This device turns the kilogram’s new definition into a real mass | Science News for Students

This device turns the kilogram’s new definition into a real mass

The tabletop system measures 10 grams to an accuracy of a few ten-thousandths of a percent
Jul 5, 2019 — 6:45 am EST
a photo of a mini Kibble balance

Small enough to fit on a table, this mini Kibble balance measures smaller units of mass, such as a few grams, with amazing accuracy. A Lego figurine perched on the top right holds a 1-gram mass for scale.

Leon Chao/NIST

Earlier this year, the kilogram got a new definition. With it, scientists can now measure mass very accurately with a type of scale that uses electromagnets. A new tabletop version of that device should now make accurate such measurements of mass more accessible to the masses.

Until this spring, the kilogram was defined as the mass of a special metal cylinder kept in a vault near Paris, France. But researchers did away with that standard on May 20. Now the kilogram is pegged instead to a fundamental constant of physics. That number is known as the Planck constant. (A fundamental constant is a number that doesn’t ever change.)

With the new definition, scientists can use a device called a Kibble balance to directly measure masses via the Planck constant. The device is named after Bryan Kibble, the physicist who invented it.

But a full-scale Kibble balance is extremely complex. It requires its own laboratory space and costs millions of dollars to build. And it needs a host of PhD-level scientists to run it.

Now a team at the National Institute of Standards and Technology in Gaithersburg, Md., has created a scaled-down version of the device. They are designing it to measure smaller masses, about 10 grams (0.35 ounce). When the prototype’s kinks are worked out, the apparatus should be accurate to a few ten-thousandths of a percent. The researchers described the tabletop device in the June IEEE Transactions on Instrumentation and Measurement.

The new, suitcase-sized Kibble balance is just over half a meter (1.6 feet) tall, with a price tag of around $50,000. That puts it within reach of labs that frequently need to weigh small things. For example, companies must accurately dole out small doses of the drugs they make.

Traditional balances work by comparing the weights of masses in two different pans. But a Kibble balance compares a mass to the electromagnetic force needed to hold up that mass. Certain measurements, such as voltage and resistance, can be tied back to the Planck constant. That allows a series of equations to connect that quantity to the object’s mass.

The Planck constant has the same value everywhere. That means researchers can directly measure out masses anywhere and anytime. They no longer have to compare things against the Parisian cylinder.

Fun with Legos inspired the new instrument, says Leon Chao. He is a mechanical engineer on the team. The researchers had made a Lego Kibble balance to help teach the public how the instruments work, he says. That experience gave them the idea to make a small-scale real one.

a LEGO model of the Kibble balance
This Lego model of a Kibble balance is at the National Institute of Standards and Technology (NIST). It helped inspire Leon Chao and others to build a real one at a similar scale.

Power Words

(more about Power Words)

constant     Continuous or uninterrupted. (in mathematics) A number that is known and unchanging, usually based on some mathematical definition. For example, π (pi) is a constant equal to 3.14. . . and defined as the circumference of a circle divided by its diameter.

electromagnetic force     ( also known as electromagnetism ) One of the four fundamental forces of nature. It’s the force that causes electrically charged particles to interact. The regions over which these interactions occur are known as electromagnetic fields.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

equation     In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

fundamental     Something that is basic or serves as the foundation for another thing or idea.

Kibble balance     A measurement device invented by Bryan Kibble in 1975 and initially known as the watt balance. It balances the force exerted by a wire carrying electric current through a magnetic field with the weight of some unknown mass. It can be used to measure the current going through the wire. It also can be used to calculate Planck's constant, a fundamental constant of nature.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

mechanical     Having to do with the devices that move, including tools, engines and other machines (even, potentially, living machines); or something caused by the physical movement of another thing.

mechanical engineer     Someone trained in a research field that uses physics to study motion and the properties of materials to design, build and/or test devices.

pH     A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

prototype     A first or early model of some device, system or product that still needs to be perfected.

resistance     (as in drug resistance) The reduction in the effectiveness of a drug to cure a disease, usually a microbial infection. (as in disease resistance) The ability of an organism to fight off disease. (as in exercise) A type of rather sedentary exercise that relies on the contraction of muscles to build strength in localized tissues. (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.

standards     (in research) The values or materials used as benchmarks against which other things can be compared. For instance, clocks attempt to match the official standard benchmark of time — the second, as calculated by the official atomic clock. Similarly, scientists look to identify a chemical by matching its properties against a known standard for a particular chemical. (in regulations) A limit above which something may not be used, sold or considered safe.

voltage     A force associated with an electric current that is measured in units known as volts. Power companies use high-voltage to move electric power over long distances.


Journal: L. Chao et al. The design and development of a tabletop Kibble balance at NIST. IEEE Transactions on Instrumentation and Measurement. Vol. 68, June 2019, p. 2176. doi: 10.1109/TIM.2019.2901550.