Gyroscope. Toy gyroscope spinning on a piece of string. A gyroscope is a heavy spinning disc mounted so that its axis is free to adopt any orientation. Its special properties depend on the principle of conservation of angular momentum. The fact that it will stay upright as long as it is spinning fast enough demonstrates the property of gyroscopic inertia, that is that the direction of the spin axis resists change. Gyroscopes are used in stabilisation devices and compasses in boats and aircraft.