B 1321
Issue 5, Oct 1970

Clock No. 6

This was one of the standard half minute pulse clocks used by the GPO.  The movements are described in B 1311.

General purpose wall mounted clock. Now superseded by Clocks No. 78A.

Drawing - P/T 26.

Overall dia. Dia. of dial Depth (back to front)
22 in 18 in 3 in


RAF Sector Clocks
Two modified versions of the Clock No. 6A were also produced for the RAF by the GPO and these were called the Clock, Coloured Dial (5 Minute) and Clock, Coloured dial (2½ Minute) and are shown on GPO Drawings AD 5076 and AD 5812.

These were fitted in Royal Air Force and Royal Observer Corps Operations Rooms during WW2 and continued in use for aircraft plotting until 1960.  They were originally called Colour Change Clocks but are generally known as Sector Clocks.

Taken from the ROC Heritage web site:-
The dial of a Sector Clock had a very specific design to aid the complex efforts of RAF controllers. Radar would pick up approaching hostile aircraft well out to sea but once over the land, radar was, in the early years, “blind”.  Aircraft were then tracked by numerous Royal Observer Corps Posts with information sent to ROC and RAF Operations Rooms.  This included the altitude, bearing, and strength of approaching hostile or indeed allied aircraft.

Incoming reports at RAF Operations Rooms would be shown by colour coded flags of either red, yellow or blue according to time of receipt.  ROC Operations Rooms used coloured arrows plotted on the Operations Table according to the position of the Minute Hand on the Coloured segment of the Sector Clock at a succession of 5 Minute intervals.  This was later reduced to 2.5 Minute (pictured below) intervals as aircraft speeds increased during the end of WW2 and start of the Cold War.  In addition, a metal plaque contained information on - aircraft type, height, number of aircraft, hostile or allied and designated track also was used to provide further information to the operations crew.  This method allowed RAF interceptor squadrons to remain on the ground until necessary, thus saving enormous amounts of time and fuel.  Fighter aircraft could then stay in the air for longer periods when required. Because of the speed of incoming enemy raiders, the RAF response had to be in minutes.

Without this very unique time coding system devised by the RAF, the British might not have been so successful in holding control of the skies over Britain in 1940 during the crucial Battle of Britain as well as later in the war.

In 1938 the RAF requested that the GPO install Master Clocks, 30 second clocks and Colour Change Clocks in the operations blocks of the following sector stations: Biggin Hill. Duxford, Hornchurch, Kenley, Northolt, North Weald and Tangmere. Clocks were also ordered for Bentley Priory in the same year.

2.5 minute Sector Clock


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Last revised: November 28, 2023