In 1967, the specific frequency from the emission spectrum of caesium-133 was chosen to be used as the definition of the second by Le Système international d’unités (SI, The International System of Units). With the help of the advancement of atomic clock, this definition clearly stated that a 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”. However, long before the SI, the duration of second had been first demonstrated in clocks dated back to the early second half of the 16th century.
As the technology and craftsmanship of clock-making and watch-making further developed, time becomes to be more tangible to men. The following drawings show the assembly of a typical mechanical watch. This small simple mechanism has revolutionized human’s relationship with the universe. Ever since then knowing time is no more the privilege of the Roman Catholic Church, but all normal citizens began to have the tools to measure time and sense time lucidly. This small duration of a second, as one of the fundamental units of measurement, helped human to understand the material world in a more accurate and scientific manner.
Alf of Clock Mechanisms (1950)
3D Model of Simple Mechanical Watch Movement
Mainspring and Mainwheel
Escapement
Atomic Clock Assembly (1955, Britain's National Physical Laboratory)