See this slave clock connected to the Shepherd master clock yourself to the left of the gate at the Royal Observatory.
This slave clock was installed at the gates to the Observatory and was the first clock ever to show Greenwich Mean Time (GMT) directly to the public.
From 1852 to 1893, the Shepherd master clock was the heart of Britain's time system. Its time was sent by telegraph wires to London, Edinburgh, Glasgow, Dublin, Belfast and many other cities. By 1866, time signals were sent from the clock to Harvard University in Cambridge, Massachusetts via the new transatlantic submarine cable. In terms of the distribution of accurate time into everyday life, it is one of the most important clocks ever made.
"This clock keeps in motion a sympathetic galvanic clock in the Chronometer-room, which, therefore, is sensibly correct; and thus the chronometers are compared with a clock which requires no numerical correction. [...] The same Normal Clock maintains in sympathetic movement the large clock at the entrance-gate, two other clocks in the Observatory, and a clock at the London-bridge Terminus of the South-Eastern Railway. [...] It sends galvanic signals every day along all the principal railways diverging from London." George Airy, the seventh Astronomer Royal