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Different Systems of Units

A system of units is an agreed set of base quantities, such as length, mass and time, together with named units for measuring them and rules for deriving other units from them. Speed, for instance, is derived as a length divided by a time. Three systems account for nearly all everyday measurement today.

The International System of Units (SI), the modern form of the metric system, is the official system of almost every country and the language of science, medicine and engineering worldwide. Its units scale by powers of ten with prefixes such as kilo, centi and milli, so converting within the system only means moving a decimal point.

The United States customary system grew out of English units used in the colonies. It measures length in inches, feet, yards and miles, weight in ounces and pounds, and volume in fluid ounces, cups, pints, quarts and gallons. Since 1893 these units have been legally defined in terms of metric ones; an inch is exactly 2.54 centimeters.

The British imperial system was standardized by the Weights and Measures Act of 1824. It shares most length and weight units with the US system but differs for volume: an imperial gallon is about 4.546 liters while a US gallon is about 3.785 liters, so an imperial pint is roughly 20% larger than a US pint. The UK has largely gone metric but still uses miles on road signs and pints in pubs.

History of the Pound

The pound descends from the Roman libra, a unit of about 327 grams that gives the pound its abbreviation, lb. As the libra spread across the Roman Empire, local versions appeared in different regions and trades, and medieval Europe ended up with dozens of pounds of different weights.

England alone used several. The Tower pound, used at the Royal Mint, was the basis of the money pound sterling, which was originally a pound of silver cut into 240 pennies. The merchants' pound and the troy pound of 12 ounces, still used for precious metals, served other trades. The avoirdupois pound of 16 ounces, from the Old French for goods of weight, became the standard for general commerce in the fourteenth century and is the pound used today.

Physical standards defined the pound for centuries. After the Houses of Parliament fire of 1834 destroyed the official standards, Britain made a new platinum Imperial Standard Pound in 1855. The United States, meanwhile, adopted the metric kilogram as its legal reference in 1893 under the Mendenhall Order and defined the pound as a fixed fraction of it.

Because slightly different definitions were in use, the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed in 1959 on the international pound of exactly 0.45359237 kilograms. That definition still holds. Since the kilogram itself was redefined in 2019 using the Planck constant, the pound now ultimately rests on a fundamental constant of nature rather than on a metal cylinder.

Brief History of the Metric System

The metric system was created in France during the 1790s, when revolutionary reformers wanted to replace the hundreds of local units in use with a rational system based on nature and the number ten. The meter was defined as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris, and surveyors spent years measuring the arc between Dunkirk and Barcelona to fix it. The gram was tied to the mass of a cubic centimeter of water.

France made the system compulsory in 1840, and it spread through Europe and Latin America during the nineteenth century. In 1875, seventeen nations signed the Metre Convention, which set up the International Bureau of Weights and Measures (BIPM) near Paris. Platinum-iridium prototypes of the meter and kilogram, made in 1889, became the world's reference standards.

In 1960 the General Conference on Weights and Measures renamed the modernized system the International System of Units, abbreviated SI from its French name. Over time the definitions moved from objects to constants of nature: the meter has been defined by the speed of light since 1983, and in 2019 the kilogram, ampere, kelvin and mole were redefined using fixed values of the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant. Today SI has seven base units, shown below, from which every other unit is derived.

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