This is a Ground Zero Indicator, or GZI. It is a circular metal box in which four pieces of light sensitive paper could be positioned, creating four shadowgraphs or rudimentary cameras. It was manufactured for the Royal Observer Corps (ROC) and was used from 1960 until their stand down in 1991.
This Ground Zero Indicator can be found in York Cold War Bunker, where it is displayed alongside other pieces of equipment designed to be used in a nuclear attack. It is a very simple piece of equipment, yet the information it provided would have been vital for the country’s survival. It was used by members of the Royal Observer Corps, an organisation which trained volunteers to collect and communicate data in the event of a nuclear attack. The ROC built over 1500 small underground ‘monitoring posts’, and each of these had its own GZI, indicating how seriously they took the threat of nuclear war.
If there had been a nuclear attack on Britain during the Cold War, GZIs would have been used to record the height and position of bomb bursts. The white case has four pin pricks positioned in line with the four points of a compass. If there were a nuclear explosion, the bright light would enter the pinhole and immediately burn a mark onto the light sensitive paper held inside. This paper also contains a graph, allowing you to work out the height of the bomb blast and the degree of bearing from the monitoring post. This information would be passed to sector controls like York Cold War Bunker, where a Triangulation Officer would add it to a map containing bearings from other GZIs in the region. This would create a triangle known as the ‘triple cut’; the bomb had to be somewhere inside that area. By measuring the size of the black mark on the page they could also estimate the distance of the bomb from the post.
Observers would have performed this task in dangerous conditions, without protective clothing and at all times of day or night. For this reason, they needed to be able to work fast, and would train blindfolded to ensure they could do it in the dark! As the papers used in the GZI were light sensitive they had to be replaced at least every twelve hours because they would turn black, and used papers had to be protected from sunlight by storing them in a little black pouch. Thankfully the Royal Observer Corps’ Ground Zero Indicators never had to be used in Britain, but this one’s survival is a reminder of how close we came to nuclear war.