Video: How we measure weight

Hello again. I’m Doctor Bruce Warrington, Australia’s Chief Metrologist.

Today I want to talk about the kilogram – the unit for mass (or weight), which has a long history.

One of the earliest units of measure, from over 2000 years ago, was based on a grain – a single barley seed - still used today to measure the mass of bullets, arrows and some medicines.

In 1824 the English Imperial pound was defined as 7000 grains exactly (interestingly that’s about 70 jelly babies). Around the same time in France, the new metric system proposed the kilogram, based on the mass of a cube of pure water 10cm on each side.

In 1875 17 countries signed a treaty to adopt the metre and kilogram. A special kilogram was made out of a platinum alloy to be the international reference, known as ‘Big K’, and copies were distributed to countries using the metric system. Australia joined formally in 1947, and here is our copy of ‘Big K’ kept at the National Measurement Institute in Sydney.

Just last year in 2018 the nations of the world agreed to a new definition for the kilogram, based on a fixed constant of nature – Planck’s constant. The best measurements of mass can now be made using an electromagnetic Kibble balance, and we no longer risk ‘Big K’ being lost or damaged.

Thanks for listening, and stay tuned for more on the basic units of measurement.