Realization of the second with 3
Caesium atomic clocks at the PTB
in Braunschweig, Germany.
These three instruments define
UTC (PTB) as basis of civil time in
Germany. They are also part of a
worldwide network of atomic clocks
in some 50 institutions defining
UTC, Coordinated Universal Time.
Copper lining on the walls shields
the instruments against electro-
magnetic interference.

[s] ‘The SECOND is the duration
of 9 192 631 770 periods of the
radiation corresponding to the
transition between the two hyperfine
levels of the ground state of the
caesium 133 atom.’