Caesium standard

The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK[1] and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

By definition, radiation produced by the transition between the two hyperfine ground states of caesium-133 (in the absence of external influences such as the Earth's magnetic field) has a frequency, ΔνCs, of exactly 9192631770 Hz.

That value was chosen so that the caesium second equaled, to the limit of measuring ability in 1960 when it was adopted, the existing standard ephemeris second based on the Earth's orbit around the Sun.

And of these, all but the mole, the coulomb, and the dimensionless radian and steradian are implicitly defined by the general properties of electromagnetic radiation.

The official definition of the second was first given by the BIPM at the 13th General Conference on Weights and Measures in 1967 as: "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

From the definition of the second it follows that the radiation in question has a frequency of exactly 9.19263177 GHz, corresponding to a wavelength of about 3.26 cm and therefore belonging to the microwave range.

Before 1967 the SI units of time and frequency were defined using the tropical year and before 1960 by the length of the mean solar day[5] In 1983, the meter was, indirectly, defined in terms of the caesium standard with the formal definition "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

[9] The 2019 revision replaced these with an assigned value for the Boltzmann constant, k, of 1.380649×10−23 J/K, implying: The mole is an extremely large number of "elementary entities" (i.e. atoms, molecules, ions, etc).

[10] The 2019 revision simplified this by assigning the Avogadro constant the exact value 6.02214076×1023 elementary entities per mole, thus, uniquely among the base units, the mole maintained its independence from the caesium standard: Prior to the revision, the ampere was defined as the current needed to produce a force between 2 parallel wires 1 m apart of 0.2 μN per meter.

[11] In any case, this convention entailed the following exact relationships between the SI electromagnetic units, elementary charge, and the caesium-133 hyperfine transition radiation: From 1967 to 1979 the SI optical units, lumen, lux, and candela are defined using the Incandescent glow of platinum at its melting point.