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In November this year at the General Conference on Weights and Measures, it was decided to change the way the kilogram is measured in favour of defining a kilogram in terms of an electric current.

In a secured vault outside of Paris within the Bureau of Weights and Measures (BIPM), a small cylinder of platinum-iridium alloy sits protected by a triple walled glass enclosure.  Since 1889, this international prototype kilogram (IPK) or “Le Grand K”, has served as the base unit of mass for all other weights manufactured around the world.

The kilogram is the base unit of mass in the International System of Units (ISI) and is the only unit still defined by a physical artefact.  All weights used to calibrate scales can be traced back to this original kilogram.

Originally, the kilogram was defined as the mass of a litre of water at room temperature. Similarly, the metre was originally defined as one ten-millionth of the distance from the North Pole to the Equator, measured along the meridian passing through Paris.

It soon became apparent that these definitions were not accurate enough. The BIPM was formed in 1875 to fix the problem and several prototype standards for the base units of measurement were commissioned to be made and stored here for reference.

The international prototype meter was made of the same platinum-iridium alloy and had an X-shaped cross section for maximum stability. Like the kilogram, copies were made and shared with the national metrology labs of other nations. The US holds Metre Bar No. 27 and has used it as the standard for all units of length.

The copies of the kilogram and meter are occasionally compared with the prototypes in Paris and any changes in their accuracy are measured. Although these copies have sat undisturbed, their accuracies have varied over the years. The differences in measurement are very small, but were enough to cause scientists to look for more stable definitions of the base units.

The metre was redefined in 1960

In 1960 the meter was redefined as the distance light travels in a vacuum during a time interval of 1/299,792,458 of a second. The speed of light in a vacuum is a constant of nature and therefore can be measured anywhere in the world repeatably and used to reconstruct the meter to extreme precision.

In 2018 its time for the kilogram to be redefined

Now, the kilogram too has been redefined in terms of a physical constant known as Planck’s constant. Planck’s constant is a measure which relates a photon’s energy to its frequency. Recent breakthroughs in science have led to incredibly accurate measurements of this constant. This, in turn, has allowed scientists to use a device known as a Kibble Balance to more accurately define the kilogram in terms of Planck’s constant.

A Kibble Balance works like a normal two-pan balance except instead of measuring one mass against another, it measures mass against an electromagnetic force. This electromagnetic force is generated by a coil surrounding a permanent magnet. Due to the recent discoveries, we can measure the resulting electromagnetic force with extreme accuracy using Planck’s constant.

Scientists employ Kibble Balance to reconstruct Kilogram

Now scientists can use a Kibble Balance to reconstruct their own kilogram anywhere in the world, without having to compare them to the IPK.

The IPK will still be stored and maintained by the BIPM for research purposes, but will no longer be needed for use as a reference standard. Most people will not notice any difference unless you are working in advanced physics. However, for those working in a measurement field around the world, this is extremely big news.

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