## Scientists about to redefine the kilogram

## What is a kilogram, exactly?

Every object we know has mass, and we usually measure its mass by putting it on a weighing scale to find how many kilograms it weighs. Have you ever wondered how heavy is a kilogram?

A kilogram weighs 1000 grams, and a gram is originally defined as exactly as much as “a volume of water equal to the cube of the hundredth part of the metre, at the temperature of melting ice”.

In 1889, the mass of the kilogram was redefined as being equal to the IPK. The International Prototype Kilogram is the only SI unit based on a physical object – specifically, a 39 millimetre-high, platinum-iridium alloy cylinder kept in a vault at the International Bureau of Weights and Measures in Sèvres, France. This is the standard against which all other kilogram weights are measured.

However as time goes by, the kilogram is literally getting heavier. This is due to a build up of pollutants over the years, adding tens of micrograms to the IPK. There are 40 official IPK replicas around the world, and they all appear to be gaining weight at different rates. This means that their masses are slowly getting out of sync.

These discrepancies affect units such as the pound, ton, or milligram are defined in relation to the kilogram, as well as non-mass units such as the ampere (for electric current), or the candela (luminous intensity).

## A new definition for the kilogram on its way

A new definition based on a mathematical foundation that can be calculated anywhere would be a better option to define the kilogram, instead of a physical item.

Jon Pratt, chief of quantum measurement at the National Institute of Standards and Technology (NIST), are part of an international effort to redefine the kilogram based on a fundamental universal constant —the Planck constant. After 16 months of hard work, NIST’s new value for the Planck constant is 6.626069934 x 10^{-34} kg∙m^{2}/s, with an uncertainty of 13 parts per billion. This means that the NIST scientists think their measurement of Planck’s constant is within 0.0000013 percent of the correct number.

## How is the Planck constant measured?

To measure the Planck constant, the researchers used a Kibble balance, a device that suspends a 1-kg weight with electromagnetism. By calculating the amount of electromagnetic energy it takes to balance the mass, researchers are able to calculate the constant with much precision.

When the International Committee for Weights and Measures announced that it would reconsider the kilogram definition, it said it would require three measurements with uncertainties below 50 parts per billion, and one below 20 parts per billion (ppb). With the new NIST measurement, the world now has at least three experiments below 20 ppb — another was conducted by a Canadian team using a Kibble balance, the third by an international group that calculates the Planck constant based on the number of atoms in a sphere of pure silicon.

All of these measurements have been submitted for consideration by an international body, which will review and average them to determine the official value of Planck’s constant. In 2018, at the next General Conference on Weights and Measures, the scientific community will draft a resolution to redefine kilogram based on this constant.

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