# New measurement will help redefine international unit of mass

Using a state-of-the-art device for measuring mass, researchers at the National Institute of Standards and Technology (NIST) have made their most precise determination yet of Planck's constant, an important value in science that will help to redefine the kilogram, the official unit of mass in the SI, or international system of units. Accepted for publication in the journal Metrologia, these new results come ahead of a July 1 international deadline for measurements that aim to redefine the entire SI in terms of fundamental constants of nature.

The new NIST measurement of Planck's constant is 6.626069934 x 10-34 kg-m2/s, with an uncertainty of only 13 parts per billion. NIST's previous measurement, published in 2016, had an uncertainty of 34 parts per billion.

The kilogram is currently defined in terms of the mass of a platinum-iridium artifact stored in France. Scientists want to replace this physical artifact with a more reproducible definition for the kilogram that is based on of nature.

Planck's constant enables researchers to relate mass to electromagnetic energy. To measure Planck's constant, NIST uses an instrument known as the Kibble , originally called the watt balance. Physicists widely adopted the new name last year to honor the late British physicist Bryan Kibble, who invented the technique more than 40 years ago.

NIST's Kibble balance uses electromagnetic forces to balance a kilogram mass. The electromagnetic forces are provided by a coil of wire sandwiched between two permanent magnets. The Kibble balance has two modes of operation. In one mode, an electrical current goes through the coil, generating a that interacts with the permanent magnetic field and creates an upward force to balance the kilogram mass. In the other mode, the coil is lifted at a constant velocity. This upward motion induces a voltage in the coil that is proportional to the strength of the magnetic field. By measuring the current, the voltage and the coil's velocity, researchers can calculate the Planck constant, which is proportional to the amount of electromagnetic energy needed to balance a mass.

There are three major reasons for the improvement in the new measurements, said physicist Stephan Schlamminger, leader of the NIST effort.

First, the researchers have much more data. The new result uses 16 months' worth of measurements, from December 2015 to April 2017. The increase in experimental statistics greatly reduced the uncertainty in their Planck value.

Secondly, the researchers tested for variations in the magnetic field during both modes of operation and discovered they had been overestimating the impact the coil's magnetic field was having on the permanent magnetic field. Their subsequent adjustment in their new measurements both increased their value of Planck's constant and reduced the uncertainty in their measurement.

Finally, the researchers studied in great detail how the velocity of the moving coil affected the voltage. "We varied the speed that we moved the coil through the magnetic field, from 0.5 to 2 millimeters per second," explained Darine Haddad, lead author of the NIST results. In a magnetic field, the coil acts like an electric circuit consisting of a capacitor (a circuit element that stores electric charge), a resistor (an element that dissipates electrical energy) and an inductor (an element that stores electrical energy). In a moving , these circuit-like elements generate an electrical voltage that changes over time, said Schlamminger. The researchers measured this time-dependent voltage change to account for this effect and reduced the uncertainty in their value.

This new NIST measurement joins a group of other new Planck's constant measurements from around the world. Another Kibble balance measurement, from the National Research Council of Canada, has an uncertainty of just 9.1 parts per billion. Two other new measurements use the alternative Avogadro technique, which involves counting the number of atoms in a pure silicon sphere.

The new measurements have such low uncertainty that they exceed the international requirements for redefining the kilogram in terms of Planck's constant.

"There needed to be three experiments with uncertainties below 50 parts per billion, and one below 20 parts per billion," Schlamminger said. "But we have three below 20 parts per billion."

All of these new values of the Planck's constant do not overlap, "but overall they're in amazingly good agreement," Schlamminger said, "especially considering that researchers are measuring it with two completely different methods." These values will be submitted to a group known as CODATA ahead of a July 1 deadline. CODATA will consider all of these measurements in setting a new value for Planck's constant. The is slated for redefinition in November 2018, along with other units in the SI.

Before they started these experiments, Schlamminger and his group went to lunch in December 2013. On a lunch napkin, each group member wrote his or her prediction of the value of Planck's constant that the group would determine through their measurements. They tucked away this napkin under their Kibble balance nearly four years ago, and they have now compared the predictions. Shisong Li, a guest researcher from Tsinghua University in China, came closest. His prediction differed by only about 5 parts per billion from the measured result. There is no word yet on how the team plans to celebrate the winner's guess.

Explore further

Important milestone reached on road to a redefined kilogram

More information: Darine Haddad et al. Measurement of the Planck constant at the National Institute of Standards and Technology from 2015 to 2017, Metrologia (2017). DOI: 10.1088/1681-7575/aa7bf2

B M Wood et al. A summary of the Planck constant determinations using the NRC Kibble balance, Metrologia (2017). DOI: 10.1088/1681-7575/aa70bf

Journal information: Metrologia

Feedback to editors

Jul 01, 2017
If they aren't using the iridium-platinum 1kg bar anymore, can I have it?

Jul 01, 2017
Since the article is about International Unit of mass, when you write Billion are you referring to one thousand million, or 10^9 (USA and UK) or one million million, or 10^12 ?

Thanks, enjoyed the article.

Jul 01, 2017
Simply drop mass. The field that we measure, i.e. mass, is not a constant, it changes as every object in the universe changes. juz say'n

Jul 02, 2017
@alvesman
Since 1974 the 10^9 definition of a billion has generally been adopted (including in the UK which used to use the 10^12 value). The word trillion is now used for 10^12.

Jul 02, 2017
@alvesman
Since 1974 the 10^9 definition of a billion has generally been adopted (including in the UK which used to use the 10^12 value). The word trillion is now used for 10^12.

Thank you. I'm asking because here in Europe, outside UK, we assume the 10^12 (a million million). https://www.youtu...2AI_ojyQ

Jul 02, 2017
@alvesman
I'm asking because here in Europe, outside UK, we assume the 10^12 (a million million).

I don't think you will find this true of the scientific community.

Jul 02, 2017
This comment has been removed by a moderator.

Jul 02, 2017
@alvesman
I'm asking because here in Europe, outside UK, we assume the 10^12 (a million million).

I don't think you will find this true of the scientific community.

The scientific community uses scientific notation and not the words thus avoiding ambiguities.

Jul 02, 2017
Thank you. I'm asking because here in Europe, outside UK, we assume the 10^12 (a million million).

As a rule of thumb, the meaning of the word changes with the language being used rather than the location; English assumes 10^9

Jul 02, 2017
Simply drop mass. The field that we measure, i.e. mass, is not a constant, it changes as every object in the universe changes. juz say'n

Jul 02, 2017
why is she explaining it?

Jul 02, 2017
OK, lesson one. See the bigger picture! Their only exist charge. Charge does not have mass. Get it?

Jul 02, 2017
The field, updated relative to a charge's center at that point in time, with the velocity vector of the charge at that instant in time. Easier to visualize if you set each axis as Lambda, i.e. space and time equal in magnitude, c - Lambda/T. Need only inspect a single point. The wave is relative, to the magnitude of lambda. Space is big. You can't see space! To see space you need a massive computer, define all images in terms of what is real. Only this law. Each point viewed is updated to it's value, i.e. a computer using only logic to define to us what it sees. Yeah, yeah, you can probably pick a set, but really, define our stream.

Jul 02, 2017
Stuff very far away can't be from some guess on our visual. Think, the speed of light is equal to the initial wavelength divided by the measured period. The velocity may be +/- R, -infinity < R < infinity. So those hazy pictures may be clouds, but aberration, ain't from any Field. Fields don't do that to fields, charges due stuff to fields. I chose R for Rufus, use it each time you calucate the Velocity. DR. E? LOL!

Jul 02, 2017
Remember, the universe is infinite. Now put that in your calculation, whut? A density function? Conditions! For other intelligent life. We are very far from reaching Par. So bet on being behind!

Jul 02, 2017
^^^^WTF is that????? Explain how charge relates to mass. Which is what you originally posted. If I put a charge on a piece of tinfoil, can I make it levitate? If the charge on a spacecraft orbiting a mass (say a comet) alters, will the orbital parameters change? (hint: no they don't). So you are making no sense whatsoever. Please tell me that this isn't more of Thornhill's uneducated BS!

Jul 02, 2017
The universe has no beginning or end in space and time, the number of pairs of these + &- charges is also infinite. If you see the largest thing in the Universe, nobody will write about it!

Jul 02, 2017
^^^^WTF is that????? Explain how charge relates to mass. Which is what you originally posted. If I put a charge on a piece of tinfoil, can I make it levitate? If the charge on a spacecraft orbiting a mass (say a comet) alters, will the orbital parameters change? (hint: no they don't). So you are making no sense whatsoever. Please tell me that this isn't more of Thornhill's uneducated BS!

You are space trying to understand itself.

Jul 02, 2017
So trying to see the universe, i.e. G? Relative to what? Dude! The entire field?

Jul 02, 2017
I understand why it's being done as it will make science more accurate but for the common person in the street that number isn't going to mean anything. People need a physical item to be able to associate it when dealing with other masses.

Jul 03, 2017
I understand why it's being done as it will make science more accurate but for the common person in the street that number isn't going to mean anything.

You never know where such numbers are used. Possibly they increase the accuracy of something like your GPS signal - which is something that is of benefit to the 'person in the street' (particularly in the street, because increased accuracy can mean the difference between autopilot software being unworkable and it becoming an everyday feature)

We all benefit from these improvements on a daily basis - but we take this stuff for granted.

Jul 03, 2017
I understand why it's being done as it will make science more accurate but for the common person in the street that number isn't going to mean anything.

You never know where such numbers are used. Possibly they increase the accuracy of something like your GPS signal - which is something that is of benefit to the 'person in the street' (particularly in the street, because increased accuracy can mean the difference between autopilot software being unworkable and it becoming an everyday feature)

We all benefit from these improvements on a daily basis - but we take this stuff for granted.

Why not Engineer any way you want; but, instrumentation has physical limits, calculation with a correct theory shall show any accuracy.

Jul 03, 2017

We all benefit from these improvements on a daily basis - but we take this stuff for granted.

I won't debate that point and that wasn't the point I was making either. Try teaching a 2nd grader or even the average adult what a kilogram is when all you can tell them is some number that means nothing to them whatsoever and they can't relate to in any manner. That's the point I'm making. Many metric measurements are like that. They don't relate to anything physical in the real world that people can readily understand. They are fine for math and science but that doesn't help the person trying to buy a kilogram of potatoes.

Jul 03, 2017
Try teaching a 2nd grader or even the average adult what a kilogram

Liter bottle of water? 10 bars of chocolate? You make it seem like people in every other country are working with a system that isn't intuitive. Guess what: it is.

And the beauty is: once you have a hold of just one of these (e.g. a meter or a kilogram) you have a hold of all the other units - larger and smaller - because they are just off by a fixed factor of 1000. No need to know about quarts, ounces, feet, miles, inches and whatnot.

And I bet you can't calculate inches to miles without looking it up.

Jul 03, 2017
Something is wrong here. If E = h.n (n = frequency, and h = plaque constant)
.....

nikola_milovic_378

yes, I see something is wrong here, but not what you think; h doesn't equal "plaque constant", whatever that is, but rather "Planck's constant". Also, as far as I am aware, f is normally the letter to rep frequency; or at least I have never personally seen 'n' being used for that.

Jul 03, 2017
Ap. I didn't insult you but if you would like I can. I can calculate them without looking it up but I don't deal with a pompass asses like you. You are now blocked along with the rest of the jerks on here that can't add anything to a conversation except insult others.

Jul 03, 2017
Something is wrong here. If E = h.n (n = frequency, and h = plaque constant)
.....

nikola_milovic_378

yes, I see something is wrong here, but not what you think; h doesn't equal "plaque constant", whatever that is, but rather "Planck's constant".

Built upon probabilities, quanta, wavelets, selective, QM non-causal. Handy, if you think like that, your idea of mass and energy. Don't hang your PhD upon non-causality! So we have the tools, let us not be lazy, we may define causality!

Jul 03, 2017
Ap. I didn't insult you

Neither did I you.

If you want to argue what kind system is more intuitive and easier to learn for kids (never mind adults) I'll argue SI units take that one. No contest.