Basic unit of time (abbr. s or sec), corresponding to one 86,000th part of the mean solar day, i.e. the duration of rotation, about its axis, of an ideal Earth describing a circle round the sun in one year, at a constant speed and in the plane of the equator. After the Second World War, atomic clocks became so accurate that they could demonstrate the infinitesimal irregularities (a few hundreths of a second per year) of the Earth's rotation on its own axis. It was then decided to redefine the reference standard; this was done by the 13th General Conference on Weights and Measures in 1967, in the following terms: "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 fundamental state of the atom of caesium 133". Conventionally, the second is subdivised into tenth, hundreths, thousendths (milliseconds), millionths (microseconds), thousand-millionths (nanoseconds), and billionths (picoseconds).
Process of bringing the hands of a watch or clock to the position corresponding to the exact time.
Resilient bearing which, in a watch, is intended to take up the shocks received by the balance staff and thus protects its delicate pivots from damage.
Skeleton watch: watch in which the case and various parts of the movement are of transparent material, enabling the main parts of the watch to be seen.
Timekeeping instrument which can be used for measuring intervals of time. When this is done, the time display is partly or wholly lost until the hands are reset.
In a watch or clock, automatic or hand-operated mechanism that strikes the hours, etc, or rings and alarm bell (v. repeater).
