TECHNICAL INFORMATION
392
Calibration of Volumetric Ware
TECHNICAL INFO. SYSTEMS OF WEIGHTS AND MEASURES The metric system of weights and measures
As its name implies, the metric system was intended to be based on the meter, a fundamental unit of length. All of the essential features of the system were contained in a proposal submitted to the French National Assembly in 1791. The metric system thus was originally a national system and was not given international recognition until 1889. The meter was to be equal to one ten-millionth part of a quadrant of the earth’s meridian. The kilogram was to be the mass of a volume of pure water at its temperature of maximum density occupying a space of one cubic decimeter (one one-thousandth of a cubic meter.) To propose such a system is one thing, but to make it practical, material standards are necessary. The French measure of length in 1791 was the “toise,” and the actual determination of the meridianal distance was made in terms of it. From the value found, a standard platinum bar was constructed called the “Metre des Archives,” and the meter was defined as the length between the centers of the end faces of this bar at the temperature of melting ice. To determine the mass of a cubic decimeter of water the principle of Archimedes was used, namely, that a submerged body is buoyed up by a force equal to the weight of the water displaced. This was accomplished by hydrostatic weighing of a carefully measured bronze cylinder from which, by comparison, a platinum weight called the “Kilogramme des Archives” was adjusted. During the years immediately preceding 1889, the metric system was studied exhaustively by an international commission. Redeterminations of the kilogram showed that there was a slight error in the original standard, but it was decided that the International Kilogram should be based on the “Kilogramme des Archives” in its actual state. Thus, the kilogram became a fundamental rather than a derived unit and the definition of the kilogram as the mass of one cubic decimeter of water was abandoned. The name originally proposed for the volume of one cubic decimeter was liter. Due to the error in the original standard kilogram, it was decided in 1901 to redefine the liter in terms of the International kilogram. This definition was used until the Twelfth General (International) Conference on Weights and Measures. The delegates to this conference decided to redefine the liter as a “special name for the cubic decimeter.” It was agreed that the terms “liter,” “milliliter” and “mL” might be continued, except in association with measurements of the highest precision, where “cubic centimeter” or “cm 3 ” must be used. The difference in volume between the old and the new meanings of liter is so small as to be negligible in most technical work, being less than 3 parts in 100,000. Hence, it can be expected that volumetric apparatus will continue to be marked with the familiar “liter” or “milliliter” for some time to come. United States and British Weights and Measures The basic units of the United States and British systems have been officially defined in terms of metric equivalents. In glass volumetric apparatus only the units of liquid measure are of importance.
Prototype Kilogram 20, replica Photo courtesy NIST
Fig. 1
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