'Si' on the new SI: NIST backs proposal for a revamped system of measurement units

October 27, 2010
In support of a proposal to revise the International System of Units (SI) so that it would be based on constants of nature, NIST efforts such as the watt balance experiment (pictured) offer new ways of determining an accurate value of the Planck constant h, thereby contributing to a definition of the kilogram based on physical constants. Credit: Steiner/NIST

Taking the first steps of what would be a major historical advance in the science of measurement, the National Institute of Standards and Technology is participating in a worldwide effort to recommend major revisions to the International System of Units (SI), the modern metric system that is the basis of global measurements in commerce, science and other aspects of everyday life. The new SI, which would be based on seven constants of nature, would enable researchers around the world to express the results of measurements at new levels of consistency and accuracy.

The most significant change in the possible future revision of the SI would be in the kilogram, the only one of the SI's seven base units* still defined in terms of a material "artifact": a 130-year-old platinum-iridium cylinder maintained at the International Bureau of and Measures in France. The kilogram artifact poses long-term problems because its mass changes slightly over time. The proposed revision "puts the SI on a firm foundation," says Ambler Thompson, a NIST scientist involved in the international effort. "We get rid of the last artifact."

In the current SI, it's not just the unit of mass that depends on the kilogram. The definitions of the ampere (electric current), mole (amount of substance) and candela (luminous intensity) ultimately depend on the platinum-iridium artifact. For example, a mole is currently defined as the number of carbon-12 atoms whose total mass is 12 grams.

The new proposal defines the kilogram in terms of the Planck constant h**, an important constant in , which is expressed in units containing the kilogram. Efforts at NIST such as the watt balance experiment and determinations of the mass of one mole of silicon atoms offer new ways of determining an accurate value of h, thereby contributing to a more reliable definition of the kilogram.

The new SI would specify agreed-upon values of the seven constants, according to the results of an analysis published by CODATA (Committee on Data for Science and Technology)of all of the relevant data. Fixed values of constants would then define all base units. For example, the ampere would be formally defined in terms of the electric charge of a proton, the kelvin (temperature) in terms of the Boltzmann constant***, and the mole in terms of the Avogadro constant. However, before the revised SI could be implemented, additional experiments are needed to obtain more accurate values for some of the constants, especially the Planck, Avogadro, and Boltzmann constants.

The Consultative Committee for Units, of which NIST is a member and which is one of 10 advisory committees of the International Committee for Weights and Measures (CIPM), submitted a draft resolution for a revised SI for consideration by the CIPM during its meeting in Paris, France, earlier this month. The CIPM, whose membership includes Willie May, director of NIST's Material Measurement Laboratory, approved the draft with only a few minor editorial changes on Oct. 15, 2010. The CIPM will soon submit the resolution for consideration at the General Conference on Weights and Measures (CGPM), the international diplomatic body that has the authority under the Meter Convention to adopt such a sweeping resolution during its October 2011 meeting. If the resolution passes and all the technical requirements are in place, a new SI could be in practice later in the decade.

Explore further: NIST improves accuracy of 'watt balance' method for defining the kilogram

More information: * The seven SI base units from which all others are derived are the second (time), the meter (length), the kilogram (mass), the ampere (electric current), the kelvin (thermodynamic temperature), the mole (amount of substance) and the candela (luminous intensity).
** Current accepted value: 6.626 068 96(33) x 10-34 kg m2/s
***The Boltzmann constant relates the change in the temperature of a system (such as a group of atoms and molecules) to the change in its thermal energy.

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5 / 5 (1) Oct 27, 2010
It's great that we can finally rid ourselves of these arcane 'artifacts' but I'm not at all optimistic that some parts of the world will ever ditch the equally arcane units of miles, gallons, pounds, foot-pounds and cubits!
5 / 5 (1) Oct 27, 2010
The US has only seemed to be untouched by metric/SI. Since the decline of overt public metrification, the simple realities of global science and business have meant that more and more US companies use metric and SI measurements in their operations and just convert to pounds and ounces for public convenience.
5 / 5 (1) Oct 28, 2010
It's great that we can finally rid ourselves of these arcane 'artifacts' but I'm not at all optimistic that some parts of the world will ever ditch the equally arcane units of miles, gallons, pounds, foot-pounds and cubits!
Since even the British had - to many people's great surprise - their "decimal day" in 1971 we can stay patiently optimistic.
1 / 5 (2) Oct 28, 2010
... indeed, but menting is not the real measure ...
not rated yet Oct 28, 2010
Wow, really? I never considered electrical current to depend on the kilogram. Coulomb/second = Charge/time= (charge of a certain amount of protons)/time. I don't see how this seemingly arbitrary number of protons could be influenced by mass. Unless it's not arbitrary (most likely isn't) and is the result of how many protons are in a kilogram of some material. I don't know, I'll look it up later. Learn something every day don't we.
not rated yet Oct 30, 2010
there is conversion and conversion. In Canada, the metric system is used ( together with the other guy ) so you can buy 970g of chocolate or a drink of 237ml ( not accurate ). So a conversion that does not use the metric system properly does more harm than good.
5 / 5 (1) Nov 01, 2010
@trekgeek1: The formal definition of the coulomb is the quantity of charge moved by 1 ampere in 1 second. An ampere is the current required to exert 2x10^-7 newtons between 2 infinitely long conductors 1 meter apart, with a bunch of other geometrical constraints. The newton is defined as the force required to accelerate a 1kg mass by 1m/sec. So the coulomb itself is based on the mass of that platinum-iridium rod in France. NIST is basically saying that it's a stupid way of doing it, and to start with the charge on the electron (and other physical constants), rather than going through mass/acceleration/force/current/charge.
not rated yet Nov 02, 2010
And the fundamental constants were determined how?
In this new system they will be *defined* to have some value. They cannot be measured without reference to other physical obervables.

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