles in a silicon sphere. Both can ultimately be traced back to the Planck constant and thus a natural constant. display

The new measuring methods for the ampere, the Kelvin and the molar were also officially recognized at the General Assembly. All electrical units including the ampere will in future be applied to the quantum effects or by counting electrons per time Elementary charge of the electron returned. The mole is defined by the Avogadro constant of a specified substance and the Kelvin by the Boltzmann constant. It indicates how the thermal energy of a gas and thus the movement of the gas particles depends on the temperature.

More precision and measuring reliability

The new unit definitions will come into force on May 20, 2019. Nothing will change for our everyday life, but it will change for science and technology. For the new system of units is now, so to speak, a universal language on which all research worldwide is based. The big advantage: The calibration of measuring instruments becomes much more precise and thus more accurate and reliable measurements are possible. "We are no longer tied to the limitations of objects, but have universally accessible units that will pave the way to even greater precision and drive scientific development, " says Barry Inglis Director of the International Committee of Measures and Weights.

Constants of nature apply everywhere. Thus, the new SI is, so to speak, a universal language to which the world community has now communicated.

(PTB / National Physics Laboratory, 19.11.2018 - NPO)