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A Step Closer to the New Kg

The guys and gals at the International Bureau of Weights and Measure (BIPM)* have a problem: Their kilogram is shrinking.

Well, it may be shrinking. It might also be growing. Either way, it appears to be changing. The problem is the world's only exact definition of a kilogram is a cylinder (made of platinum-iridium) that's under lock and key at the BIPM. It lives in a carefully-managed environment and has only been exposed to the world three times since its creation in 1889.

Copies of the original kilogram cylinder were made, and when the original was measured against a copy in 1989, the mass of the original was 50 micrograms less than the copy -- a difference about equal to the size of a grain of sand.
[Photo caption: Master Optician Achim Leistner at the Australian Centre for Precision Optics holds a 1-kg silicon crystal sphere.]

Though it's a small difference, it could have big implications for scientists who rely on precision and accuracy. Luckily, a group of scientists on a mission are working to fix the problem, and they just announced big progress.

There are seven "base" units in the International System of Units (SI), otherwise known as the metric system, including the meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), candela (light intensity) and mole (amount of substance). Of the seven, the kilogram is the only one that is still fundamentally defined by a physical object.

For example, the SI definition of a meter is "The length of the path travelled (sic) by light in vacuum during a time interval of 1/299 792 458 of a second." This definition will always be true and testable no matter where you are.

By contrast, the SI definition of kilogram is "The unit of mass; it is equal to the mass of the international prototype of the kilogram." Unless you had access to the prototype (and you don't), you couldn't measure an exact kilogram in your lab.
[Photo caption: A computer-generated image of the kilogram cylinder.]

Scientists working on the Avogadro project are trying to use a measurement known as Avogadro's constant to define a kilogram and rescue the world from the shrinking cylinder. Crudely, Avogadro's constant deals with the number of atoms in an element relative to how much mass that element has when it contains a specific number of molecules, atoms, or other elementary particles. (That specific number is called Avogadro's number.)

Understanding exactly what Avogadro's constant is is not so important. What is important is knowing that the relationship between the two quantities (the number of atoms and the mass) is always the same and is represented by a really ugly-looking number, called Avogadro's constant: 6.0221367 x 10^23 mol^-1. (Told you.)

A handful of scientists coordinated by the Physikalisch-Technische Bundesanstalt have worked to measure Avogadro's constant exactly. To do this, they created two 1 kg crystal spheres made of a silicon isotope, known as 28Si. They polished the spheres and tested their perfection.

Unfortunately, they found some unexpected traces of cooper copper and nickel on the surface of the spheres possibly left over from the polishing phase. The spheres' level of contamination is slightly over the level of contamination acceptable for a new fixed measurement of Avogadro's constant.

The spheres, however, mark significant progress in achieving exact measurements and once spheres with next to no contamination are made, they'll be one step closer. Combining an accurate way to measure Avogadro's constant with other constants could give the world an accurate way to define the kilogram forever.

The paper describing the two spheres can be found in January's Physical Review Letters.

[The International Bureau of Weights and Measures in Sèvres, France.]

*Along with the International Bureau of Weights and Measures (BIPM), two other organizations were created to maintain the International System of Units. They are called the General Conference on Weights and Measures and the International Committee for Weights and Measures. The international prototype of kilogram that is shrinking is housed at the BIPM.


  1. I understand why you can be perplexed by the use of the word "travelled" (even though my dictionary states that that is an acceptable spelling of the word), but "unexpected traces of cooper and nickel"? I know what nickel is with respect to the elements. What's "cooper"? (Yes, I slay me.)

  2. Cooper is a misspelling of the word "copper". Excellent catch!


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