English: Precision pendulum regulator clocks at the United States Naval Observatory in 1905 which generated the time signal sent out at noon every day to all parts of the US by telegraph lines, serving as the primary time standard used to set accurate clocks throughout the US. These two clocks were synchronized to a third precision pendulum clock, the "star clock", which was set by the time of passage of stars overhead. From the source:

One of the most important functions of the Naval Observatory is found in the daily distribution of the correct time to every portion of the United States. This is effected by means of telegraphic signals, which are sent out from Washington at noon daily, except Sundays. The original object of this time service was to furnish mariners in the seaboard cities with the means of regulating their chronometers; but, like many another governmental activity, its scope has gradually broadened until it has become of general usefulness. The electrical impulse which goes forth from the Observatory at noon each day, now sets or regulates automatically more than 70,000 clocks located in all parts of the United States, and also serves, in each of the larger cities of the country, to release a time-ball located on some lofty building of central location. The dropping of the time-ball--accompanied, at some points, with the simultaneous firing of a cannon--is the signal for the regulation by hand of hundreds of other clocks and watches in the vicinity.

The clocks which contain the mechanism for sending the time signal, cost upwards of $800 each. As a precaution against accident, two of these clocks are provided; but only one is used in sending the signal. At three and a-quarter minutes before noon, the signal clock is switched into the telegraph circuit which covers the entire country; and from that moment until the sending of the signal, all business is suspended throughout the 350,000 miles of telegraph lines over which it is to be flashed.