The Shepherd Gate Clock is mounted on the wall outside the gate of the Royal Greenwich Observatory building in Greenwich, Greater London. The clock, an early example of an electric clock, was a slave mechanism controlled by electric pulses transmitted by a master clock inside the main building. The network of master and slave clocks was constructed and installed by Charles Shepherd in 1852. The clock by the gate was probably the first to display Greenwich Mean Time to the public, and is unusual in using the 24-hour analog dial.
The original idea for the clock network came from the Astronomer Royal, George Airy. With the arrival of the railway network, a single time standard was needed to replace the various incompatible local times then in use across the country. Airy proposed that this standard time would be provided by the Royal Observatory. His idea was to use what he called "galvanism" or electric signalling to transmit time pulses from Greenwich to slave clocks throughout the country, and perhaps to Europe and the colonies too. The new undersea cable recently installed between Dover to Calais in 1851 raised the possibility of sending time signals between England and France - this would allow longitude differences to be measured very accurately, for the first time.
In 1849 Charles Shepherd Junior (1830-1905),an engineer and son of a clockmaker, patented a system for controlling a network of master and slave clocks using electricity (or galvanism, as it was called). Shepherd installed the public clocks for the Great Exhibition which opened in May 1851. In October 1851, Airy wrote to Charles Shepherd asking for proposals and estimates, including a request for the following clocks:
One automatic clock. One clock with large dial to be seen by the Public, near the Observatory entrance, and three smaller clocks, all to be moved sympathetically with the automatic clock.
Airy also wanted the existing Greenwich time ball to be electrically operated, so that its descent at 13:00 was synchronised with the master clock inside the observatory.
By August 1852 Shepherd had built and installed the network of clocks and cables in the observatory. Costs were considerably higher than the original estimates. Shepherd had estimated £40 for the master clock and time ball apparatus, and £9 for each sympathetic clock. The total costs included £70 for the master clock, and £75 for the wall clock by the gate.
Shepherd was later appointed to oversee the construction of a telegraph network for the Indian Government in 1853.
The master clock, at first called the Normal Clock or Master Clock, but later known as the Mean Solar Standard Clock, sent pulses every second to sympathetic or slave clocks in the Chronometer Room, the Dwelling House (Flamsteed House), and at the gate (the Gate Clock). A pulse was sent to the time ball at 13:00. The signals were also transmitted along cables from Greenwich to London Bridge. From London Bridge a time signal was distributed at less frequent intervals by telegraph wires to clocks and receivers in London, Edinburgh, Glasgow, Dublin, Belfast and many other cities. From 1866 time signals were also sent to US Harvard University via the new transatlantic submarine cable.
Airy's report to the Observatory's Board of Visitors in 1853 explained the function of the Shepherd master clock:
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. It drops the Greenwich Ball and the Ball on the Offices of the Eastern Telegraph Company in the Strand.
All these various effects are produced without sensible error of time; and I cannot but feel a satisfaction in thinking that the Royal Observatory is thus quietly contributing to the punctuality of business through a large portion of this busy country.
The Gate Clock originally indicated astronomical time, in which the counting of the 24 hours of each day starts at noon. The clock was changed in the 20th century to indicate Greenwich Mean Time, in which the counting of the 24 hours of each day starts at midnight. Currently, the Gate Clock continues to show Greenwich Mean Time, and it does not show summer time. The clock is now controlled by a quartz mechanism inside the main building. The master clocks are still on display, but are not functional.
The Timeball Museum in Deal contains another slave clock once connected to the Greenwich master clock.