The Ferranti oscillogyro is described from the point of view of the vacuum engineer, illustrating the tenet that small vacuum systems can provide the vacuum engineer with problems as severe as those presented by large ones. The oscillogyro is a mechanical instrument, about 40 mm diameter and 45 mm long, which, with suitable electronics, can detect rotation as slow as 0.01° h −1. The gyroscopic sensitive element (the ‘beam’) rotates at 10, 500 rpm and as it must not be subject to air damping, it is enclosed in an evacuated volume. The whole vacuum tank rotates with the beam. The vacuum required is only modest, but there can be no pumping after initial manufacture. The total vacuum volume is 3 ml, the design lifetime 5 years and hence the maximum permissible total outgassing and leakage is about 10 −13 mbar / s −1. The choice of materials is limited by the need for thermal expansion matching and high mechanical tolerances. Evacuation is through a tube conductance 0.04 / s −1.