Scientists Are Redefining the Kilogram

For decades, the unit of mass kilogram, has been defined as the weight of a small block of metal sitting in France. The block of metal is called Le Grand K, it's used to make sure all the world's scales are on the same page by pulling it out and using it for recalibration every now and then. This might seem very low tech—and it is—but for a long time it was a pretty reliable way to define the kilogram, especially back in the late 1800s when people still thought the aether was real. It was also not the only unit to be defined this way. The meter, for instance, was the length a particular stick. It was a system that worked pretty well until 1992, when scientists discovered that Le Grand K was shrinking.

Scientists from the National Institute of Standards and Technology (NIST) have announced that they have succeeded in taking a highly precise measurement that will be used to redefine the kilogram.
Having units suddenly change on you is more than a little annoying, especially if you are performing very precise experiments that require very accurate measurements and not everyone is working with the same frame of reference. With the exception of the kilogram, most of the other units have already been redefined in terms of fundamental physical constants to ensure they never change. The meter stick, for instance, was replaced by a measurement of the speed of light, and the second (that is, the unit of time) has been defined in terms of electron transition frequencies in cesium atoms. But the kilogram has been a stubborn holdout, mostly because the physical constant you'd use define weight in natural terms, Planck's constant, is really small and difficult to measure accurately.
So the International Committee for Weights and Measures (CIPM) came up with a plan, which was to perform three independent measurements of the constant, all of which would need to agree, and two of which would need to be the result of significantly different experiments. One of those experiments was the Avogadro Project, completed in 2015, which involved creating a silicon sphere that weighs one kilogram and counting the number of molecules inside it, exactly.
The second experiment uses a watt balance, which calculates mass very accurately by comparing it to a voltage and a current. The watt balance used to calculate Planck's constant was built by the US National Institute of Standards and Technology (NIST). In a paper published yesterday in the journal Review of Scientific Instruments, the team announced that their machine has taken its first full measurement of Planck's constant.
That measurement of Planck's constant is h = 6.62606983 x 10-34 J s, with a margin of error of +/- 22 on those last two digits. It is a very very small number. It's also in agreement with the Avogadro Project, which recorded ...82 and ...76, very close to the ...83 from the watt balance, well within the margin of error. That is to say, close enough.

Together, these measurements meet the standards set by CIPM, which means that the kilogram will be officially redefined in 2018. At long last, scientists will see some stability return to their measurements, and Le Grand K will be put in a museum, where it belongs.