International system of units overview

There are almost 200 countries, but only three — the U.S., Myanmar and Liberia — have not adopted the International System of Units, the official metric system of weights and measures.

Ancient Babylonian and Egyptian records, as well as the Bible, indicate that length first was measured with the forearm, hand or finger. Time was measured by the periods of the sun, moon and other heavenly bodies. Volume was measured with stones and seeds, which served as standards, and “carat” still is used as a unit of weight for diamonds and other gems. Carat was derived from the seed of the carob tree. The uniqueness of these seeds is that their weight is practically the same from one seed to another. In the metric system of units, the weight of a carat equals 200 mg (0.007 oz.).

In the 1790s, the French Academy of Sciences deduced an invariable standard for all measures and weights and established a Commission of Weights and Measures. The commission assigned the name meter to the unit of length. This term was derived from the Greek word “metron,” meaning a measure. The physical standard to represent a meter was to equal one ten-millionth of the distance from the North Pole to the equator along Earth’s meridian running near Dunkirk, a city and port in France on the Strait of Dover, and Barcelona, Spain.

The commission was asked to conduct accurate measurements of the length of the meridian and prepare physical standards for a meter and a unit of mass, the kilogram. France approved the two platinum prototypes Dec. 10, 1799, forming the basis of the metric system.