Gyrocompass

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Cutaway of Anschütz gyrocompass

The following description refers to the gyrocompasses used on ships. Aircraft also use gyrocompasses, but use different, faster means to counter the precession of the compass. One of those means is to use a magnetic compass to continually adjust the gyrocompass. Also, some gyrocompasses are driven by compressed air instead of electricity. An astrocompass can also be used to ascertain true North.

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. Gyrocompasses are widely used on ships. They have two main advantages over magnetic compasses:

they find true North, i.e. 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. Because of the law of conservation of angular momentum, such a wheel will maintain its original orientation. Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is rotating once every 24 hours. Such a rotating gyroscope cannot be used for navigation. 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.

One method uses friction to apply the needed torque: the gyroscope in a gyrocompass is in fact not completely free to reorient itself; 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. This is considered to be a point of minimum potential energy.

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.

Since the operation of a gyrocompass crucially depends on its rotation on Earth, it won't function correctly if the vessel it is mounted on is fast moving, especially in an East to West direction.

The gyrocompass was patented in 1885 by the Dutch Martinus Gerardus van den Bos; however, his device never worked properly. In 1889, Captain Arthur Krebs designed an electric pendular gyroscope for the experimental French submarine Gymnote. It allowed the Gymnote to force a naval blockade in 1890. In 1903, the German Herman Anschütz-Kaempfe constructed a working gyrocompass and obtained a patent on the design. In 1908, Anschütz-Kaempfe and the American inventor Elmer Ambrose Sperry patented the gyrocompass in Britain and the US. When Sperry attempted to sell this device to the German navy in 1914, Anschütz-Kaempfe sued for patent infringement. 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. Anschütz-Kaempfe won the case in 1915.