3. Length

A length standard can be realized in different ways, either by the distance between two lines or points, or by one of the dimensions of a given body, or by any other length such as, for example, a light wavelength. Different types of basic standards were successively adopted, depending on the degree of sophistication of the technical processes available.

Since 1983, the definition of the metre has been based on the speed of light. This is the result of the invention of lasers, which can deliver very monochromatic and intense radiation. On the one hand, we know how to stabilize the frequency and therefore also the wavelength in vacuum of some of these radiations, which today constitute wavelength standards of exceptional quality. Secondly, the frequencies of these same radiations can be measured by direct comparison with a microwave frequency reference, using a frequency...