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Unit of the month – the kilogram

In preparation for the SI changes that are coming in on 20 May 2019, we are looking at a unit a month

Following the recent decision, taken by measurement scientists from around the world, to revise the International System of Units (SI), on the 20 of each month we are looking at one of the seven SI base units. You'll be able to find out where they are used in everyday life, how they are defined now, and the changes that will come into force on 20 May 2019. The timetable is;

20 November 2018 the metre (m)
20 December 2018 the candela (cd)
20 January 2019 the ampere (A)
20 February 2019 the kelvin (K)
20 March 2019 the second (s)
20 April 2019 the mole (mol)
20 May 2019 the kilogram (kg)








Mass is a quantity that has been used by humanity for thousands of years. The need to trade goods is a fundamental part of any civilisation and the methods to do so are constantly developing. Our first records of using measurement standards in trade come from ancient Mesopotamia. The ancient Egyptians employed stones to check the weight of goods, later these were replaced by standards cast from bronze. Amazingly, until 20 May 2019 we did something very similar. The International Prototype of the Kilogram is also a material measure, a cylinder made from a platinum-iridium alloy.

This inconspicuous object (as platinum is a heavy element the cylinder is only 39 mm in height with a diameter of 39 mm), is kept secure in a vault at BIPM in Paris, and has been a reference for mass measurements around the whole world for over hundred years.

Since 1889, the kilogram was defined as the mass of the International Prototype. Each country that was a signatory to the Metre Convention received its own copy and maintained it as their National Porotype. Each (in theory) identical national copy was periodically compared with the International Prototype. Over the years, measurements done in these comparisons showed that masses of the various artefacts were diverging. The last comparisons showed that the spread had grown to 50 μg – about the same mass as a fly’s wing. Although it seems very little to the ordinary person buying flour in a shop, this is a huge amount for science. In the field of medicine, for example, 50 μg is a daily dose of D-vitamin for a newborn baby.

From 20 May 2019 the International Prototype of the Kilogram is no longer the reference for international mass measurement. It will be replaced by h, the Planck constant, a fundamental constant of nature that represents the quantum of electromagnetic action, which relates a photon’s energy to its frequency and is present in the majority of quantum mechanics. Since mass and energy are equivalent (think of Einstein’s famous equation E = mc2) it also relates mass to frequency. But, to ‘make a kilogram from the Planck constant’ we use a ‘Kibble balance’. The Kibble balance was developed by Dr Bryan Kibble at NPL in 1970s and is used to realise a mass based on the Planck constant using very accurate quantum electrical standards.

In practice, the Kibble Balance works by putting a weight on a plate attached to the top of a wire coil suspended in an external magnetic field. Gravity forces the coil down through the external magnetic field. But, if we adjust the current in the coil, as we just learnt, this causes a force that can push the coil upwards. In this way, we can adjust the current until the magnetic force upwards balances the downwards gravitational force.

Balancing these forces is just a beginning of a very complicated process, that involves measuring temperature, time, gravity, currents and voltages, needed to establish the unknown mass. These measurements, along with lots of physical equations and quantum effects like the Quantum Hall and Josephson effects, are where the Planck constant comes into play. There are also a lot of very important conditions that have to be fulfilled to precisely measure this unknown mass.

Definition of the kilogram until 20 May 2019:

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Definition of the kilogram from 20 May 2019:

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and νCs

As you can see, this definition is a lot more complicated. But, by using constants of nature allows scientists to measure the kilogram with precision like never before. The new definition also means that the mass of a kilogram won’t be changing with time, unlike the international prototype.

20 May 2019