Weights and Measures

Weights and Measures, measurements of length, capacity, and weight, using standard units. The principal early standards of length were the palm or hand breadth, the foot, and the cubit, which is the length from the elbow to the tip of the middle finger. Such standards were both changeable and perishable, and only within modern times have definite unchanging standards of measurement been adopted.

In the United States and Britain, the everyday units of linear measurement have been the inch, foot, yard, and mile. Until recently in Britain, the English units of length were defined in terms of the imperial standard yard, which was the distance between two lines on a bronze bar made in 1845 to replace an earlier yard bar that had been destroyed by fire in 1839. Because the imperial standard yard bar has been shrinking at the rate of 1.5 millionths of an inch per year, the United States adopted a copy of the international prototype meter as the national standard of length in 1889. Until 1960, all U.S. measurements of length were derived from a standard meter (meter prototype number 27). In 1960 the meter was redefined in terms of wavelengths of light from a krypton-86 source. In 1983 it was again redefined as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second.

English units of weight (ounces, pounds, and tons) are now also derived from the metric standard of mass, which is the international prototype kilogram. This is a solid cylinder of platinum-iridium alloy maintained at constant temperature at Sèvres near Paris. A copy, as exact as possible, of this standard is maintained by an agency of the U.S. Department of Commerce (see National Institute of Standards and Technology).

Most countries have converted or are in the process of converting their local systems of weights and measures to the metric system. Some old terms, however, may continue in use.

See also CGS System; International System of Units; Metric System.