Metrology is the science that has as its purpose the identification of the most suitable and precise methods to carry out the measurement of any physical quantity, of which it also defines the unit of measurement, and to express and use in a correct way the result of the measurement itself. It therefore deals only with physical quantities; so much so that, to have the right to be defined as such, the properties of an object or phenomenon must be measurable, that is, it must be possible to define units and methods of measurement.
Metrology is a multidisciplinary science that for its purposes must be confronted with both purely theoretical aspects (eg basic physics, mathematics and statistics) and practical (eg mechanical technology, electronics), up to the proper management (eg management of laboratories, cost-benefit analysis).
The need to establish uniform and reproducible methods for measuring goods and products has led authorities to define samples of measurement units since ancient times. Often based on artefacts, measuring parts of the human body (thumb, foot) and mainly localised, they were totally inadequate for the development not only of science and technology but also of modern industry and international trade.
The foundation of metrology as a science is the work of the spirit of the Enlightenment and its purpose since the beginning is to harmonize the locally distributed samples in a single system. Initially only two units of measurement were adopted, for length and mass. In 1795 the French National Assembly deliberated the definition of the meter as the forty millionth part of a terrestrial meridian and that of the kilogram as the mass of an appropriate volume of purified water. In 1799 the metric system progressed with the acquisition of samples of units of measurement, respectively a bar and a block of platinum. Already at its inception it was clear that the realization of the sample was quite different from the definition of the unit of measure.
During the nineteenth century the metric system spread first in Europe, then in South America: many countries now used it, but each one had its own samples, different from each other. In order to standardize the various national standards, in 1875 the International Metric Convention founded the Bureau International des Poids et Mesures (BIPM), which was entrusted with the task of preserving the primary prototypes of the meter and the kilogram, to make copies and to compare national standards with the primary prototypes. The International Committee of Weights and Measures (CIPM) and the General Conference of Weights and Measures (CGPM) were also set up, with the task of deciding on changes in the system of fundamental units. These institutions still exist today and continue their mission.
The International Metre Convention, which now has 51 member states, officially adopted the metric system, which, with the inclusion of the astronomical second as a unit of time, was named the MKS System (from the initials of metre, kilogram and second). During the twentieth century, new units of measure were included and the definitions of the existing ones were continuously revised. In 1948, with the introduction of the ampere as the fundamental electrical unit, the MKSA System was established, then replaced in 1960 by the International System (SI) in force today. In this system are included seven fundamental units of measurement, for as many physical quantities: the meter (m) for length, the kilogram (kg) for mass, the second (s) for the duration of time, the ampere (A) for electric current, the kelvin (K) for the thermodynamic temperature, the mol for the amount of substance and the candle (cd) for the luminous intensity.
In the last century there has been an evolution of the principles behind the choice of fundamental units: from units based on artifacts to units rooted in the natural world and physical laws. Already in 1870 James C. Maxwell argued the need to define the units of measure linking them to physical quantities of atoms and molecules, rather than to the motion or size of the Earth. Max Planck, going further, believed that units should be defined not by the properties of specific atoms but rather by fundamental constants, such as the speed of light in vacuum c, the universal gravitation constant G, Planck’s constant h, the elementary charge e, and so on. The main difficulty of a system of units of measurement based in this way was its impracticality: these fundamental units were extremely advantageous for the microscopic world, but were not very adaptable to the needs of everyday life in the macroscopic world.
Currently, Maxwell’s point of view is taken for granted and, thanks to the increased ability to relate the scale of the microscopic world with the human one, metrology proceeds in the direction indicated by Planck. Metrology is however a prudent science: changes to the fundamental units proceed at a steady but slow pace and are adopted only when there is consensus on the maturity of scientific and technological knowledge involved and after evaluation of costs and benefits for science and society.