In mid-November 2018, did you hear the bell toll? The longtime king was dead! No, a person didn’t die, it was Le Grand K! Le Grand K is a cylinder of platinum-iridium alloy locked away in the cellar in the International Bureau of Weights and Measures in Sevres, France. It weighs exactly one kilogram, and it has set the standard for weights around the world for the last 130 years. Le Grand K emerges once about every 40 years from its protected environment to make sure all our weights around the world are standardized. No matter where you are on the planet, a kilogram is a kilogram is a kilogram — precisely.

On a trip to Europe, I remember walking through old city squares in Switzerland where there were publicly displayed rods from the Middle Ages defining the accepted measures of a foot and a yard. Sometimes this was based on the size of the duke’s or king’s foot, which was hardly precise, but it was certainly standardization. This was important for commerce to make sure merchants measured amounts of products the same way. I imagine they hoped for a king with small feet.

Most of our key measures are now defined by physical constants. For example, the meter is defined as the distance light travels in one 299,792,458th of a second in a vacuum. Because the speed of light never changes, the length of a meter is absolutely precise. Other measurements, including the kelvin (temperature) and candela (light brightness) are already defined by natural physical constants.

The kilogram will now be defined by a physical constant called Planck’s constant, a number that’s important in quantum mechanics. It has the value of 6.62607015 X 10-34 kilogram meters squared per second. That sounds complicated, but the key element is that kilograms are part of this unwavering physical constant. Just like the speed of light can define a meter, Planck’s constant can precisely define the kilogram. Finding the precise measure of a kilogram uses a specialized scale that can determine mass as a balance to an electric force. This is heavy stuff (ha), but suffice to say this leads to an incredibly precise definition of a kilogram. This will hardly have an impact on you weighing your produce at the grocery store, but this precise definition will be useful to those measuring extremely small amounts, like we do in science all the time.

Now that the new definition of the kilogram has been approved by scientists from 60 nations at the 26th General Conference on Weights and Measures, it went into effect in May. Le Grand K has abdicated! The King is dead — long live King Planck’s!

Next up for re-definition is the second, which has long been defined by radiation emitted from a clock made with radioactive cesium. Don’t hold your breath, this will take some time (pun intended!) and will likely occur 10 or more years from now. Can you stand the excitement?

Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at

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