Science & Technology

How the kilogram has changed

Down To Earth tells you how the kilogram's definition has changed

Last Updated: Tuesday 21 May 2019

The kilogram (kg) is no longer what it is used to be. Its definition has changed. It still means the same amount of mass as before — your body will still weigh the same as before. But the standard used to define a kilogram has changed.

Why did this happen? From 1889, a platinum-iridium cylinder called the International Prototype Kilogram or IPK was used as standard for a kg.

This cylinder was locked in a jar in the headquarters of the International Bureau of Weights and Measures in Sèvres, Paris and an exact replica of this, called the National Prototype Kilogram had been in use to the define the Indian standard.

But over the years, scientists have questioned using IPK as metrics for a kilogram because of its limitations. The accuracy of IPK was always a limitation as even with the added weight of dust particles or loss of atoms here and there could change the entire scenario. Also, IPK could only be used as a standard under the earth’s gravity and not, for instance, on Mars.

Because of these reasons, in November 2018, 60 countries agreed that the kilogram should be redefined in terms of natural constants and the Planck constant was chose as its standard

So what is a Planck constant?

Planck constant is a quality that relates light’s particle energy to its frequency. The value of the Planck constant has been accurately measured as 6.626069 x 10^ (-34) kilograms per second per square meter. As the units of time (seconds) and distance (metres) have already been defined in terms of natural constants, scientists are able to define kilogram precisely from the equation. A machine called the kibble balance is then used to obtain this practically. Scientists believe this change will be better for technology, retail and health. 

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