The International System of Units is a modernized
version of the metric system, established by international
agreement, that provides a logical and interconnected
framework for all measurements in science, industry,
and commerce. The system is built on a foundation
of seven basic units, and all other units are derived
from them. (Use of metric weights and measures was
legalized in the United States in 1866, and our
customary units of weights and measures are defined
in terms of the meter and kilogram.)
Length.
Meter. Up until 1983, the meter was defined
as 1,650,763.73 wavelengths in a vacuum of the orange-red
line of the spectrum of krypton-86. Since then,
it is equal to the distance traveled by light in
a vacuum in 1/299,792,45 of a second.
Time.
Second. The second is defined as the duration
of 9,192,631,770 cycles of the radiation associated
with a specified transition of the cesium-133 atom.
Mass.
Kilogram. The standard for the kilogram is a
cylinder of platinum-iridium alloy kept by the International
Bureau of Weights and Measures at Paris. A duplicate
at the National Bureau of Standards serves as the
mass standard for the United States. The kilogram
is the only base unit still defined by a physical
object.
Temperature.
Kelvin. The Kelvin is defined as the fraction
1/273.16 of the thermodynamic temperature of the
triple point of water; that is, the point at which
water forms an interface of solid, liquid, and vapor.
This is defined as 0.01°C on the Centigrade
or Celsius scale and 32.02°F on the Fahrenheit
scale. The temperature 0°K is called “absolute
zero.”
Electric
Current. Ampere. The ampere is defined as that
current that, if maintained in each of two long
parallel wires separated by one meter in free space,
would produce a force between the two wires (due
to their magnetic fields) of 2 × 10-7 newton
for each meter of length. (A newton is the unit
of force that when applied to one kilogram mass
would experience an acceleration of one meter per
second per second.)
Luminous
Intensity. Candela. The candela is defined as
the luminous intensity of 1/600,000 of a square
meter of a cavity at the temperature of freezing
platinum (2,042°K).
Amount
of Substance. Mole. The mole is the amount of
substance of a system that contains as many elementary
entities as there are atoms in 0.012 kilogram of
carbon-12.
Source:
Department of Commerce, National Bureau of Standards.
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