History of the Metric System

In 1790, shortly after the conclusion of the French Revolution, a committee of French scientists initiated a study that by 1799, would become the standard system of measurement in France. That system was the metric system. Througout the next century it would become the standard system of measurement of the scientific community as well.

In 1875, nineteen countries signed a treaty in 1875 which bound them to the metric system. That treaty provided for an international bureau of standards located in Sevres, France. Meter bars and kilogram masses were cast from a platinum-iridium alloy and international prototype standards were chosen. The bars and masses that remained were distributed among treaty member nations to be used as their national prototypes standards.

President Harrison accepted, on behalf of the United States, its national prototype in 1890 and with the authority of Congress, our National Bureau of Standards adopted the metric system. It immediately became legal standard for defining our yard (length) and pound (mass). Today, however, the metric system has yet to replace these units.

Units of Length

The elemental standard metric unit of length is the meter [m]. It was originally intended to be 1/10,000,000 the distance from the equator to the North Pole. Once that length was marked off on the prototype bar however, a discrepancy was discovered. That flaw was tolerated until 1960 when the meter was re-defined as the length of exactly 1,650,763.73 wavelengths of the orange-red specral emission of krypton-86. At that time, this was a convenient measurement because that wavelength was reproducable in any laboratory and was not subject to change. In 1983, that definition was replaced. A meter is now defined as the length of the path of light travelled - in a vacuum - during 1/299,792,458 of a second.

Units of Mass

The elemental standard metric unit of mass - the kilogram [kg], was intended to be the mass of water, at maximum density (4°C) that would fill a cube with edges 10 centimeters (cm) in length. That volume would be the equivalent of 1000 cubic centimeters (cm3).

Units of Volume

While the kilogram prototype was being defined, the elemental metric unit of volume - the liter [L] was determined to be that volume occupied by 1 kilogram of water at its maximum density (4°C).

Units of Time

The elemental standard unit of time, the second [s] is the same in the English and metric measurement systems. While metric relationships for length, mass, and volume are decimal based, time units follow a sexagesimal system, that is, it is based on the number 60. The measurement of a uniform, periodic motion was the key to measuring time. Until 1956, the rotation of the Earth was used to officially determine time. This 24 hour period of time is called the solar day and the second was equivalent to 1/86,400 of a mean solar day. Because irregularities in Earth's rotation caused a variation of the the length of a solar day, ephemeris time became the official method for determining the time unit. It was based on the revolution of the earth around the sun and was accepted to be exactly 1/31,556,925.9747 of the solar year 1900.

Today the second is defined using an atomic standard of time-interval. It is based on the transition between two energy levels of a cesium 133 atom in its ground state at 0 K (absolute zero). Its duration is 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of that ground state.

Units of Temperature

Temperature is a measure of the average kinetic energy of molecules in any given system. Historically, temperature measurements were obtained using scales that had two reporoducible temperature points (i.e. freezing point, boiling point). Freezing point is the temperature of an ice-water mixture at one standard atmosphere of pressure (14.7 psi, 101.3 kPa) while boiling point is the temperature of the steam generated by the boiling water.

Fahrenheit Scale. This scale is based upon 180 equal divisions (degrees) between the freezing and boiling points. The designated freezing point of 32°F and boiling point of 212°F were selected by Gabriel Fahrenheit, an 18th century German physicist. Temperatures are recorded as °F.

Celsius Scale. This temperature scale is based upon 100 equal divisions (degrees) between the freezing and boiling points. It was also known as the centigrade scale. Its selected freezing and boiling points are 0°C and 100°C, respectively. They were chosen in 1742 by Anders Celsius, a Swedish scientist. The Celsius scale is used by scientists worldwide and temperatures are recorded as °C.

Kelvin Scale. Since we use average kinetic energy as the definition of temperature, our scale can obviously display a zero or the point at which kinetic energy is zero. That point is known as absolute zero and is how zero is defined using the Kelvin Scale. The units of the Kelvin temperatue are Kelvin K and not degrees Kelvin.

Elemental Units

As with any system of measurement, there needs to be some quantities that are considered to be basic or elemental. By agreement, six elemental units in the Système International (SI) were chosen: the meter (length), the kilogram (mass), the second (time), the ampere (current), the Kelvin degree (temperature), and the candela (luminance).

Metric Prefixes

Prefix Power Symbol Length Volume Mass
tera 1012 T terameter teraliter teragram
giga 109 G gigameter gigaliter gigagram
mega 106 M megameter megaliter megagram
kilo 103 k kilometer kiloliter kilogram
hecto 102 h hectometer hectoliter hectogram
deca 101 da decameter decaliter decagram
100 meter liter gram
deci 10-1 d decimeter deciliter decigram
centi 10-2 c centimeter centiliter centigram
milli 10-3 m millimeter milliliter milligram
micro 10-6 μ micrometer microliter microgram
nano 10-9 n nanometer nanoliter nanogram
pico 10-12 ρ picometer picoliter picogram

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