Early Measurements and Standards
Some of the earliest surviving measuring devices include gold scales recovered in present-day Greece from the tombs of Mycenaean kings. Although these are merely decorative, their survival shows that the measurement of mass has been around for over four millennia. The tombs of Egyptian pharaohs — the pyramids — were constructed by builders using more than simple rulers: the pyramids are regular, symmetric and aligned with the Earth's axis. Details of the measuring devices these builders employed are sketchy, although some instruments are illustrated in hieroglyphic drawings found on tomb walls, and documents found within the tombs.
Standards of measurements have existed since the appearance of organized societies. Babylon, Egypt, and the city states of Greece all had standards against which commercial measuring devices had to be compared and approved. By about 500 B.C., Athens had its own central depository of official weights and measures — the Tholos — and some fifth-century-B.C.
||The round base of the Tholos (fifth century B.C.) in Athens, where the city's standards were kept, survives near the original stoa (market), under the shadow of the Acropolis.
Athenian weights and volume measures survive to this day. Merchants were required to take their weights and linear and volume measures to the Tholos to compare them with the official standards. Merchants caught short-changing their customers were subject to fines, just as they are today.
Although we might suggest that the Egyptians had discovered the art of measurement, it is really only with the Greeks that the science of measurement begins to appear. The Greeks' knowledge of geometry, and their early experimentation with weights and measures, soon began to place their measurement system on a more scientific basis. By comparison, Roman science, which came later, was not as advanced, although people like Heron devised a number of ingenious measuring devices — including an odometer for measuring distance and a surveying instrument which measured azimuth and altitude angles. Not surprisingly, several of the most important Roman measuring devices were developed for practical purposes such as road and aqueduct construction.
Every major civilization has developed a set of standards based on the physical world. In England, three barleycorns defined the inch from 1305 on. In the 1360s, the English adopted the length of Edward III's arm as a national standard for length; this later evolved into the Elizabethan yard. As science and technology advanced, so did the sophistication of measurement standards. By the late eighteenth century, British makers of scientific instruments were taking great care to ensure the precision of secondary standards. In order to accomplish this, they used microscopes, carefully duplicated the composition of the originals, and controlled temperature during the manufacturing process.
During the eighteenth century, French scientists concluded that a length standard based on an unchanging physical quantity was highly desirable. The metric system, proclaimed in 1795 — or Système International (SI), as it is now officially called — grew out of a 1734 attempt to measure the distance between the Equator and the North Pole. The metre was defined as one ten-millionth of that distance. As is well known, the French measurement was very slightly in error. It resulted, nevertheless, in a carefully-made bar that became the standard metre. This standard lasted until 1961, when the metre was redefined in terms of light emitted from the element Krypton. It has since been redefined again. The French also adopted volume and mass standards based on the metre — mass being defined as the amount of water held in a standard volume at a standard temperature.