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time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, audible signals (even signal guns) have limited range. Busy ports used a visual signal, the dropping of a ball, to allow mariners to check the chronometers used for navigation. The advent of electrical telegraphs allowed widespread and precise distribution of time signals from central observatories. Railways were among the first customers for time signals, which allowed synchronization of their operations over wide geographic areas. Special purpose radio time signal stations transmit a signal that allows automatic synchronization of clocks, and commercial broadcasters still include time signals in their programming. Today, GPS navigation radio signals are used to precisely distribute time signals over much of the world. There are many commercially available radio controlled clocks available to accurately indicate the local time, both for business and residential use. Computers often set their time from an internet atomic clock source. Where this is not available, a locally connected GPS receiver can precisely set the time using a software application. There are several such applications available to download from the internet.
==Audible and visible time signals==
One sort of public time signal is a striking clock. These clocks are only as good as the clockwork that activates them, but they have improved substantially since the first clocks from the 14th century. Until modern times, a public clock such as Big Ben was the only time standard the general public needed.
When more accurate time signals were required in navigation, a number of traditional audible or visible time signals were established so navigators could check their marine chronometers. These public time signals were formerly established in many seaport cities.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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