HDid you already weigh yourself this morning, look at the thermometer, or perhaps even read the electricity meter? And have you noticed no change? However, from this Monday (May 20, 2019) new rules apply as to how the world will be measured. From now on, the more than one hundred member countries of the Meter Convention unanimously voted in Versailles on November 16, 2018, receiving from the seven basic physical units of the International System of Units (SI) – seconds, meters, kilograms, amps, Kelvin, Mol and Candela – even the last one a new foundation, and indeed the most stable thing that physics has to offer: natural constants. These quantities are – as far as we know today – immutable for all eternity. Ideal prerequisite for calibrating the coordinate system with which we measure time, space, mass, temperature or electricity. Another good news: In everyday life you will notice nothing of this new regulation.
The reason for the completed remediation of the SI system is due to deficiencies in previous definitions of kilogram, kelvin, ampere and mole, the unit of chemical mass. These variables were in contrast to meter and second until yesterday on idealized, but impractical measurement requirements, unstable material properties or artificially created artifacts set. These historically grown conventions had long since ceased to meet the precision requirements of the present, and certainly not of modern metrology, which can count atoms and weigh molecules.
The Urkilogramm loses in mass
The kilogram required the greatest need for action. Because the unit of the mass was the only one of the SI basic sizes, which was still set on a reference object – a cylinder made of a platinum-iridium alloy. All the weights of the world, including those of the store around the corner, related to this Urkilogramm, which is kept since 1889 in a vault in Sèvres near Paris. A scare the guards of the Urkilogramms when they found that their gem has become easier over time against some of its duplicates, by 50 micrograms. Because of its "volatility" was the metrologists the Paris Urkilogramm therefore long a thorn in the eye. To make matters worse, central physical quantities such as energy, pressure or force and thus the amperes, which are linked to the fragile original kilogram, were therefore not reliable enough.
Rescue by the immutable
So it was high time for a general overhaul of the SI system, which was officially introduced in 1960 and whose regulations are committed to almost all countries of the world. Exceptions: the United States, Liberia and Burma. In recent years, thanks to sophisticated experiments, it has been possible to determine many natural constants very precisely. Out of seven of these Immutable, the values are now so well known that they can be used to set the base units of kilograms, amps, kelvin, and moles. And if the natural constants are determined more accurately, then the units will also benefit accordingly.
The idea of defining units of measurement over naturally occurring quantities has a long tradition. It goes back to the French Revolution. At that time, one wanted to bring order to the then prevailing thicket of different weight and length units. The measure of length should no longer be arbitrarily based on the physical dimensions of a person such as cubit or foot, but on the dimensions of the earth. Thus, the length of a meter was defined as the ten-millionth part of the distance from the North Pole to the Equator along the meridian that runs through Paris. The same was done with the jumble of existing mass units. From then on, a cubic decimeter of water served as a reference. For practical reasons, the original meter and the original kilogram were produced. The two artifacts were soon not only in France the "measures of all things".
The Urkilogramm moves to the museum
With the industrial revolution and growing international trade relations, many states realized that a unified cross-border measurement system would be very useful. In 1875, 17 leading industrial nations – including the German Reich – agreed to introduce a system of units based on meters, seconds and kilograms. It was sealed in a diplomatic contract, the "Meter Convention". In the course of technical and scientific progress, the system of units has been expanded. The Ampere, the Kelvin and the Candela were added in 1954, and the Mol in 1971. With inventions and refined measurement techniques, the definitions have also changed over time. Thus, the original meter soon after the invention of the laser in the early sixties had its day. The same happened for the second, which is no longer attributed to the earth's rotation, but to the precise ticking of the atomic clock.
Just in time for this year's World Metrology Day, the original kilogram has also become obsolete as a macroscopic standard. From now on, the cylinder in Sèvres is nothing but a museum piece. But that also comes at a price: If you could explain clearly yesterday what a kilogram is, it will not be that easy anymore. Because now you have to know what's up with natural constants like Planck's quantum of action. Fortunately, you do not need this special knowledge when you ask the baker for a kilogram of loaf of bread or read the temperature on the thermometer.
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