Weight is over as nations agree to redefine the kilo
THE kilogram will no longer be measured against an actual weight after scientists voted to start using an electromagnetic current instead.
Since 1889, a kilogram has been defined by a single lump of platinum-iridium which is housed inside three glass bell jars at the headquarters of the International Bureau of Weights and Measures (BIPM) outside Paris.
But the master copy, known as Le Grand K, has been picking up microparticles of dust, or losing mass in cleaning, causing dismay for scientists using it to measure other weights.
Now, after a week-long meeting at the Palace of Versailles, representa- tives of 60 nations agreed to redefine the kilogram based on the unchanging value of the Planck constant.
Instead of checking it against an actual weight, scientists can now find an exact kilogram by measuring the amount of electricity needed to lift it, using a special set of scales known as the Kibble balance.
Scientists have wanted to define a constant value for the kilogram that is derived from laws of physics, in the same way they have done for other standard units.
For example, a metre is not defined as 100cm but “the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second”.
Describing what impact the new kilogram would have, the BIPM said: “In the same way that if you replaced the decaying foundations of a house with robust new ones.
“It may not be possible to identify the difference from the surface, but some substantial changes would have taken place to ensure the longevity of the property.”
Martin Milton, director of the BIPM, said: “The redefinition is a landmark moment in scientific progress.
“Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges.”
The new definitions agreed by the BIPM will come into force on May 20 2019.