E1C01 09/14/2010
15:40:35 Page 25
The Bureau International de l’Heure (BIH) in Paris maintains the primary standard for clock
time. Periodically, adjustments to clocks around the world are made relative to the BIH clock so as to
keep time synchronous.
The standard for cyclical frequency is derived from the time standard. The standard unit is
the hertz (1 Hz ¼ 1 cycle/s). The cyclical frequency is related to the circular frequency
(radians/s) by
1 Hz ¼
2p rad
1 s
ð1:14Þ
Both time and frequency standard signals are broadcast worldwide over designated radio stations for
use in navigation and as a source of a standard for these dimensions.
Length
The meter is the standard unit for length. New primary standards are established when our
ability to determine the new standard becomes more accurate (i.e., lower uncertainty) than the
existing standard. In 1982, a new primary standard was adopted by the CGPM to define the
unit of a meter. One meter (m) is now defined exactly as the length traveled by light in
1/299,792,458 of a second, a number derived from the velocity of light in a vacuum (defined as
299,792,458 m/s).
The I-P system unit of the inch and the related unit of the foot are derived exactly from the
meter.
1 ft ¼ 0:3048 m
1 in: ¼ 0:0254 m
ð1:15Þ
Temperature
The kelvin, K, is the SI unit of thermodynamic temperature and is the fraction 1/273.16 of the
thermodynamic temperature of the triple point of water. This temperature scale was devised by
William Thomson, Lord Kelvin (1824–1907), and forms the basis for the absolute practical
temperature scale in common use. This scale is based on polynomial interpolation between
the equilibrium phase change points of a number of common pure substances from the triple
point of equilibrium hydrogen (13.81 K) to the freezing point of pure gold (1337.58 K). Above
1337.58 K, the scale is based on Planck’s law of radiant emissions. The details of the standard have
been modified over the years but are governed by the International Temperature Scale–1990 (11).
The I-P unit system uses the absolute scale of Rankine (
R). This and the common scales of
Celsius (
C), used in the metric system, and Fahrenheit (
F) are related to the Kelvin scale by the
following:
C
ð Þ ¼ K
ð Þ À 273:15
F
ð Þ ¼
R
ð Þ À 459:67
F
ð Þ ¼ 1:8 Â
C
ð Þ þ 32:0
ð1:16Þ
Current
The SI unit for current is the ampere. One ampere (A) is defined as that constant current which, if
maintained in two straight parallel conductors of infinite length and of negligible circular cross
1.5 Standards 25
15:40:35 Page 25
The Bureau International de l’Heure (BIH) in Paris maintains the primary standard for clock
time. Periodically, adjustments to clocks around the world are made relative to the BIH clock so as to
keep time synchronous.
The standard for cyclical frequency is derived from the time standard. The standard unit is
the hertz (1 Hz ¼ 1 cycle/s). The cyclical frequency is related to the circular frequency
(radians/s) by
1 Hz ¼
2p rad
1 s
ð1:14Þ
Both time and frequency standard signals are broadcast worldwide over designated radio stations for
use in navigation and as a source of a standard for these dimensions.
Length
The meter is the standard unit for length. New primary standards are established when our
ability to determine the new standard becomes more accurate (i.e., lower uncertainty) than the
existing standard. In 1982, a new primary standard was adopted by the CGPM to define the
unit of a meter. One meter (m) is now defined exactly as the length traveled by light in
1/299,792,458 of a second, a number derived from the velocity of light in a vacuum (defined as
299,792,458 m/s).
The I-P system unit of the inch and the related unit of the foot are derived exactly from the
meter.
1 ft ¼ 0:3048 m
1 in: ¼ 0:0254 m
ð1:15Þ
Temperature
The kelvin, K, is the SI unit of thermodynamic temperature and is the fraction 1/273.16 of the
thermodynamic temperature of the triple point of water. This temperature scale was devised by
William Thomson, Lord Kelvin (1824–1907), and forms the basis for the absolute practical
temperature scale in common use. This scale is based on polynomial interpolation between
the equilibrium phase change points of a number of common pure substances from the triple
point of equilibrium hydrogen (13.81 K) to the freezing point of pure gold (1337.58 K). Above
1337.58 K, the scale is based on Planck’s law of radiant emissions. The details of the standard have
been modified over the years but are governed by the International Temperature Scale–1990 (11).
The I-P unit system uses the absolute scale of Rankine (
R). This and the common scales of
Celsius (
C), used in the metric system, and Fahrenheit (
F) are related to the Kelvin scale by the
following:
C
ð Þ ¼ K
ð Þ À 273:15
F
ð Þ ¼
R
ð Þ À 459:67
F
ð Þ ¼ 1:8 Â
C
ð Þ þ 32:0
ð1:16Þ
Current
The SI unit for current is the ampere. One ampere (A) is defined as that constant current which, if
maintained in two straight parallel conductors of infinite length and of negligible circular cross
1.5 Standards 25