Gyrocompasses were first successfully developed at the beginning of the 20th century as a solution to the problems of magnetic variation and deviation that are inherent in magnetic compasses. They use the properties of spinning gyroscopes which keep the compass pointing in a fixed direction, usually Earth’s true north. Although they were much more expensive than magnetic compasses and were more difficult to maintain, navies throughout the world soon adopted them.

This is the master compass of the Admiralty gyrocompass AP1005, one of a series developed by the Sperry Gyroscope Company for the Royal Navy as standard gyrocompass systems. The master compass contains the gyroscopes that lie at the heart of the system. The top of the unit has a degree ring marked in degrees (0-360 by 1 degree), which is visible through a glass dome that can be lifted off. The direction information provided by the master unit was fed off to repeater units throughout the ship.