Different Systems of Units
Historically, many different systems of units have been used, where a system of units is defined as a collection of units of measurement with rules that relate them to each other. A unit of measurement is a defined magnitude of a quantity that it used as a standard for measurement for the same kind of quantity, such as measurements of length, weight, and volume.
In the past, many systems of measurement were defined on a local level, and could be based on factors as arbitrary as the length of a king's thumb. While this may work on a local level, when considering trade and science, having systems of units based on factors that others may not be able to relate to makes interaction difficult. As such, the development of more universal and consistent systems developed over time. Today, some of the systems of units in use include the metric system, the imperial system, and the United States customary units.
The International System of Units (SI) is the standard metric system that is currently used, and consists of seven SI base units of length, mass, time, temperature, electric current, luminous intensity, and amount of substance. Although SI is used almost universally in science (including in the US), some countries such as the United States still use their own system of units. This is partly due to the substantial financial and cultural costs involved in changing a measurement system compared to the potential benefit of using a standardized system.
History of the Pound
In the eighth and ninth centuries of the Common Era (CE), Arab civilization flourished in the Middle East and Spain. The Arabs used coins as a measurement of units of weight since a minted coin could not easily be cut or shaved to reduce its weight, and thus provided a measurable standard. They used a coin called a silver dirhem as a basic measure of weight. Ten dirhems comprised a Wukryeh, which was translated into Latin as an "uncia" – the origin of the word "ounce."
Over time, trade spread from the Mediterranean area to Europe. As a result, a pound, 16 ounces of silver, or 7200 grains, became a commonly used measure in many regions. While England also adopted this measure, a shortage of silver caused King Offa to reduce the measurement of the pound to 5400 grains in order to use smaller coins. Eventually, when William the Conqueror became King of England, he retained the 5400-grain pound for minting coins, but reverted to the 7200-grain pound for other purposes.
The avoirdupois weight system was adopted during the reign of Queen Elizabeth in the 16th century. It was a system based on the weight of coal, and its name was derived from the French phrase "avoir de pois" (goods of weight or property). Since 1959, the avoirdupois pound has been officially defined in most English-speaking countries as exactly 0.45359237 kilograms.
Brief History of the Metric System
In 1668, John Wilkins proposed a decimal system in which length, area, volume, and mass were linked to each other based on a pendulum that had a beat of one second as a base unit of length. In 1670, Gabriel Mouton proposed a decimal system that was instead based on the circumference of the earth, an idea supported by other prominent scientists but that did not take hold for approximately another 100 years.
By the mid-eighteenth century, it was clear to nations who traded and exchanged scientific ideas that standardization of weights and measures was necessary. In 1790, Thomas Jefferson presented the "Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States," which advocated for a decimal system in which units were related to each other by powers of ten. A committee formed in France came to a similar conclusion. Although Congress considered Jefferson's report, it was not adopted in the US.
In 1795, the metric system was formally defined in French law. The spread of the metric system did not occur quickly, but areas that were annexed by France during Napoleon's reign were the first to adopt it. By 1875, two-thirds of the European population, and nearly half the world's population had adopted the metric system.
The International System of Units, currently the most widely used system of measurement, was published in 1960. It has been adopted by all developed countries except for the United States, though it is used heavily in science and the military within the US.