A gyroscope is made up of a rotor spinning about an axis. Any attempt to change the spinning axis requires work as it is an attempt to change the angular momentum. As long as the system keeps spinning the orientation of the axis of rotation will remain constant unless an outside force or couple is applied. The rotor is often kept spinning by driving it with an electric motor. Mounting the spinning rotor in a frame which isolates it from any external couple results in a system capable of providing a constant reference direction which is used for measuring the orientation or maintaining the stability of airplanes, spacecraft, and submarines – vehicles in general. Various gyroscopes are used as sensors in inertial guidance systems.