The gyro instruments include the heading indicator, attitude indicator and turn coordinator (or turn-and-slip indicator). Each contains a gyro rotor driven by air or electricity and each makes use of the gyroscopic principles to display the attitude of the aircraft. The primary trait of a rotating gyro rotor is rigidity in space, or gyroscopic inertia. Newton's First Law states in part: "A body in motion tends to move in a constant speed and direction unless disturbed by some external force". The spinning rotor inside a gyro instrument maintains a constant attitude in space as long as no outside forces change its motion. This stability increases if the rotor has great mass and speed. Thus, the gyros in aircraft instruments are constructed of heavy materials and designed to spin rapidly (approximately 15,000 rpm for the attitude indicator and 10,000 rpm for the heading indicator). The heading indicator and attitude indicator use gyros as an unchanging reference in space. Once the gyros are spinning, they stay in constant positions with respect to the horizon or direction. The aircraft heading and attitude can then be compared to these stable references. For example, the rotor of the universally mounted gyro remains in the same position even if the surrounding gimbals, or circular frames, are moved. If the rotor axis represents the natural horizon or a direction such as magnetic north, it provides a stable reference for instrument flying. Another characteristic of gyros is procession, which is the tilting or turning of the gyro axis as a result of applied forces. When a deflective force is applied to the rim of a stationary gyro rotor, the rotor moves in the direction of the force. When the rotor is spinning, however, the same forces causes the rotor to move in a different direction, as though the force had been applied to a point 90° around the rim in the direction of rotation. This turning movement, or procession, places the rotor in a new plane of rotation, parallel to the applied force.