It's official. What has been the physical definition of the kilogram (kg) for over 130 years, "Le Grand K" will be retired on May 20th, 2019. This decision was made on November 16th, 2018 by researchers from the  Bureau of Internation Weights and Measures at the 26th General Conference on Weights and Measures (CGPM) in Versaille, France. This change in definition will forever change the way we measure mass, at least for the foreseeable future.

What is Le Grand K? 

Known as Big K in English, this moniker refers to the physical cylinder of a platinum alloy that is used as the definition of a kilogram. The kilogram is the last of the SI units to be defined by a physical object. Big K is officially known as the  International Prototype of the Kilogram, or IPK. Since 1889, the IPK and its official copies, have been the standard for defining the kilogram. Every weight and scale that is certified and calibrated is traceable back to Big K. This helps to ensure a standard unit of mass measurement around the world, which is crucial in trade, research, engineering, and other industries.

Four of the five U.S. national prototype kilogram artifacts, including K20 (front) and K4 (back). Credit: J. Lee/NIST

Changing the Definition

So why did the BIPM decide to change the definition of the kilogram? Simply put, by the very nature of a physical object, its mass is not absolutely stable. Since 1889, the IPK has diverged from its copies by about  50 micrograms (µg). This is significant because three other SI units depend on the kilogram reference including the ampere, candela, and the mole. The new definition would use a fundamental constant of nature to define the kilogram called Planck's Constant, keeping the definition of the kilogram stable.

Comparisons between the official copies and IPK. Credit: Bureau International des Poids et Mesures

Planck's Constant

Next, I'm going to discuss  Planck's Constant and why it was chosen for the new definition of the kilogram. Bare with me on this, we're getting into a bit of physics. Planck's Constant relates the frequency of a photon to its energy as expressed in the formula E = hf. E is the amount of energy the photon has, h is Planck's Constant, and f is the frequency of the photon. Energy is also related to mass in the more commonly known formula E = mc² (mass x speed of light squared). You can see the relation between Planck's Constant and mass as m = hf/c².

This is why researchers chose Planck's Constant for the new definition of the kilogram. Planck's Constant has some uncertainty in the last handful of digits in its value (it's an incredibly small number, like 33 zeros after the decimal small). The purpose of the CGPM was to adopt an ISO standard for Planck's Constant and set a definite value. Planck's Constant is now 6.62607015 × 10 ^–34 J s. I encourage you to check out Veritasium's video on the subject as Derek has an educational physics background and can explain this much better than I can.

The Effects of Changing the Definition of the Kilogram

Now that your head is spinning (maybe not for some of you but I know mine is), let's talk about what this will change. In short, nothing. That's entirely the point of this redefining of the kilogram. Stability in the definition is paramount. Researchers over the last decade have measured Planck's Constant to find the most accurate value possible. By setting a standard value, the SI base unit of mass is no longer tied to a varying physical object. However, the value was chosen based on other fundamental constants like Avogadro's Constant. This means that most of us won't see any major changes in our day-to-day lives, but electrical engineers will have to adapt their calculations a bit. This also means that anyone, anywhere in the world, will now be able to produce precision weights (if they have a  Kibble balance of course) based on a fixed constant that will never change.

Conclusion

Now that the kilogram is defined by Planck's Constant, that means that the little platinum-iridium cylinder in Paris is no longer equal to one kilogram. What once was the most important object in the metrology world is now useless, apart from its monetary and educational value. For the time being,  BIPM has stated that the Big K is going nowhere.

"There are no plans to change the storage conditions for the IPK. It will remain at the BIPM and it will not be on display for the general public."

In memory of Big K, our team has decided to offer an additional 5% discount on all of our already-discounted weights through the remainder of 2018. Use the coupon code BIGK at checkout. We have a  large selection of avoirdupois and metric weights and weight sets and also offer NIST traceable certificates. Chat with us or give us a call if you need help with choosing the right weights for your application.