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Tech by VICE

This Perfect Silicon Sphere is the Most Precise Kilogram Ever

The General Conference of Weights and Measures comes closer to finding their new standard.

by Rachel Pick
Jul 16 2015, 5:01pm

Photo: Veritasium/YouTube

The search for a new standard measure of the kilogram has been under way for several years, and it seems we may have found it. A sphere of isotope-enriched silica—potentially the most perfectly round object ever created—may soon replace the previous measure.

One might think it's easy enough to just cut off a kilogram's worth of material, but what scientists are searching for is something with a mass so precise it comes down to a matter of atoms. In fields like engineering and pharmacology, this kind of intense precision is vital. Every atomic crystal of silica is perfectly identical, so it's an ideal substance for the new standard.

The previous international standard, a cylinder of platinum and iridium forged during the 1880s and known as "Big K," has been mysteriously losing weight over the years. The mass discrepancy between Big K and its six official siblings has now reached 50 micrograms, too big for ultraprecise measurements. Disturbed by the slim-down, the General Conference of Weights and Measures unanimously voted to find a new standard in 2011, with a projected date of implementation sometime in 2018.

Since then, the search has been on, and one of the proposed methods to find the "one true kilogram" is to use Avogadro's number as a measurement of accuracy. (Avogadro's number, also known as the mole, is used to translate atomic mass into grams.) When Motherboard first covered the issue in 2014, the Avogadro project was still little more than conceptual.

Since then, the baseball-sized silicon sphere has proved itself, calculating Avogadro's number with an error of only 0.000000018, according to a paper published in the Journal of Physical and Chemical Reference Data. This is the most accurate value of the mole ever recorded.

With a few more years left before the target implementation date, it's possible that even more accurate kilogram measurements could be forthcoming. But for now, the silicon sphere is our strongest contender.