What is the new definition of KG

Constants of nature Resolved: the kilogram is redefined

Problems with the original kilo

The original kilo is a 3.9 centimeter high and 3.9 centimeter thick metal cylinder made of 90 percent platinum and 10 percent iridium. Since 1889 the reference standard for the unit of measurement, the kilogram, has stood under three glass bells in a safe of the "International Bureau of Weights and Measures" (BIPM) in Paris. For eternity, one might think. But the original kilo - nobody knows why - is getting lighter and lighter. It has lost 50 micrograms in the past 129 years compared to its 70 official copies worldwide. That doesn't sound like much at first, but in our high-tech world, where measurements are taken in nanometers (millionths of a millimeter) or femtoseconds (millionths of a billionth of a second), it is becoming more and more of a problem.

No place for handcrafted items

In such a world there is no longer any room for a man-made macroscopic reference object such as the original kilogram, especially since this has long played an outsider role in the International System of Units (SI) of physical quantities. The metric units for length or time have long been derived from natural constants such as the speed of light or electron charges.

Today, around one meter corresponds to the distance that light travels in a vacuum during the 299,792,458th part of a second. One second in turn is defined as 9,192,631,770 times the period of radiation oscillations at energy level transitions of the cesium isotope 133Cs.

Watt Libra and Avogadro Project

In the future, the kilogram will also be defined with the help of such natural constants. Two methods are considered to be particularly promising: the so-called watt balance and the Avogadro project.

With the watt balance, for example, mechanical weight on one side is balanced out by electromagnetic force on the other. Put simply, current runs through a coil and a force is generated that compensates for the weight of the item to be weighed. The electrical energy used can be precisely determined using the so-called Planck constant. This natural constant, which describes the relationship between the energy and frequency of a photon, is in a sense the smallest amount of energy that can be imagined. It enables an exact physical conversion of electrical energy into kilograms.

The Avogadro project does not compare mechanical and electromagnetic performances. Rather, the scientists are trying to determine the kilogram as precisely as possible by counting the atoms of a high-purity silicon sphere.

On May 20, 2019, the original kilo finally had its day

It has not yet been determined which of the methods will be used to determine our kilogram in the future. The Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig assumes that both the Watt balance and the Avogadro method will replace the original kilo on May 20, 2019. Then the new system of units should come into effect. According to the PTB, states could then choose the method to be used in the future to determine a kilogram.

That we could then become noticeably lighter or heavier than before when weighing on the bathroom scales cannot be assumed, however. The changes are not noticeable in daily life, says PTB spokesman Jens Simon. However, deficiencies in the previous system would be eliminated and a universal language would be created for a highly technical world.