The New Kilogram

BIPM/NIST

OLD: The International Prototype of the Kilogram used to define a kilogram.

To find out exactly how much a kilogram weighs, you need to go to the International Bureau of Weights and Measures in Paris, France. There, behind a triple-locked door, inside a vault, and under three glass jars, is the International Prototype of the Kilogram. This small metal cylinder 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 in Maryland. “It’s the mass of everything around the world that would change.”

To accurately measure anything—mass, distance, time, etc.—you need a unit of measure. That’s a quantity that everyone agrees to use. For years, the units of measure we use were based on physical objects. But since 1948, scientists have been redefining them using universal constants, which are numbers that can be measured and used anywhere in the world.

Exactly how much does a kilogram weigh? To find out, you need to go to the International Bureau of Weights and Measures. It’s located in Paris, France. You’d pass through a triple-locked door, open a vault, and lift three glass jars. Beneath them you’d find the International Prototype of the Kilogram. The mass of this small metal cylinder is 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 an expert on measurement at the National Institute of Standards and Technology in Maryland. “It’s the mass of everything around the world that would change.”

People measure everything from mass to time to distance. To do that accurately, they need units of measure. A unit is a quantity that everyone agrees to use, like an inch or a second. For years, units of measure were based on physical objects. But in 1948, scientists started changing that. They began to use numbers called universal constants instead of objects. Universal constants are numbers that can be measured and used anywhere in the world.

The kilogram is the only unit left defined by a physical object. But that won’t be true for much longer. Starting on May 20, the kilogram will be based on Planck’s constant. The constant helps explain the energy of small particles of light called photons. It might sound strange to use something related to energy to measure mass, but the two concepts are related. That’s where a Kibble balance comes in.

A Kibble balance acts like a traditional balance scale. You place the object you want to measure on one end of the scale. That object’s mass exerts a force on the scale. On the other end is a complex system of magnets and electric coils. Using Planck’s constant, you can adjust the magnets and coils to precisely determine the object’s mass.

Using Kibble balances and Planck’s constant, labs around the world have weighed objects within an accuracy of 10 micrograms. This led scientists from 60 countries to unanimously vote to approve the new kilogram definition.

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

The kilogram is the only unit still defined by a physical object. But that won’t be true for much longer. Starting on May 20, the kilogram will be based on a universal constant instead of a metal cylinder. The number is called Planck’s constant. Scientists usually use it to explain the energy of light particles.

It might sound strange to use a number related to energy to measure mass. But the two concepts are connected. A special instrument called a Kibble balance helps scientists weigh objects using energy.

A Kibble balance acts like a traditional balance scale. You place an object you want to measure on one end. That object’s mass exerts a downward force on the scale. On the other end is a system of magnets and electric coils. They generate an upward force to balance the object’s mass. Scientists use Planck’s constant to calculate how much force the magnets and coils are using. This lets them precisely measure the object’s mass.

Labs around the world have used Kibble balances and Planck’s constant to weigh objects. The results are extremely accurate. This led scientists to propose defining the kilogram based on the constant. Scientists from 60 countries voted unanimously to approve the idea.

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

Use this information to investigate the densities of the elements in the chart. Round answers to the nearest hundredth. Use 3.14 for pi. Round all answers to the nearest whole number. Record your work and answers on our answer sheet.

A 1-kilogram cylinder has a radius of 2 centimeters and a height of 7.58 cm. What is the cylinder made of?

A 2-kilogram cylinder has a radius of 3 cm and a height of 26.20 cm. What is the cylinder made of?

A cylinder-shaped metal necklace has a mass of 0.115 kg. It has a height of 51 cm and a diameter of 0.4 cm. The necklace’s label says it is solid gold. Is the label correct?