The metrology renovation challenges one true kilogram of science


The atoms of the silicon-28 balloon were counted to amplify the Avogadro constant and again to determine the molar. A copy of Le Grand K, the standard of a kilogram, is reflected in the ball reflection.


Adrian Cho

Like an aging monarch, Le Grand K intends to show off to the modern one. For the past 130 years, this platinum-iridium alloy sharp cylinder has been the standard of world mass. Retained in the Threshold and locked away from the International Weight and Measurement Units (BIPM) in Sèvres, France, every 40 years weighs the same weights worldwide. Now, in a revolution that is much more bloodthirsty than the one who paid the king of Louis XVI, it would give it a throne to one real kilogram.

When the 26th General Conference on Weight and Measure (CGPM) meets next week in Versailles, France, representatives of 60 Member States are expected to vote to redefine the International Unit System (SI) so that the four basic units – kilograms, amperes, kelvins and moles – are indirectly determined by physical constants fiat confirms. They combine the other three basic units – the second, the meter and the candle (the visible brightness of light) – already defined in this way. The rewriting removes the last physical artifact which is used to define the unit, Le Grand K.

The purpose of the transfer is to make the units more stable and allow researchers to develop more accurate and flexible techniques for converting standard units into units of measure. "It is the beauty of redefinition," says BIPM physicist Estefanía de Mirandés. "You are not limited to one technique." But even supporters of fragile changes admit that they may be confused. "Cooler heads have said," What are we going to do by teaching people to use this? "Says Jon Pratt, physicist at the US National Standardization and Technology Center (NIST) in Gaithersburg, Maryland.

The new SI generalizes the compromise already used to further determine the meter according to the speed of light. Until 1983, the speed of light was measured independently by meters and seconds. However, that year, the 17th CGPM determined the speed of light to exactly 299,792,458 meters per second. The meter then came to be measured: the distance light goes 1 / 299,792,458 seconds. (The second was tied to the fluctuations in the microwave radiation of cesium-mining in 1967)

The new SI plays the same game with other units. For example, it defines a kilogram of Planck as a standard that pops up the whole quantum mechanics. Standard is now fixed at exactly 6.62607015×10-34 kilograms per square meter. Since a kilogram appears in this assay, any experiment that previously measured the constant would be a means of measuring pounds instead.

Such experiments are much more difficult than the speed of the luminous intensity, which is the physics of the undergraduate studies. One technique uses a device called the Kibble Balance, which is a bit like a fair scale. The mass on the other side is equilibrated by electric force produced by the electric winding on the other side, depending on the magnetic field. To balance the weight, the current must pass through the coil. Scientists can compare mass at present with the independent voltage generated when they remove the pulp and move the coil up and down in the magnetic field.

Metric Editing

Voting is expected to redefine the basic units of the metric system to the fixed physical standards.

The metric system Amount Define the standard
Kilogram Mass Planck's standard
meter Speed Speed ​​of light
Second Time Cesium radiation frequency
Ampere current Electron charge
Kelvin Temperature Boltzmann's standard
Mole Amount of substance Avogadro constant
candela The intensity of light Frequency frequency efficiency


The real difficulty is to increase current and voltage by quantum-mechanical devices that make it electronically charged and according to Planck's constant. Now that the new SI has corrected these constants, the balance can be used to pick up the slowness whose mass is exactly one kilogram. Redefining also makes quantum techniques the SI standards for measuring voltages and currents, says James Olthoff, NIST physicist. Until now, SI has determined the amperage impracticably between the power of infinitely long power cord wires separated by a meter.

But the application of complex new definitions blurs to anyone without physics, says Gary Price, metrology in Sydney, Australia, who always reported to the Australian National Standardization Committee. In fact, he says that the new SI does not fulfill some of the basic conditions of the unit system, which is to be defined as the mass of mass, which measures the masses, the length of the length being measured, and so on. "The new SI does not have any weights or measures," says Price.

Metrologists considered more intuitive redefinitions, "says Olthoff. For example, you can set a pound to a large amount of specific atom. But such a standard would be impractical, Olthoff says. Quite ironically, scientists have already dropped atoms with slightly circular 1 kg silicon-28 spheres to determine the exact value of a molar previously defined as 12 grams of carbon-12 atoms to be measurable.

If adopted, the new SI will enter into force in May 2019. In the short term, the little one will change, Pratt says. NIST continues to apply pressure standards by calibrating the weights of its kilogram – even now it does so with its Kibble balance. Finally, Pratt says that scientists could develop table balances that companies could use to calibrate their own micrograms.

Next is another way of thinking. Metrologists develop more accurate atomic clocks that use optical frequencies at higher frequencies than the current cesium standard. They should form the basis for another finer definition, De Mirandés says, perhaps in 2030.

With Le Grand K, BIPM keeps that time and calibrates it regularly as a secondary mass rule, De Mirandés says. It is quite valuable to the late French king.


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