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Universal Units Reflect Their Earthly Origins

For the past 2 centuries, scientists, surgeons, shipping companies, and shoppers have relied on a common system of measurements: the…

For the past 2 centuries, scientists, surgeons, shipping companies, and shoppers have relied on a common system of measurements: the metric system. De basisenheder van dit systeem hebben hun oorsprong in Earth’s spatial dimensions, timescales, en materialmasses. As our ability to make measurements becomes ever more precise, these units have been redefined, one by one, in terms of fundamental physical constants rather than material objects-except for the kilogram.

In an era where units are increasingly defined by the Properties of atomic physics, Earth’s properties remain important in metrology, the science of measurement.

This basic unit of mass remains tethered to a 1

39-year-old metal cylinder about the size of a plum that sits in air under three bell jars in France.

Scientists around the world rely on the International System of Units as a common basis on which to record and report.

Physics-based definition is officially adopted. their findings. The units used in this system are based on the meter, kilogram, second, kelvin, ampere, mol, and candela. Selv om alle videnskabelige områder, herunder geovidenskaberne, bruger disse enheder hver dag, kan mange geoscientists ikke forstå, hvor vigtige jordens egenskaber var i udviklingen af ​​disse enheder, og hvordan den nye definition opretholder sine bånd til terrestriske dimensioner.

The change in definition shows Earth’s properties remain important in metrology, the science of measurement.

Measuring the Earth

Earth, which at the time was thought to represent imperishability and stability , was the initial base for the International System of Units (SI) of length, time, and, indirectly, mass. Geodesy, the science of the shape of the Earth, its orientation in space, and its gravity field, was key in the definition of the metric system during the French Revolution.

In 1791, the French Academy of Sciences defined the meter as 1 / 10,000,000 the length of a quadrant of Earth’s meridian. Siden 1983 har måleren imidlertid været defineret som længden af ​​banen, som er rejst ved lys i en vakuum i et interval på 1 / 299.792.458 eller en sekund. Hence, c the speed of light in a vacuum, was fixed to a given value, and the definition of the meter now derived from that of the second.

Translating this concept into practical measurements requires some Metode dat is zowel precies en repeatable. Verschillende methoden concurreren voor de praktische realisatie van de definitie van de meter (ook wel mise en pratique genoemd). Today, the SI meter is often derived from the wavelength of an iodine-stabilized red helium-neon laser.

The SI unit of time, the second, was originally defined as 1 / 86,400 of the mean solar day. Subsequently, clocks reached a precision that allowed monitoring irregularities in the Earth’s rotation and revolution. In 1967, de Algemene Conferentie over Gewichten en Measures (CGPM) veranderde de definitie van de tweede tot de duur van 9,192,631,770 perioden van de straling die overeenstemt met de overgang tussen de twee hyperfine niveaus van de grondstaat of the cesium-133 atom. “

By this definition, the above frequency (abbreviated in metrology to Δν ( 133 Cs) hfs ) is exactly equal to 9,192,631,770 transitions per second, and the second is defined accordingly.

The Drifting Kilogram

The kilogram is unique in that the standard is still based (for now) on a manufactured object rather than on a physical constant.

In 1799, the kilogram was defined as the mass of 1 cubic decimeter or water at a temperature of 4 ° C. This unit is unique in that the standard is still based (for now) on a manufactured object rather than on a physical constant. The prototype kilogram from 1799 (the “kilogram of the archives”) and the present artifact from 1875 (the International Prototype Kilogram, or IPK) were manufactured to be consistent with this definition. The IPK is a cylinder of 39-millimeter height and diameter, made of 90% platinum and 10% iridium. At present, the IPK is held at the International Bureau of Poems and Mesures in Sèvres, France.

Forty replicas of the IPK were manufactured in 1884, and 34 of them were distributed to the signatories of the Meter Convention. The United States was allocated prototype numbers 4 and 20; Belgium received numbers 28 and 37, and Switzerland got number 38 and, more recently, number 89. These replicas have been used as national standards ever since.

The kilogram is scheduled to join the other SI units, and its prototypes are scheduled To become museum items when the kilogram is officially redefined in terms of the Planck constant. Deze verandering in definities is noodzakelijk omdat het gebruik van de IPK, een fysieke artifact, verschillende problemen veroorzaakt. Det er ingen måde at sikre dens langsiktige stabilitet, det kan være ødelagt eller ødelagt, og det udgør logistiske problemer når det skal sammenlignes med eksempler på andre nationale metrologiske institutter (NMIs).

Sammenligninger af massen af IPK to those of official copies and the national prototypes in 1889, 1948, 1989, and 2014 indicated that the IPK seems to have lost about 50 micrograms over 100 years (five parts in 100 million). Det er også mulig at alle prototyper viser en fælles masse drift, som ikke kan opdages ved intercomparisons. We thus face a strange situation: By definition, the mass of the IPK is invariant, but there is no means to check its stability using an absolute reference!

The instability of the IPK propagates to other base units that are tied to the kilogram, such as the candela (luminous intensity), the mole (number of atoms in a mass of material), and the ampere (electric current). It also influences the derived quantities such as force, density, and pressure. Consequently, for the past 25 years, several NMIs have been working to replace the IPK with a definition based on a fundamental constant of nature [ Richard et al., 2016].

Based in Physics, Measured in Geodesy

Although Earth’s properties are not stable enough to serve as a basis for the SI, geodesy has not said its last word. De nye definities van het tweede en de meter, eerder afgeleid van geodesy, richt nu op laboratoriumfysica experimenten.

 The Smurfs try their hand to measure gravitational acceleration. (left) The Smurfs try their hand to measure gravitational acceleration, perhaps inspired by (right) the FG5 # 202 absolute gravimeter at the Royal Observatory of Belgium’s underground geophysical monitoring station near Membach. Credit: (left) © Peyo – 2018, Lic. Lafig Belgium –; (right) Kris Vanneste, Royal Observatory of Belgium

On 16 November 2018, the 26th CGPM will ratify the revised SI based on seven constants: the frequency of the ground state hyperfine splitting of the cesium 133 atom Δν ( 133 Cs) hfs the speed of light in vacuum c Planck’s constant h the elementary charge e Boltzmann’s constant k Avogadro’s constant N A and the luminous efficacy K cd .

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