The Admiralty Fire Control Clock was a mechanical computer that helped with the accurate firing of shells in a naval battle. The calculations needed for accurate fire at sea are very complex- you have two targets, both moving and possibly at different speeds and directions. Added to that guns’ ballistic characteristics were affected by the wind, air pressure, the age of the barrels and the swell of the sea. With so many variable and with naval battles being fought at ever increasing distances, mechanical computers began to be introduced before the Great War to help reduce the burden of making incredibly complex mathematical calculations in very short time scales during battle. These devices relied on large amounts of data input and the data added could change if sub calibre practice ammunition was being used rather than full size ammunition. Today we are looking at a metal plate that would have been displayed in the transmitting station for the clock for use when sub-calibre ammunition was being used:

The upper half of the plate tells the operator which pieces of data need to be entered as normal, and which need to be changed to reflect the ammunition in use:

The mark number of the Admiralty Fire Control Clock indicates that this data plate comes from the 1920s and was from the No 5.0 pattern of AFCC, it would be updated later in the interwar period as technology improved.

The lower half of the plate shows the alternative range that should be set for the ballistic qualities of the sub calibre ammunition, here much longer ranges are substituted into the machine than would be the case with full size ammunition that would be set to the true range:

With this piece of technology going out of service eighty to ninety years ago, this plate is surely a rare survivor today.