Kilogram reinvented from today: Scientists formally change the definition6 min read

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The kilogram has been reinvented! Scientists formally change the measurement based on a lump of metal to a calculation based on the speed of light

  • Le Grand K has been stored under secure lock and key in France since 1889 
  • It is the the defining mass against which all other kilograms are measured 
  • Definition of kilogram formally changed from today after November 2018 vote  
  • From now it’ll be measured by the ratio of energy to frequency of the photon

The official definition of a kilogram has changed today – just six months after scientists voted for a brand new interpretation. 

Previously, Le Grand K – a hunk of metal – was the defining object by which all other kilogram measurements were made and utterly reliant upon.  

Scientists have been striving for a way to sustainable replace it in the unfortunate event it was destroyed. 

Stored under secure lock and key in France since it was made in 1889, the carefully calibrated alloy cylinder has now been retired.

It has served as the global standard for weighing things for 130 years, with dozens of copies stored across the globe to standardise the weights of individual nations.  

Now, scientists will measure the kilogram via the Kibble balance. 

Based on the Planck constant theory, this instrument tracks tiny changes in electrical current to calculate the gravitational force acting on a mass – the two components of weight. 

WHAT IS LE GRAND K?

Le Grande K is the defining mass against which all other kilograms are measured. 

It is a carefully calibrated alloy cylinder that has been held under lock and key in France since it was made in 1889.

Dozens of copies have been made and stored around the world to standardise the weights of individual nations.

Le Grande K is stored in the Louis XIV Pavillon de Breteuil, a building that also houses the International Bureau of Weights and Measures.

The weight is so precious that it is only taken out once every 40 years to make copies for other nations.

‘One key reason for doing this work is to provide international security,’ Ian Robinson, head of engineering measurement at the NPL, told the Luxembourg-based magazine Delano.

‘If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world’s metric weights system.

‘There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all.’

The Kibble balance calculates weight using small changes in any electrical current.

It measures the electric current required to produce an electromagnetic force that is equal to the gravitational force acting on a mass.

Scientists have spent decades developing a new global weight standard

The device, known as a Kibble balance, measures tiny changes in electrical current to calculate the gravitational force acting on a mass - the two components of weight

Scientists have spent decades developing a new global weight standard. The device, known as a Kibble balance, measures tiny changes in electrical current to calculate the gravitational force acting on a mass – the two components of weight

HOW WOULD A KIBBLE BALANCE MEASURE THE PLANCK CONSTANT?

A Kibble balance would redefine the kilogram by giving scientists the most precise measurement yet of the Planck constant.

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It comprises a wire coil inside a magnetic field that is suspended from the arm of a balance.

A kilogram mass is also placed on this arm exerting a force downwards due to gravity.

An electrical current is passed through the coil generating a force, the strength of which depends on the size of the current, the strength of the field and the length of the coil.

The value of current is varied until the downward force from the kilo mass is balanced by the force from the coil in the magnetic field.

The mass is then removed and the coil is moved in the field, which induces a voltage in the coil.

By tracking the current and voltage the Planck constant can be measured in terms of mass, length and time. 

This precise current measurement is used to produce the most accurate calculation yet of Planck constant, which is then in turn used to define a kilogram.

Planck’s constant – one of the fundamental constants of nature – can be combined with certain properties of light and Einstein’s e=mc2 to give the new kilo. 

Using these machines as an international standard would save the need to keep Le Grand K and its copies under tight security.

‘We are going to create a method for weighing the kilogram completely accurately until the end of time,’ Mr Robinson said.

‘We will have released ourselves from a single point of failure.’

Later this month, delegates at the international General Conference on Weights and Measures, held in France, are expected to vote to retire Le Grande K.

The Louis XIV Pavillon de Breteuil in Paris houses the International Bureau of Weights and Measures

The Louis XIV Pavillon de Breteuil in Paris houses the International Bureau of Weights and Measures

The Kibble balance is widely expected to replace it, allowing the kilo to join a wide range of modernised standard measurements.

The metre, once standardised using an alloy bar stored in Paris, has been defined as the distance travelled by a light particle in 1/299,792,458 of a second since 1983.

For more than a century, the second was for decades measured as 1/86,400 of an average day.

As Earth’s rotation is variable, the unit has now been updated to be the time taken for a caesium atom to vibrate precisely 9,192,631,770 times.

HOW CAN THE PLANCK CONSTANT REDEFINE THE KILO?

The Planck constant is a fundamental constant of nature which sets a limit on the accuracy with which we can measure physical systems.

It depends on the SI units of length, mass and time: The metre, kilogram and second, respectively.

The second and metre are already defined by universal constants.

The metre, once standardised using an alloy bar stored in Paris, is now defined as the distance travelled by a light particle in 1/299,792,458 of a second.

The second is defined as the time taken for a caesium atom to vibrate precisely 9,192,631,770 times.

Because we already have precise measurements for the second and metre, they can be used in conjunction with a fixed value of the Planck constant to redefine the kilogram.

This would remove the need for Le Grand K, also known as the International Prototype Kilogram.

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