Page 95 - 4498

independently of any other base units. The metre is defined in terms of the
speed of light, so depends upon the definition of the second. The
definitions of the other base units are more complicated.

Unit Unit Quantity Definition Dime
name symbol name nsion

symbo
l

metre m length The path travelled by light in L
vacuum during a time interval
of 1/299792458 seconds. This
fixes the speed of light to

exactly 299792458 m/s.
kilogram kg mass The mass of the International M
Prototype Kilogram, a cylinder
of platinum-iridium alloy kept

at the International Bureau of
Weights and Measures near
Paris

One second equals T
9192631770 periods of the
radiation due to the transition
second s time between the two hyperfine

levels of the ground state of
Cesium 133.

The constant current which, if I

maintained in two straight
parallel conductors of infinite
length, of negligible circular

electric cross-section, and placed 1 m
ampere A
current apart in vacuum, would
produce between these

conductors a force equal to
−7
2×10 newtons per metre of
length
thermodyn  The fraction 1/273.16 of Ө
kelvin K
amic the thermodynamic

95