Common Unit Systems
A unit system, or system of measurement, is a system comprised of interrelated units of measurement. Various unit systems have existed throughout history, and their importance remains evident today, as seen by their widespread use within society.
In the past, unit systems were defined locally, and often, highly arbitrarily. As such, the length of a unit could vary significantly from region to region. For example, since some units in the past were often based on parts of the body, the unit of the 'foot' could have a different definition based on the size of the foot of the king or feudal lord of a given region. Since travel was more limited in the past, local definition and use of units could be seen as being more practical. However, with the advent of globalization, particularly the growth of commerce and science, the arguable need for a universal system of measurement became more apparent.
Standardized units of measurement facilitate communication between different cultures and countries that may otherwise choose to use local systems, potentially leading to confusion and miscommunication. The International System of Units (SI), the most widely used system of measurement today, was developed in an effort to provide a standardized, more rational system that could be used worldwide. Despite efforts to implement SI globally, there are still a few unit systems in common use, including the United States customary units, and the imperial system of measurement.
Although striving for standardization is important, since it is difficult to entirely eliminate the local use of historical unit systems, it is equally important to accept that other systems of measurement exist and be able to use them, or at least relate and convert them to the preferred unit system.
The three common unit systems that are in use today are the International System of Units, United States customary units, and the imperial system of units.
International System of Units
The International System of Units (SI) is the modern form of the metric system and is comprised of seven base units that use twenty metric prefixes to denote decimal multiples or submultiples of the base unit. SI is intended as a coherent, rational system of measurement.
It is a system that was stringently conceived and is defined based on invariant constants of nature including the speed of light, the triple point of water, and a physical prototype. In order to maintain reliability and precision, SI requires the precise definition of constants in addition to precise of measurement standards.
SI is the most widely used system of measurement, and the evolution of the system is still ongoing today.
SI Base Units
| Unit Name | Symbol | Quantity |
|---|---|---|
| Meter | m | Length |
| Kilogram | kg | Mass |
| Second | s | Time |
| Ampere | A | Electric Current |
| Kelvin | K | Temperature |
| Mole | mol | Amount of Substance |
| Candela | cd | Luminous Intensity |
Metric Prefixes
| Prefix | Symbol | Factor |
|---|---|---|
| exa | E | 10¹⁸ |
| peta | P | 10¹⁵ |
| tera | T | 10¹² |
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| hecto | h | 10² |
| deca | da | 10¹ |
| (none) | (none) | 10⁰ |
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | μ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
| femto | f | 10⁻¹⁵ |
| atto | a | 10⁻¹⁸ |
History of SI
The International System of Units (SI) is the most widely used unit system in the world. It is the modern version of the metric system which was developed over time while borrowing units or ideas from other systems, in large part by a committee of the French Academy of Sciences, beginning in 1791.
The process of metrication, including the eventual definition and adoption of the International System of Units was a slow process. France officially introduced the metric system in 1799, and the system spread throughout Europe during the 19th century. By the 1970s, metrication in the form of SI was complete in almost all countries around the world. Notable exceptions to this include the United Kingdom, the United States, Liberia, and Myanmar.
United States Customary Units
United States customary units (UCS) are a system of measurements used in the United States (US). UCS originated from English units (not to be confused with imperial units) which were used by the British Empire starting as early as 1495 and continued to be used in the UK up until replacement by the imperial system in 1824.
While UCS is primarily used for commercial, social, and personal applications, the US uses the International System of Units (SI) in many other areas including science, medicine, industry, the government, and the military.
UCS uses the unit degrees Fahrenheit to measure temperatures for everyday purposes. Like most of the rest of the world however, UCS uses degrees Celsius and kelvins within scientific contexts.
Imperial System
The imperial system of measurements, also known as British Imperial, was defined in 1824, replacing the previous English units known as the Winchester Standards, which were in effect from 1588 to 1825.
Although the United Kingdom has technically adopted the International System of Units (SI), in practice, the UK is much like the United States in that it uses SI in many areas such as industry, healthcare, government, science, and others, but still commonly uses a system derived from English units (the imperial system) in everyday life.
The imperial system is also still in use in Canada as well as some other countries previously under the control of the British Empire.