How are the units of measurement defined?
Historically, units of measurement were defined by physical objects or properties of materials. For example, the metre was defined by the length between lines engraved on a metal bar and the kilogram is still defined as the mass of a single cylinder of platinum-iridium metal – the International Prototype of the Kilogram (IPK).
In these two examples, the definition was also the physical form – the realisation – of the unit. However, these physical representations can change over time or in different environments. So, over the years, the definitions have been improved to be more stable and reproducible, and to meet the needs of today’s research and technological applications.
During the last century, scientists measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy. They discovered that these were more stable than physical objects. It became clear that these constants of nature could offer a new and more stable foundation for the SI.