How basic physical units of length, mass and time were defined

From the beginnings of the Scientific Revolution in the mid-16th century science was international in character, though for long essentially European. Major discoveries were made in Italy, France, England, Holland and in many other countries. Scientists exchanged information through informal correspondence and through the great national academies such as the Royal Society in London (founded in 1660) and the Academy of Sciences in Paris (founded in 1666). Increasingly, especially in the physical sciences, the emphasis was on the quantitative rather than qualitative observations. To avoid confusion it was important that observers at widely different centers should be able to express their results in the same terms. This need for precision was further made necessary by advances in technology, especially in mechanical engineering.

The basic physical units are length, mass and time and to obtain absolute, reproducible values recourse was made to unvarying natural features. At the beginning of the 20th century the standard length was the meter, defined as one ten-millionth of the distance between the equator and the north pole. This was inscribed on a platinum bar in Paris, maintained at 0°c. From this as many sub-standards as were required were derived. The unit of mass was the gram defined as the mass of a cubic centimeter of water at 4°C (its maximum density). For time, the second was defined as 1/86,400th part of the mean solar day. On these basic units of centimeter, gram and second was based the C.G.S. system, from which many other units - of force, acceleration, power, etc. - were derived. For measuring temperature two fixed points were defined: the freezing-point and the boiling-point of water at standard atmospheric pressure.

The need to ensure that the wealth of instruments in general use all conformed to the same standards was one of the factors that led to the foundation of the National Physical Laboratory in England (1900), the National Bureau of Standards in America (1901) and the Kaiser Wilhelm Institutes in Germany (1910). With the need for ever greater precision new standards have been introduced. Thus in 1960 the meter was redefined in terms of the velocity of light and in 1983 in terms of the wavelength of emission from krypton gas. (‘Science, A History of Recovery in the Twentieth Century’, by Trevor Williams)