Tuesday, June 18, 2019 | ePaper

Definition of kilogram set to change

  • Print


Pallab Ghosh :
Scientists are set to change the way the kilogram is defined. Currently, it is defined by the weight of a platinum-based ingot called "Le Grand K" which is locked away in a safe in Paris.
Researchers are expected to vote to get rid of it in favour of defining a kilogram in terms of an electric current. The decision is to be made at the General Conference on Weights and Measures, in Paris.
But some scientists, such as Perdi Williams at the National Physical Laboratory in the UK, have mixed feelings about the change.
"I haven't been on this project for too long but I feel a weird attachment to the kilogram," he said.
"I think it is such an exciting thing and this is a really big moment. So I'm a little bit sad about [the change]. But it is an important step forward and so the new system is going to work a lot better. It is also a really exciting time, and I can't wait for it to happen."
Why kill off the kilogram?
Le Grand K has been at the forefront of the international system of measuring weights since 1889. There are also several close replicas.
But the master kilogram's days are numbered. Its weight has changed over the years because it has deteriorated. The kilogram, like the pope, is infallible, so other weights have to be adjusted accordingly.
In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering - those responsible for maintaining the system plan to overturn Le Grand K's increasingly flawed rule.
How wrong is Le Grand K?
The fluctuation is about 50 parts in a billion, less than the weight of a single eyelash. But although it is tiny, the change can have important consequences. Coming in is an electrical measurement which Dr Stuart Davidson, head of mass metrology at NPL, says is more stable, more accurate and more egalitarian.
"We know from comparing the kilogram in Paris with all the copies of the kilogram that are all around the world that there are discrepancies between them and Le Grand K itself," he said.
"This is not acceptable from a scientific point of view. So even though Le Grand K is fit for purpose at the moment, it won't be in 100 years' time."
How does the new system work?
Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, such as old cars. The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. There is, therefore, a direct relationship between electricity and weight.
So, in principle, scientists can define a kilogram, or any other weight, in terms of the amount of electricity needed to counteract its force.
Here's the tricky part
There is a quantity that relates weight to electrical current, called Planck's constant - named after the German physicist Max Planck and denoted by the symbol h.
But h is an incredibly small number and to measure it, the research scientist Dr Bryan Kibble built a super-accurate set of scales.
The Kibble balance, as it has become known, has an electromagnet that pulls down on one side of the scales and a weight - say, a kilogram - on the other.
The electrical current going through the electromagnet is increased until the two sides are perfectly balanced.
By measuring the current running through the electromagnet to incredible precision, the researchers are able to calculate h to an accuracy of 0.000001%.
This breakthrough has paved the way for Le Grand K to be deposed by "die kleine h".
What are the advantages of the new system?
Every few decades, all the replica kilograms in the world have to be checked against Le Grand K. The new system, if it is adopted, will allow anyone with a Kibble balance to check their weights anytime and anywhere, according to NPL's Dr Ian Robinson.
"It feels really good to be at this point. I feel it is the right decision. Once we've done this it will be stable for the foreseeable future," he said.

(The author is the Science correspondent, BBC News).

More News For this Category

Reduced Timetables

Carl O'Brien :Niall Muldoon, Ombudsman for Children, said safeguards need to be put in place to ensure that the use of a reduced timetable is time-limited, in the best

Global Search for Education

Global Search for Education

C. M. Rubin :Since 2009, Design for Change, based in India, has focused on creating learning environments that balance academic programs with character development. Their program focuses on problem-solving

Teaching kids about sustainable living

Teaching kids about sustainable living

Marianne StengerĀ  :Today's children will be tomorrow's leaders in the battle against climate change, so it's vital that kids understand the concept of sustainable living from an early age.Research

How to Talk to Children about Climate Change

How to Talk to Children about Climate Change

James L. Payne :We smile at seeing those young faces waving placards out in the rain, urging action on the problem of climate change. But our smile is tinged

Imbalanced Assessment in Higher Education

Imbalanced Assessment in Higher Education

Mili Saha :A widespread issue is the poor standard we maintain in our higher education system. I often talk to my colleagues or students teaching in different universities or

Confronting Inequity

Confronting Inequity

Adam Alvarez :Unlike educational equality, which may focus on school outcomes alone, equity is more justice-centered and tends to refer to school outcomes and structures in relation to access

Teens Are Not Alright

Teens Are Not Alright

Cathy Vatterott :Often when I am invited to middle and high schools to work with teachers, it is because school leaders believe their teachers' homework practices are causing students

Preparing for School

Preparing for School

Anna Kelsey-Sugg :It might seem a little early to start discussing your child's first day of school, but preparing children well in advance for this big step in their

Ageless Rewards of Reading Aloud

Ageless Rewards of Reading Aloud

Margaret Robson Kett :Readers love to share what they're reading, and there are few pleasures for story lovers like being read to.From toddlers on someone's lap, listening to a

Education with Virtual Reality

Education with Virtual Reality

Annabel Astbury :Over the next few years we will see an explosion of virtual reality in consumer entertainment and educators cannot miss the potential that it offers, especially in