Finding the Enemy
This tiny structure, a Position-Finding (PF) Cell, performed a major role in the period 1891-1918. It was used to pinpoint the position of enemy ships and to relay that information to the coast defence guns at Langdon Battery, 1km to the east.
An observer at the window used a receiving instrument with telescope to locate a suspect ship. The instrument calculated the bearing to the ship. This cell received bearings transmitted electrically from an identical cell some distance away. The distance between the two cells was known, and so it was easy to fix the position of the ship by triangulation. The data was transferred electrically to dials next to the guns, enabling the gun crews to aim and fire at the ship.
By taking repeated readings, the ship's course and speed could be calculated, the gun aimed at a predicted position and fired, even when the gun crew couldn't see the ship.
( photo captions )
- This drawing, dated 1897, is a section through a PF Cell showing details of its fittings and construction.
- A receiving instrument from a PF cell. The telescope allowed tracking of the target while the electrical wire brought data from another PF Cell (the transmitting cell). The data was combined on a map under the telescope and the target position fixed.
- This PF Cell was later adapted for a depression range finder (DRF), which worked out the target position by trigonometry from just one PF Cell. A DRF (pictured left) was mounted on the surviving concrete pillar in the building.