Gyrocompass
The gyro compass points to True North rather than magnetic north and is used in surveying and navigation. There operation is based on a fast spinning mass (the gyroscope) which will stay in one place. By using a north seeking device the instrument is made to point to true north. This can be a lengthy process which is normally aided by pointing the compass in the right direction ('slewing' the compass).
The compasses in my collection are from Wild Heerbrugg and were designed for use on a theodolite. They are relatively small and cannot hold there true North pointing for very long. They were designed to be used when setting up an instrument and obtaining a reference direction. They are used in combination with a magnetic (box) compass for roughly pointing the instrument; then with the gyrocompass the exact alignment can be made.
