Abstract :
The speaking clock, introduced recently by the General Post Office, provides an accurate time service which is continuously available to telephone subscribers in the London area. A subscriber connected to an automatic exchange on dialling the code T-I-M, or a subscriber connected to a manual exchange on asking for ¿Time,¿ is routed to the clock and hears the time announced every 10 seconds. Each announcement is followed by three audio-frequency ¿pips,¿ the last of which indicates within ± 0.l sec. the exact time spoken. The announcements are made by photo-electrically reproducing words or phrases, which are selected in the correct sequence from recordings made photographically on four glass discs. The mechanism for rotating the discs, for building up the announcement, and for changing from one announcementto another, is driven by a low-speed synchronous motor. The frequency of the a.c. supply to this motor is directly controlled by a seconds-beating free pendulum. Every hour the clock is checked automatically against a signal transmitted from Greenwich Observatory, and any small error is corrected. Should the error exceed the prescribed limits the service is transferred to a duplicate .standby clock. Facilities are provided to connect up to 100 simultaneous calls to the installation.
