gyrocompass

A form of gyroscope which is set to maintain a N-seeking orientation as an aid to navigation.

A gyrocompass is a compass which finds North by using an (electrically powered) fast spinning wheel and friction forces in order to exploit the rotation of the Earth. the direction of Earth's rotational axis, as opposed to magnetic north, they are not affected by metal in a ship's hull.

A gyrocompass is essentially a gyroscope, a spinning wheel mounted so that the wheel's axis is free to orient itself in any way. Suppose it is spun up with its axis pointing in some direction other than the North Star. Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is rotating once every 24 hours. The crucial additional ingredient needed for a gyrocompass is some mechanism that results in applied torque whenever the compass's axis is not pointing north. if for instance a device connected to the axis is immersed in a viscous fluid, then that fluid will resist reorientation of the axis. This friction force caused by the fluid results in a torque acting on the axis, causing the axis to turn in a direction orthogonal to the torque (that is, to precess) toward true North (to the North star). Once the axis points toward the North star, it will appear to be stationary and won't experience any more friction forces. This is because true North is the only direction for which the gyroscope can remain on the surface of the earth and not be required to change.

Another, more practical, method is to use weights to force the axis of the compass to remain horizontal with respect to the Earth's surface, but otherwise allow it to rotate freely within that plane. In this case, gravity will apply a torque forcing the compass's axis toward true north. Because the weights will confine the compass's axis to be horizontal with respect to the Earth's surface, the axis can never align with the Earth's axis (except on the Equator) and must realign itself as the Earth rotates. But with respect to the Earth's surface, the compass will appear to be stationary and pointing along the Earth's surface toward the true north pole. Sperry argued that Anschütz-Kaempfe's patent was invalid because it did not significantly improve on the earlier van den Bos patent. Albert Einstein testified in the case, first agreeing with Sperry but then reversing himself and finding that Anschütz-Kaempfe's patent was valid and that Sperry had infringed by using a specific damping method.