BOSTON — All the world's measurements can be traced back to what's known as SI Units. From just seven base units, all other units of measurement can be derived. Over the years, the definition of the measurements — Ampere, Candela, Kelvin, Kilogram, Meter, Mole, and Second — have been redefined so that they're based on phenomena that occur in nature such as Planck's Constant. The last SI unit based on a physical artifact, the kilogram, has finally been redefined.

Until today, all measurements of mass had to be traced back to a single object (International Prototype of the Kilogram) housed at the International Bureau of Weights and Measures (BIPM) outside Paris. The Prototype has been there since it was cast as the standard for the kilogram in 1884.

Having an artifact as the world standard for a measurement unit has two fundamental problems. The first is that it can change value over time. But, because it's the world standard, we have no way of knowing how it changes because, well, it's the standard. If the world standard changes, all measurement traced to it also change.

Kibble Balance NIST
A Kibble Balance uses
electrically created force
to measure mass.
Source: NIST

The second problem is that you can’t bring every mass-measurement device to France every time it needs calibration. Thus, national standards labs such as NIST in the U.S. and NPL in the U.K. have their own kilogram standards that they periodically bring to France for comparison against the Prototype. The national standards are then used to compare other standards, which may then be used to compare to yet another level of standards, which may then are used to calibrate measurement instrumentation. Therefore, you need a documented chain of calibrations back to the Prototype to have confidence in a mass measurement.

The new kilogram definition does away with the comparisons to the Prototype. Now, National labs will be able to create their own standard for the kilogram, because its reproducible, albeit difficult to realize. Metrologists will need to have the right equipment and follow strict procedures to produce their own kilogram standards, but it's still better than relying on a single artifact. Don't worry, you needn't bring your bathroom scale in for calibration because of the new definition. The kilogram won’t change, just the way we define it.

Starting in May 2019, the definition of the kilogram will be:
The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m² s−1, where the metre and the second are defined in terms of c and Δv.

The process for realizing a kilogram relies on a tool known as a Kibble Balance. It uses coils and magnets where current in the coil exerts a known force and that current can be measured to sufficient accuracy. Because the definition of current has already been established in terms of magnetic-field force, it can be used to derive an upward force from which the mass of an unknown object can be compared. Then the forces are equal, the mass of the unknown object can be measured. The voltage and current measurements are based on Planck's Constant. See Kilogram: The Kibble Balance from NIST for a detailed explanation of how a Kibble Balance works.

Because Kibble Balances already exist in national metrology labs, the kilogram can now be recreated across the planet.

— Header image courtesy of Greg L of Wikipedia, used under CC-BY-SA 3.0.