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A sector clock or colour change clock was a round colour-coded clock used at military airfields and observation posts in the United Kingdom to help track the movements of enemy aircraft and coordinate and control air defences.
During the Second World War sector clocks played a significant role in the Battle of Britain as part of the Dowding system of air defence and continued to be used by the Royal Air Force (RAF) and Royal Observer Corps (ROC) as simple clocks and keepsakes until the end of the Cold War. Two versions of the sector clock were supplied for issue by the Air Ministry; the RAF sector clock having a "Tudor Crown" RAF Warrant Officer's insignia under the "24", the ROC issue clock having no associated insignia. Clocks were either electrical or mechanical; electric clocks usually having the coloured segments pointing inwards, mechanical clocks having the segments pointing outwards.
The United States Army Air Force (USAAF) adapted the RAF sector clock using a coloured block pattern in place of triangles.
Sector clocks are sought after by collectors of militaria/aeronautica. (The clock face design also appears on wrist watches commemorating the wartime role of the sector clock).
Aircraft position was recorded along with the colour of the triangle beneath the minute hand at the time of sighting. This was reported to sector headquarters, where counters of the relayed colour were used to represent each air raid on a large table with a map of the UK overlaid with a British Modified Grid. As the plots of the raiding aircraft moved, the counters were pushed across the map by magnetic "rakes".
This system enabled "Fighter Controllers" to see very quickly where each formation was heading and allowed an estimate to be made of possible targets. The age of the information was readily apparent from the colour of the counter. Because of the simplicity of the system, decisions could be made quickly and easily.
File:Sector clock.JPG|RAF issue Sector clock. Coloured triangles span 5 minute time periods, (Outward pointing triangles indicate the model shown to have a mechanical mechanism). File:RAF Digby Ops Room.jpg|RAF Digby Operations Room with a Sector clock displaying inward pointing triangles, typical of electrical models. (This clock appears to show a key aperture below the dial and may be mechanical). File:The Royal Observer Corps, 1939-1945. CH11336.jpg|ROC issue electrical and mechanical Sector clocks in the No. 19 Group (Bromley) Operations Room, (Mechanical models were often paired with electrical types as a backup in the event of power outages). File:100 years of the RAF MOD 45163760.jpg|Electrical Sector clock above the plotting table at headquarters Fighter Command, RAF Bentley Priory. File:Royal Observer Corps necktie.jpg|An RAF Sector clock themed wrist watch, (ROC tie with matching NATO watch strap).
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