Physikalisch-Technische Bundesanstalt

The foundations of a house can carry their load only if the underground does not slide. The same applies to the foundations of all measurements, the SI base units. In the course of millenniums man has tried to give his measurements a solid basis in order that the entire building of measurement results cannot suddenly begin to sway.

It is possible that the ancient Egyptians already developed very "modern" ideas in this context. The dimensions of the pyramids allow the conclusion to be drawn that a certain section of the circumference of the earth served as a basis of their measure of length, the Egyptian ell. This knowledge then was lost for a long time. In the meantime, man used as a makeshift his own body as a reference measure, in particular for such a "tangible" unit as the length – a method which had chaotic consequences. As late as 1870 there were still 600 different square measures in the German Reich – and as many mistakes, frauds, misunderstandings and conversion problems. And this despite the fact that the idea of the ancient Egyptians had won through again in France less than 100 years earlier: to derive a new measure of length from the characteristics of the earth. The conviction was that the shape of the earth, which was considered to be invariable, was a sound basis for an invariable natural measure. The new measure of length, the metre, was defined as the ten millionth part of the quadrant of the earth’s meridian. With the Metre Convention concluded in 1875, it started its triumphant advance in the whole world – and with it the kilogram which was directly derived from the metre (as the mass of a cube of pure, distilled water with edges 1 dm in length) and the entire metric system. Until far beyond the middle of the 20th century, yet another unit was derived from the characteristics of our planet: the second whose duration followed from the earth’s rhythm (day and night, summer and winter).

However, at the end of the 19^th century some scientists expressed first doubts as to whether the characteristics of the earth could really be the best basis for a system of units which was to be valid as long as possible and without changes. The dimensions were too large, and the circumference of the earth and its movement around the sun were possibly not constant. In 1870, James Clerk Maxwell demanded "not to seek the units of length, time and mass in the movement or mass of our planet but in the wavelength, frequency and mass of the imperishable, invariable and altogether similar molecules". Max Planck went even further in 1889: A system of units should not be based on atoms or molecules but on farther reaching, established facts, the fundamental constants themselves, because they were independent of special atoms or substances.

The arguments were convincing. In the time that followed, the physicists tried to fulfil the requirements – and they have been busy with this task up to this very day. The progress made is different for the different base units. As to the metre, Maxwell's requirement was the first to be met. The unit of length was defined on the basis of a molecular property: the oscillatory behaviour of a noble gas, krypton. In 1983, this definition was replaced by the definition of the metre valid today, which is based on a fundamental constant: the velocity of light. 

The success achieved in the field of the electric units is of recent date and opens up exciting prospects. Quite a number of fundamental constants are used here to maintain and disseminate these units with extremely high accuracy: the Josephson constant K_J for the dissemination of the unit "volt", and the von Klitzing constant R_K for the dissemination of the unit "ohm". Both were raised to the rank of fundamental constants only in 1990. They are closely linked with two other fundamental constants: Planck's constant h and the elementary charge e. The latter might gain in significance in the future if one succeeds in developing a standard for the SI base unit, the ampere, by means of single electron tunneling.

Today, metrologists are really content with the units second, meter, volt and ohm, because in their case traceability to fundamental constants has already been achived.

They are, however, concerned about the kilogram. It is still defined as the mass of the international kilogram prototype which has once been derived from the meter. The efforts to create a more solid base for the kilogram rank among the most interesting projects of global basic research in metrology.

Extra Internet chapter Natural constants, among others with current CODATA values of important constants.