The scientists are preparing to dethrone the "King of Kilograms" – the determining mass against which all other kilograms are measured.
Le Grand K, kept under lock and key in France since its manufacture in 1889, is being upgraded with a modernized device that measures light.
The scientists hope to be able to retract the carefully calibrated alloy cylinder to ensure the safety of the world's metric weight systems if the object is destroyed.
Held under two glasses and secured with three keys – only two of them in France – the precious metal piece is kept in an elegant building outside Paris.
It has served as the world standard for weighing things for 130 years. Dozens of copies have been stored worldwide to standardize the weights of individual nations.
Le Grand K (right) is an alloy cylinder that has been standardizing the kilo for more than a century and has been under lock and key since its manufacture in 1889 in Paris. The weight is stored under two bells (middle) Prevent dust and dirt from forming
WHAT IS LE GRAND K?
Le Grande K is the determining mass against which all other kilograms are measured.
It is a carefully calibrated aluminum cylinder that has been locked up in France since its manufacture in 1889.
Dozens of copies have been made and stored worldwide to standardize the weights of each nation.
Le Grande K is located in the Louis XIV Pavillon de Breteuil, a building that also houses the International Weights and Measures Bureau.
The weight is so valuable that it is removed only every 40 years to make copies for other nations.
The British version is stored at the National Physical Laboratory (NPL) in London, where scientists have been developing a new global weight standard for decades.
Known as the Kibble Libra, the device tracks minute changes in the electrical current to calculate the gravitational force on a mass – the two components of the weight.
It could mark the end of Le Grand K, which has been held for 130 years in the Louis XIV Pavillon de Breteuil, a building in the French town of Saint-Cloud.
"One of the main reasons for this work is to ensure international security," said Ian Robinson, head of metrology at NPL, to the Luxembourg magazine Delano.
"If the Pavillon de Breteuil had burned down tomorrow and the kilogram melted in its vault, we would have no indication of the weight system of the world.
"There would be chaos. The current definition of the kilogram is after all the weight of this cylinder in Paris. "
The Kibble scale calculates the weight with small changes in the electrical current.
It measures the electrical current required to produce an electromagnetic force corresponding to the gravitational force acting on a mass.
Scientists have developed a new global weight standard for decades. The device, known as the Kibble Libra, measures tiny changes in the electrical current to calculate the gravitational force on a mass – the two components of the weight
HOW WOULD A KIBBLE BALANCE MEAN THE PLANCK CONSTANT?
A kibble balance would redefine the kilogram by giving scientists the most accurate measurement of the Planck constant so far.
It includes a wire coil within a magnetic field suspended on the arm of a balance.
On this arm, a kilogram of weight is placed, which exerts a force due to gravity force down.
An electric current is passed through the coil, generating a force whose magnitude depends on the magnitude of the current, the strength of the field and the length of the coil.
The current value is varied until the downward force of the kilo-mass is compensated by the force of the coil in the magnetic field.
The mass is then removed and the coil is moved in the field, inducing a voltage in the coil.
By tracking current and voltage, the Planck constant can be measured in terms of mass, length and time.
This accurate current measurement is used to obtain the most accurate calculation of the Planck constant that will be used to define a kilogram in the future.
The Planck constant – one of the fundamental constants of nature – can be combined with certain properties of light and Einstein's e = mc2 to obtain the new kilo.
Le Grande K is kept locked up in the Louis XIV Pavillon de Breteuil
Using these machines as an international standard would limit the need to keep Le Grand K and its copies under strict safety precautions.
"We're going to develop a method to weigh the kilogram completely accurate by the end of time," Robinson said.
"We have freed ourselves from a single point of failure."
At the end of this month, it is expected that the delegates to the International General Conference on Dimensions and Measures in France will vote in favor of the resignation of Le Grande K.
The Louis XIV Pavilion de Breteuil in Paris is home to the International Weights and Measures Bureau
It is generally expected that the Kibble scale will replace them so that the kilo can be connected to a variety of modernized standard measurements.
The meter, which had once been standardized with an alloy rod stored in Paris, was defined as the distance a light particle has traveled since 1983 at 1 / 299,792.458 seconds per second.
For more than a century, the second decade was measured as 1 / 86,400 of an average day.
Since Earth's rotation is variable, the unit has now been updated to vibrate a cesium atom exactly 9,192,631,770 times.
HOW CAN THE PLANCK CONSTANT DEFINE THE KILO?
The Planck constant is a fundamental constant of nature that limits the accuracy with which physical systems can be measured.
It depends on the SI units length, mass and time: meters, kilograms and seconds.
The second and the meter are already defined by universal constants.
The gauge, once standardized with an alloy rod stored in Paris, is now defined as the distance that a light particle takes in 1 / 299,792.458 seconds.
The second is defined as the time required for a cesium atom to oscillate exactly 9,192,631,770 times.
Since we already have accurate readings for seconds and meters, they can be used along with a fixed value of the Planck constant to redefine the kilogram.
This would eliminate the need for Le Grand K, also known as the International Prototype Kilogram.