# Metric system

The system has changed somewhat since it was first developed; e.g., the definition of the meter has
changed, and the unit for mass is different. The meter was originally intended to be 1/10,000,000 of
the distance on the earth’s surface between the equator and either pole; however, because of
errors in the original survey for determining the meter and because of the impracticality of referring
to such a standard, the meter was later redefined in terms of the standard prepared and kept at Sèvres,
France, near Paris. Long defined as the distance between two scratches on a bar of platinum-iridium alloy,
the meter in 1960 was first redefined in terms of an atomic standard. In 1983 the meter was officially
redefined as the distance traveled by light in vacuum during 1/299,792,458 of a second.

The original unit of mass, the gram, was first defined as the mass of pure water at maximum density
that would fill a cube whose edges are each 0.01 m. The unit of mass is now the kilogram, defined
as the mass of a platinum-iridium cylinder kept at Sèvres. (A gram is now defined as a
mass 1/1,000 kg.) Other metric units can be defined in terms of the meter and the kilogram. For
example the are, the unit of area, is equal to the area of a square whose edges are each 10 m long.
The liter, the metric unit of volume, is equal to the volume of a cube whose edges are each 1/10 m long.

Fractions and multiples of the metric units are related to each other by powers of 10, allowing conversion
from one unit to a multiple of it simply by shifting a decimal point, and avoiding the lengthy arithmetical
operations required by the English units of measurement. Standard prefixes (found in the table entitled
Prefixes) have been accepted for designating multiples and fractions of the meter,
gram, are, and other units. Thus, 1,000 grams are a kilogram, 100 ares are a hectare, and 1/100 of a meter is a centimeter.

Several other systems of units based on the metric system have been in wide use. The cgs system
is based on the centimeter of length, the gram of mass, and the second of time. The mks system
is based on the meter of length, the kilogram of mass, and the second of time. Units in the mks system are
larger than the corresponding cgs units. Electric and magnetic units
have been defined for both of these systems; in fact, two different sets of electric units are defined in the cgs
system. The mks system serves as the basis for the International System of Units,
a comprehensive system of units for all physical quantities adopted in 1960 by the 11th General Conference on Weights and Measures.