Pääkellona
Pääkellona is Finnish for main clock, a term used to describe the primary time source in a system of synchronized clocks. It functions as the reference by which all subordinate clocks are set and kept in synchrony. In traditional systems, the main clock drives secondary clocks via electrical impulses, rotating couplers, or synchronous motors, and may be installed in clock towers, schools, factories, or transit hubs. The main clock is typically driven by a highly accurate time standard, such as a quartz oscillator disciplined by a radio time signal, a GPS receiver, or a connection to an official time service. Subordinate clocks, often called slave clocks, receive timing signals and adjust their hands automatically, often at regular intervals or on demand. Modern implementations often replace wired impulse or 1:1 motor drives with networked time distribution using protocols like NTP, IRIG-B, or PTP, while still maintaining a central master clock as the authoritative source.
In practice, the term emphasizes centralized timekeeping within a building or system, and it remains relevant