System of units of measurement facts for kids
A system of units is like a special set of rules and tools for measuring things. It includes different units of measurement (like meters or pounds) and explains how they are connected. These systems are super important for science and trade so everyone can understand measurements in the same way.
Some well-known systems are the International System of Units (which is the modern metric system), the British imperial system, and the United States customary system.
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History of Measurement Systems
Long ago, people made up their own ways to measure things. For example, a unit of length might be based on the length of a king's thumb or foot. Or, a weight might be based on how much water fit into a certain barrel.
The main idea was that each measurement had a standard, even if it was local. Over time, units like "cubits" (the length from elbow to fingertip) became more common. These were called "customary units."
As people started trading more and science grew, everyone realized it would be better to have a system that was the same everywhere. Changing a measurement system is hard and can cost money, so people often didn't want to change. But scientists, engineers, and businesses pushed for a more logical and worldwide system. This led most of the world to start using the metric system.
The French Revolution in the late 1700s helped create the metric system. It then spread across the globe, replacing many older units. In most systems, length (distance), mass (how much stuff is in something), and time are the basic measurements.
Later, scientists added things like electric charge or electric current as basic measurements. Other measurements, like power or speed, are made from these basic ones. For example, speed is how much distance you cover in a certain amount of time.
Historically, many different units were used for the same type of measurement. Length could be measured in inches, feet, yards, miles, and more. The ways to change between these units were often complicated.
In the metric system, units are connected in a simpler way. They use powers of ten. For example, 1 metre is 1,000 millimetres, and 0.001 kilometres. This makes calculations much easier.
How We Measure Today
Most countries today use the metric system. They have either fully switched or are almost done switching. This process is called Metrication.
However, US customary units are still widely used in the United States. They are also used a bit in Liberia and Burma. Canada uses some U.S. units because of a lot of trade with the U.S. The United Kingdom also still uses some imperial units, like miles for road signs.
In the U.S., science and the military mostly use metric units. But in homes, U.S. customary units are common. For example, drinks are often sold in litres, and medicines are measured in milligrams.
Some non-SI units are still used worldwide. For example, nautical miles and knots are used for ships and planes. Feet are also used for aircraft altitude.
The Metric System
The Metric system has changed and improved since it first started in France in 1795. It spread first to countries that didn't speak English, and then to English-speaking countries.
A cool thing about metric units is that their larger or smaller versions are related by powers of ten. This means they work perfectly with our decimal number system. This makes the metric system very easy to use.
The first metric system had two main units: the metre for length and the gram for mass. All other units, like those for area, volume, or density, were made from these two.
For a short time, France used a system called Mesures usuelles. This was a mix of the metric system and older traditional measurements.
Many versions of the metric system have been used. These include systems for science (like the cgs system) and engineering.
Today, the main international standard for the metric system is the International System of Units (often called SI). This system has seven basic units. All other units can be made from these seven. The SI basic units are the metre, kilogram, second, ampere, kelvin, mole, and candela.
Imperial and US Customary Units
Both imperial units and US customary units came from older English units. Imperial units were used a lot in the old British Empire. But now, most of those countries have switched to the metric system. The UK still uses some imperial units, but mostly for everyday things, not for science or business.
US customary units are still the main system in the United States. Even though there have been efforts to switch to metric, the older units are still very common.
While these two systems are similar, they have some differences. Units for length and area (like the inch, foot, yard, and mile) are the same. But units for mass and weight are different for amounts larger than a pound. For example, the imperial system uses a "stone" (14 pounds), which the U.S. does not.
The biggest differences are in units of volume. A U.S. fluid ounce is slightly larger than an imperial fluid ounce. However, because there are 16 U.S. fluid ounces in a U.S. pint but 20 imperial fluid ounces in an imperial pint, the imperial pint is actually bigger. This difference also applies to quarts and gallons.
Besides the common avoirdupois system for mass and weight, there were also troy and apothecaries' systems. Troy weight was used for precious metals. The troy ounce is still used for gold and silver today. The apothecaries' system was for pharmacology (medicine), but it has now been replaced by the metric system.
Natural Units
Natural units are special units of measurement that are based on universal rules of nature. They are not made up by humans. Instead, they use basic physical constants (like the speed of light) as their foundation. When you use these units, those constants become the number one.
Different systems of natural units exist, depending on which natural constants are chosen. For example:
- Planck units are based on things like the speed of light and gravity. They are used in very advanced physics.
- Atomic units are used in atomic physics. They are based on properties of electrons, like their mass and charge.
Non-standard Units
Sometimes, people use unusual measurement units in books or news. These are not official but help describe things in a relatable way.
Area
- The American football field: This is often used in the U.S. to describe the size of large buildings or parks. It can mean the length (100 yards) or the area (about 5,350 square meters).
- The football pitch (soccer field): British news often uses this. Soccer fields can vary in size, but a UEFA Champions League field is exactly 105 by 68 meters.
Energy
- A ton of TNT equivalent: This is used to describe the power of huge events like explosions, volcanic eruptions, or asteroid impacts. A gram of TNT is defined as 1,000 thermochemical calories.
- The atom bomb dropped on Hiroshima: Its energy is sometimes used in media to describe very powerful events.
- One stick of dynamite.
Units of Currency
A unit of measurement for money is called a unit of account. This is usually the currency issued by a country, like the U.S. dollar or the euro.
The ISO 4217 is an international standard. It uses three-letter codes (like USD for U.S. Dollar) to identify different currencies around the world.
Historical Measurement Systems
Many official measurement systems have been used throughout history. While most are no longer official, some of these older customary systems are still used sometimes in daily life, like in cooking.
Africa
- Algerian
- Egyptian
- Ethiopian
- Eritrean
- Guinean
- Libyan
- Malagasy
- Mauritian
- Moroccan
- Seychellois
- Somalian
- Tunisian
- South African
- Tanzanian
Asia
- Arabic
- Afgan
- Cambodian
- Chinese
- Hong Kong
- Hebrew (Biblical and Talmudic)
- Hindu
- Indonesian
- Japanese
- Korean
- Omani
- Pakistani
- Philippine
- Mesopotamian
- Persian
- Singaporean
- Sri Lankan
- Syrian
- Taiwanese
- Tamil
- Thai
- Vietnamese
- Nepalese
- Still in use: Myanmar
Europe
- Ancient Greek
- Belgian
- Byzantine
- Czech
- Cypriot
- Danish
- Dutch
- English
- Estonian
- Finnish
- French (now)
- French (to 1795)
- German
- Greek
- Hungary
- Icelandic
- Irish
- Italian
- Latvian
- Luxembourgian
- Maltese
- Norwegian
- Polish
- Portuguese
- Roman
- Romanian
- Russian
- Scottish
- Serbian
- Slovak
- Spanish
- Swedish
- Switzerland
- Turkish
- Tatar
- Welsh
North America
- Costa Rican
- Cuban
- Haitian
- Honduran
- Mexico
- Nicaraguan
- Puerto Rican
South America
- Argentine
- Bolivian
- Brazilian
- Chilean
- Colombian
- Paraguayan
- Peruvian
- Uruguayan
- Venezuelan
Ancient Systems
- Arabic
- Biblical and Talmudic
- Egyptian
- Greek
- Hindu
- Indian
- Mesopotamian
- Persian
- Roman
See also
- Conversion of units
- History of the metric system
- ISO 31
- Level of measurement
- Medieval weights and measures
- Megalithic yard
- Petrograd Standard
- Pseudoscientific metrology
- Unified Code for Units of Measure
- Weights and measures
- In Spanish: Sistema de unidades para niños
