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The Definition of a Kilogram Has Changed! Here is What it Means

18 November 2018
The Definition of a Kilogram Has Changed! Here is What it Means

The real prototype is-and will continue to be-kept in Paris at the International Bureau of Weights and Measures (BIPM), the arbiter of global measurement standards since the Meter Convention in 1875.

Well, the kilogram update is not going to affect the common man.

In 2013, scientists discovered that the kilogram Prototype had gained tens of micrograms of mass from surface contamination. It will be replaced by the Planck constant - the fundamental constant of quantum physics.

"Those units, those constants chosen now, include everything we know, everything we have always known and provide that springboard for us to go pursue those things that we don't know", said Jon Pratt of the US National Institute of Standards and Technology.

In a similar way, the SI unit for the ampere will now be based on the constant for the charge of the electron.

Scientists in Paris on Friday approved a change which will alter how a kilogram is defined, the first change to the system of measuring mass which has been in use since 1889. The current SI unit for the ampere is impractical to realize, and as a result, precision electrical measurements have been based for 30 years on quantum measures such as the quantum Hall effect and Josephson junctions that generate quantum voltages. "This will pave the way for far more accurate measurements and lays a more stable foundation for science".

Suffice to say that the updated definition will, in time, spare nations the need to occasionally send their kilos back to France for calibration against the "Grand K" Scientists instead should be able to accurately calculate an exact kilo without having to measure one lump of metal against another.

"So when you just get it [the International Prototype of the Kilogram] out of the vault, it's slightly dirty and the whole process of cleaning or handling or using the mass can change its mass".

In fact the changes were so small they were about 50 parts out of a billion, or the weight of an eyelash.

The new system will allow us to determine the pounds of anyone who has a device called the balance of the Kibble.

Uptill now, all the replica kilograms had to be checked against Le Grand K after every few decades. The new definition means that the weight of a kilogram no longer has to be compared to a block of platinum. The machine measures the weight of an object by the strength of the electric current and voltage required to produce a compensating force, equalising one force with another.

"This is the most important decision that the BIPM has made in maybe 100 years, which may be a slight exaggeration, but at least since 1960 when they adopted the International System of Units", Terry Quinn, emeritus director of the BIPM, told Engadget last year. According to its original design, the kilogram was "equal to the mass of a liter of water".