STANDARDS

NGSS: Core Idea: PS1.A

CCSS: Literacy in Science: 4

TEKS: 6.4A, 7.4A, 8.4A, C.3F

The New Kilogram

A physical object has long defined the kilogram—but that’s about to change

COURTESY OF CURT SUPLEE/NIST

ESSENTIAL QUESTION: What are units of measure, and why are they important?

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.

MEASURING UP

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.

CHANGING MASS

The Grand K has been taken out of its vault only a handful of times since 1889. Nonetheless, it now has a different mass than once-identical copies housed in labs around the world. The difference is only about the mass of an eyelash, but it’s enough to matter. No one knows how it happens.

“[The Grand K] is a material object, and all material objects change over time,” says Schlamminger, the researcher at NIST. This change in mass presents a problem for researchers at the cutting edge of science, where accuracy is key. Metrologists from all over the world decided that they needed a way to base all measurements on universal constants.

The speed of light in a vacuum, for example, is always 299,792,458 meters (983,571,056 feet) per second. Researchers have used this constant since 1983 to define the length of a meter as the distance it takes light to travel in 1/299,792,458 of a second (see Measuring: Then and Now). But finding a universal constant for the kilogram proved harder.

The Grand K almost never leaves its vault. It has been taken out only a handful of times since 1889. Exact copies of it are kept in labs around the world. But the masses of it and its copies are no longer the same. They differ by about the mass of an eyelash. No one knows exactly how the change happened.

“[The Grand K] is a material object, and all material objects change over time,” says Schlamminger, the researcher at NIST. Accuracy is key for researchers working at the cutting edge of science. This change in mass is a problem. Metrologists from all over the world made a decision. They needed to base all measurements on universal constants.

For example, take the speed of light in a vacuum. It’s always 299,792,458 meters (983,571,056 feet) per second. Since 1983, researchers have used this constant to define the length of a meter. It’s the distance light travels in 1/299,792,458 of a second (see Measuring: Then and Now). But finding a universal constant for the kilogram was harder.

FINDING A CONSTANT

After decades of research, scientists recently settled on something called Planck’s constant to define the weight of a kilogram. This universal constant helps describe the energy of small particles of light called photons. How does a concept involving energy help measure mass? The answer involves a device called the Kibble balance.

The Kibble balance essentially acts like a traditional balance scale, says Darine El Haddad. She runs the Kibble balance program at NIST. On one end of the scale is the object whose mass you want to determine. On the other end is a complex system of magnets and electric coils. By calibrating the magnets and coils using Planck’s constant, technicians can determine the weight of the object on the other side.

Scientists studied the problem for decades. Recently, they chose something called Planck’s constant to define a kilogram’s weight. This universal constant helps describe the energy of photons. These are small particles of light. How does an idea involving energy help measure mass? The answer involves a device called the Kibble balance.

The Kibble balance acts like a regular balance scale, says Darine El Haddad. She runs the Kibble balance program at NIST. To find an object’s mass, you put the object on one end of the scale. A complex system of magnets and electric coils is on the other end. Technicians adjust the magnets and coils using Planck’s constant. That tells them the weight of the object on the other side.

COURTESY OF J.L. LEE/NIST

NIST TEAM: Metrologists at NIST build and maintain Kibble balances.

Various labs have independently measured the mass of the same object to within 10 micrograms of each other using their Kibble balance systems. The tests were so successful that scientists representing nations around the world agreed last fall that Planck’s constant and the Kibble balance would replace the physical kilogram kept in Paris. It was the last metric unit to be replaced by a universal constant.

“It was an emotional moment,” says Schlamminger. “Countries that don’t get along very well voted together. Science can still unify and transcend politics.”

Different labs have tested their Kibble balance systems. They measured the mass of the same object to within 10 micrograms of one another. Scientists from nations around the world looked at the successful tests. Last fall, they made an agreement. Planck’s constant and the Kibble balance would replace the metal kilogram kept in Paris. It was the last metric unit to be replaced by a universal constant. “It was an emotional moment,” says Schlamminger. “Countries that don’t get along very well voted together. Science can still unify and transcend politics.”    

CORE QUESTION: What issue with the Grand K led metrologists to change the definition of the kilogram?

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