Scientists fear kilograms don't weigh as much as they used to

 

When is a kilogram not a kilogram? When it starts to weigh less. It came into existence more than two centuries ago and has become the standard unit of weight around the world, from the shopping malls of Europe to the souks of the Middle East, but scientists believe that the reign of the kilo as we know it is about to come to an end.

A group of experts meeting in London today want to redefine the kilogram so that it is no longer based on the mass of a solid cylinder of platinum-iridium alloy that sits beneath three layers of protective glass sealed in a locked vault in Sèvres, France.

This metal block, known as the International Prototype Kilogram, has been used since it was first registered with the International Bureau of Weights and Measures (BIPM) in 1889 as the definitive unit of mass against which all other kilograms are measured.

In the past 122 years, it has been brought out of storage just three times to calibrate the national prototype kilograms used by countries around the world. However, scientists now believe it is time to redefine the kilogram because there is evidence that the precise mass of the international prototype in Sèvres is not as constant as it should be.

"We think it is losing weight, and we don't know why," says the BIPM's Michael Stock, who is due to attend the meeting at the Royal Society in London that will look again at the kilogram. "From the three times we have had it out to make calibrations, we have had indications that it is not perfectly stable. It seems to have lost about 50 micrograms and there is no real explanation."

The loss of mass, amounting to fifty-millionths of a gram, is equivalent to a small grain of sand, but for metrologists, who make a religion out of measuring things with extreme accuracy, the change represents a disturbing disparity from the expected.

Cleaning, polishing or touching the international prototype is strictly controlled as they can all alter its weight. One theory for the loss of mass is that over the years, the platinum-iridium alloy may have emitted some of the gas that had been incorporated into the metal block when it was made in London in 1879, Dr Stock said.

"There are no real problems now but if it continues, then we may run into problems in 10 or 20 years' time because measurements are getting even more precise. We need to anticipate the problems and, from time to time, we have to improve our definitions of the standard units of measurement – if you need to make an accurate measure of length, you need a good ruler," he said.

The kilogram is one of the seven "base units" on which all other units of measurement in science are derived. The other six are the metre, second, ampere, kelvin, mole and candela, measuring, respectively, length, time, electric current, temperature, chemical amount and light intensity.

What makes the kilogram different is that it is the only international standard unit of measurement that is based on a physical object rather than a fundamental physical constant.

The metre, for instance, is no longer defined as the distance between two scratches on a metal bar, but on the distance travelled by light in a vacuum in 1/299792458 of a second, whereas the second is the duration of 9,192,631,770 cycles of radiation emitted by a particular electronic transition in an atom of caesium-133.

Getting the unit of the kilogram right is important for other units of measurement, such as the volt and the ampere, which are used to measure electric potential and current. Physicists believe that redefining the kilogram based on something that is as immutable as a physical constant, rather than relying on a physical object, could improve the precision of electrical measurements fifty-fold.

The favoured option to be discussed at the Royal Society is for the kilogram to be based on the Planck constant, which represents the sizes of the quanta in quantum physics, and is as reliable as the speed of light in a vacuum.

"Our experiments are moving forward; however, it is too early to implement the new definition of the kilogram just yet," Dr Stock said.

In practice, the kilogram would be based on the electric power needed to counter perfectly a kilogram pulled by the Earth's gravity by levitating it in mid-air. The idea, called a watt balance, was first proposed by British metrologist Bryan Kibble in the 1970s when he worked for the National Physical Laboratory in Teddington, London.

Length and time: the history of measurement

The metre

In 1781, the French Academy of Sciences defined the metre as one ten-millionth of the length of the Earth's meridian along a quadrant, or the distance from the Equator to the North Pole. The academy organised an expedition to measure the distance from Montjuic in Barcelona to Dunkirk, considered to be half the length of the meridian. A miscalculation meant that this metre fell a fifth of a millimetre short of a true metre. In 1983, scientists adopted the latest definition of a metre based on the speed of light. The metre today is defined as the length travelled by light in a vacuum during the time interval of 1/299,792,458 of a second.



The second

The origins of the 12-hour day and night can be traced to the ancient Egyptians. This gives us a starting point for the origin of dividing the hours of the day up into a series of one-sixtieths – an invention of the ancient Babylonians in about 300BC – and hence the invention of the second.

The first clocks to depict seconds appeared in the latter part of the 16th century and a modern definition appeared in 1956 based on the movement of the Earth around the Sun. The latest definition is based on the duration of 9,192,631,770 cycles of radiation emitted by a particular electronic transition in an atom of caesium-133. Scientists want to refine this still further by basing the definition on the optical transition of atoms, which is at even higher frequencies than electronic transitions, and even more precise.

Voices
A Russian hunter at the Medved bear-hunting lodge in Siberia
Save the tigerWildlife charities turn to those who kill animals to help save them
News
Davis says: 'My career has been about filling a niche - there were fewer short actors and fewer roles – but now I'm being offered all kinds of things'
PeopleWarwick Davis on Ricky Gervais, Harry Potter and his perfect role
News
i100
Sport
Frank Lampard will pass Billy Wright and equal Bobby Charton’s caps tally of 106 caps against
sportFormer Chelsea midfielder in Etihad stopgap before New York contract
PROMOTED VIDEO
Arts and Entertainment
The first film introduced Daniel Radcliffe to our screens, pictured here as he prepares to board the train to Hogwarts for the first time.
booksHow reading Harry Potter helps children grow up to be gay-friendly
Arts and Entertainment
A scene from Aladdin is performed at the Tony Awards in New York in June
theatreBrit producer Lythgoe makes kids' musical comedy a Los Angeles hit
Sport
Usain Bolt of Jamaica smiles and shakes hands with a competitor after Jamaica won their first heat in the men's 4x100m relay
sport
News
Chancellor George Osborne, along with the Prime Minister, have been 'complacently claiming the economy is now fixed', according to shadow Chancellor Ed Balls
i100... which is awkward, because he is their boss, after all
Life and Style
A small bag of the drug Ecstasy
Health
Life and Style
Floral-print swim shorts, £26, by Topman, topman.com; sunglasses, £215, by Paul Smith, mpaulsmith.co.uk
FashionBag yourself the perfect pair
News
news
News
ebookA unique anthology of reporting and analysis of a crucial period of history
News
Netherlands' goalkeeper Tim Krul fails to make a save from Costa Rica's midfielder Celso Borges during a penalty shoot-out in the quarter-final between Netherlands and Costa Rica during the 2014 FIFA World Cup
newsGoalkeepers suffer from 'gambler’s fallacy' during shoot-outs
News
people
Arts and Entertainment
Standing the test of time: Michael J Fox and Christopher Lloyd in 'Back to the Future'
filmReview: A week late, Secret Cinema arrives as interactive screening goes Back to the Future
Extras
indybest
News
i100
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

(Senior) IT Support Engineer - 1st-3rd Line Support

£40000 per annum: Ashdown Group: A successful IT service provider that has bee...

Wind Farm Civil Design Engineer

£55000 - £65000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Principal Marine Mechanical Engineer

£60000 - £70000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Principle Geotechnical Engineer

£55000 - £65000 Per Annum: The Green Recruitment Company: The Green Recruitmen...

Day In a Page

Save the Tiger: Meet the hunters tasked with protecting Russia's rare Amur tiger

Hunters protect Russia's rare Amur tiger

In an unusual move, wildlife charities have enlisted those who kill animals to help save them. Oliver Poole travels to Siberia to investigate
Transfers: How has your club fared in summer sales?

How has your club fared in summer sales?

Who have bagged the bargain buys and who have landed the giant turkeys
Warwick Davis: The British actor on Ricky Gervais, how the Harry Potter set became his office, and why he'd like to play a spy

'I'm a realist; I know how hard this business is'

Warwick Davis on Ricky Gervais, Harry Potter and his perfect role
The best swim shorts for men: Bag yourself the perfect pair and make a splash this summer

The best swim shorts for men

Bag yourself the perfect pair and make a splash this summer
Has Ukip’s Glastonbury branch really been possessed by the devil?

Has Ukip’s Glastonbury branch really been possessed by the devil?

Meet the couple blamed for bringing Lucifer into local politics
Dress the Gaza situation up all you like, but the truth hurts

Robert Fisk on Gaza conflict

Dress the situation up all you like, but the truth hurts
Save the tiger: Tiger, tiger burning less brightly as numbers plummet

Tiger, tiger burning less brightly

When William Blake wrote his famous poem there were probably more than 100,000 tigers in the wild. These days they probably number around 3,200
5 News's Andy Bell retraces his grandfather's steps on the First World War battlefields

In grandfather's footsteps

5 News's political editor Andy Bell only knows his grandfather from the compelling diary he kept during WWI. But when he returned to the killing fields where Edwin Vaughan suffered so much, his ancestor came to life
Lifestyle guru Martha Stewart reveals she has flying robot ... to take photos of her farm

Martha Stewart has flying robot

The lifestyle guru used the drone to get a bird's eye view her 153-acre farm in Bedford, New York
Former Labour minister Meg Hillier has demanded 'pootling lanes' for women cyclists

Do women cyclists need 'pootling lanes'?

Simon Usborne (who's more of a hurtler) explains why winning the space race is key to happy riding
A tale of two presidents: George W Bush downs his paintbrush to pen father’s life story

A tale of two presidents

George W Bush downs his paintbrush to pen father’s life story
Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover

The dining car makes a comeback

Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover
Gallery rage: How are institutions tackling the discomfort of overcrowding this summer?

Gallery rage

How are institutions tackling the discomfort of overcrowding this summer?
Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players

Eye on the prize

Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players
Women's rugby: Tamara Taylor adds fuel to the ire in quest to land World Cup

Women's rugby

Tamara Taylor adds fuel to the ire in quest to land World Cup