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 was 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 realisation – the physical form – of the unit. However, these physical representations can change over time and are susceptible to damage or loss. So, over the years, the definitions have evolved to depend on constants of nature that are 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.