^{-10}meters or one-ten-millionth of a millimeter, used for atomic measurements and the wavelengths of electromagnetic radiation.

A unit that measures the wavelength of light and equals 0.00000001 of a centimeter. Blue light has a wavelength of about 4400 angstroms, yellow light 5500 angstroms, and red light 6500 angstroms.

For simplicity, an AU is usually rounded off to 93,000,000 miles or 149,637,000 kilometers.

2. An astronomical unit is used to describe planetary distances.Light travels this distance in approximately 8.3 minutes.

Burning one cubic foot of natural gas releases about 1,000 Btu of heat.

The number of particles greater ghan 0.5 micrometers in diameter per cubic foot of air.

Normally, the measure is the weight of contaminants per cubic meter of air.

**S**ystème**I**nternationale d’Unités**International System of Units**.

The International System is called the **SI**, using the first two initials of its French name *Système International d'Unités*.

The primary agreement is the "Treaty of the Meter" or the *Convention du Mètre*, signed in Paris on May 20, 1875.

Forty-eight nations have signed the treaty, including all the major industrialized countries. The United States is a charter member of the metric club, having signed the original document back in 1875.

Each SI unit is represented by a symbol, not an abbreviation. The use of unit symbols is regulated by precise rules.

These symbols are the same in every language of the world; however, the names of the units themselves vary in spelling according to national procedures; therefore, it is correct for Americans to write **meter** and Germans to write **Meter**, and it is also correct for the British to write **metre**, Italians to write **metro**, and Poles to write **metr**.

There is no *official* spelling of the SI units; however, the SI does provide the **names**, the **definitions**, and the **symbols** of the units which must be followed even when the spellings are different as shown below.

### The fundamental SI unit of length has numerous spellings

- meter (American English, Danish, Dutch, German, Hungarian, Norwegian, Slovak, and Swedish)
- metr (Czech, Polish, Russian, Ukrainian)
- metras (Lithuanian)
- metre (British, Australian, Canadian and New Zealand English; French)
- metri (Finnish)
- metro (Basque, Italian, Portuguese, Spanish)

### A list of elements associated with the *International System of Units* (SI)

- ampere (unit of measurement)
- are (unit of area measurement)
- atomic second
- candela (cd) (SI unit of measurement)
- centimeter (cm) (unit of measurement)
- coulomb (unit of energy measurement)
- farad (unit of measurement)
- gram (gm or g) (measurement)
- hectare (unit of measurement)
- henry (unit of energy measurement)
- hertz (unit of measurement)
- joule (unit of energy measurement)
- kelvin (K) (unit of measurement)
- kilogram (kg) (unit of measurement)
- kilometer (km) (unit of measurement)
- liter (l) (unit of measurement)
- lumen (unit of energy measurement)
- lux (unit of energy measurement)
- megohm (electronics)
- meter (m) (measurement)
- metric ton (unit of weight)
- micrometer (unit of measurement)
- millimeter (mm) (unit of measurement)
- mole (chemistry)
- nanometer (unit of measurement)
- nanotesla (physics)
- newton (unit of measurement)
- ohm (unit of energy measurement)
- pascal (Pa ) (unit of energy measurement)
- second (unit of time)
- siemens (S) (unit of energy measurement)
- tesla (unit of energy measurement)
- unit (measurement)
- volt (unit of measurement)
- watt (unit of measurement)
- weber (unit of measurement)