The New Kilogram

To find out how much a kilogram weighs, you’ll first have to fly to Paris, France. On the outskirts of the city, there is a 17th-century mansion that is home to the International Bureau of Weights and Measures. There, behind a triple-locked door, inside a vault, and under three glass jars, is the International Prototype of the Kilogram. This small, cylindrical piece of metal is the literal definition of a kilogram.

“If you dropped it and a piece broke off, it would still be a kilogram,” says Stephan Schlamminger. He’s a metrologist who studies measurement at the National Institute of Standards and Technology (NIST) in Maryland. “It’s the mass of everything around the world that would change.”

But that won’t be true for much longer. After a historic vote last November, the official definition of a kilogram is set to change. Starting on May 20, World Metrology Day, the kilogram will no longer be a hunk of metal. It will be based on a universal constant. Using this unchanging quantity observed in nature and some high-tech equipment, researchers can measure the kilogram anywhere in the world—no trips to Paris required.

It’s not easy to find out how much a kilogram weighs. First, you’ll have to fly to Paris, France. On the city’s edge, there’s a 17th-century mansion. It’s home to the International Bureau of Weights and Measures. There, look behind a triple-locked door, inside a vault, and under three glass jars. You’ll find the International Prototype of the Kilogram. It’s a small, metal cylinder. And it’s the definition of a kilogram.

“If you dropped it and a piece broke off, it would still be a kilogram,” says Stephan Schlamminger. He’s a metrologist who studies measurement at the National Institute of Standards and Technology (NIST) in Maryland. “It’s the mass of everything around the world that would change.”

But that won’t be true for much longer. After an important vote last November, the official definition of a kilogram is set to change. May 20 is World Metrology Day. Starting then, the kilogram will no longer be a hunk of metal. It will be based on a universal constant. That’s an unchanging quantity observed in nature. Researchers can use this constant and some high-tech equipment to measure the kilogram anywhere in the world. They won’t need a trip to Paris.

The International Bureau of Weights and Measures was founded in 1875 to answer a question that had troubled humanity since the beginning of civilization: How do we accurately measure something? Take mass, or the amount of matter in an object, for example.

For much of history, people relied on simple balance scales to compare the mass of one object to another. If the masses on either end of the scale were unequal, the scale would be out of balance. If the masses were roughly the same, the scale would be even.

But to determine an object’s exact mass, you need an agreed-upon unit of measure. Some societies used pieces of grain or objects called weighing stones. But having different standards of measurement in different places made trade difficult. So in 1799, a group of French scientists developed a solution: the metric system.

The unit of measure for mass became the kilogram, which was based on the mass of a liter of water at a certain temperature. In 1889, the International Prototype of the Kilogram, nicknamed the Grand K, was commissioned. The weight was made of a corrosion-resistant alloy, a metal mixture that contained 90 percent platinum (Pt) and 10 percent iridium (Ir). It would be the unit of measure “for all people, for all time”—or so metrologists thought.

The International Bureau of Weights and Measures was founded in 1875 to answer a question. People had been asking it since the beginning of civilization. How do we accurately measure something? For example, take mass, or the amount of matter in an object.

People used simple balance scales for much of history. They compared the mass of one object with another. An object was placed on either end of the scale. If the masses were unequal, the scale would be out of balance. If the masses were about the same, the scale would be even.

But to find an object’s exact mass, people need to agree on what unit to use. Some societies used pieces of grain or objects called weighing stones. But different places had different standards  of measurement. That made trade difficult. So a group of French scientists developed a solution in 1799. It was the metric system.

The unit of measure for mass became the kilogram. It was based on the mass of a liter of water at a certain temperature. The International Prototype of a Kilogram was made in 1889. It was nicknamed the Grand K. The weight was made of an alloy that wouldn’t corrode. This metal mixture contained 90 percent platinum (Pt) and 10 percent iridium (Ir). It would be the unit of measure “for all people, for all time.” At least that’s what metrologists thought.