Precession is the motion of the axis of a spinning body that results when there is an external force acting on that axis. The phenomenon finds its roots in the first law of Newtonian Physics, which states that a body in motion continues to move at a constant speed along a straight line unless it is acted upon by an outside, unbalanced force.
Precession was a concept basic to the invention of the gyrostabilizer, which Sperry initially created when he began his attempt to apply the gyro to useful purposes. He used this device to stabilize first automobiles, and later ships. He pointed out that rigidly mounted wheels did not stabilize automobiles, nor did rigidly mounted turbines succeed in reducing the roll of ships. Sperry explained that their rigid mountings prevented the wheels and turbines from moving in response to outside forces, and thus absorbing the disturbing energy of those outside forces. In short, the rigid mountings prevented the revolving masses from precessing. In explaining his devices, Sperry often had to define this concept of "precession." One of his clearest definitions went as follows:
"If I impress a force on one end of the axis of a gyroscope it will resist this impressed force but will turn in a direction at right angles to the force impressed. This motion at right angles to the impressed forces is called "precession." It will be observed that the gyroscope does not resist any forces impressed by linear motion; nothing but angular motion causes precession, and nothing but the forces impressed by angular motion are resisted. This angular motion may be about a point within the gyro or a point at any distance from the gyro, but must be angular motion."