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. 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; the axis always points to the same star. Such a rotating gyroscope cannot be used for navigation. The crucial additional ingredient needed for a gyrocompass is friction
: the gyroscope 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 force. Over time, this has the effect that the axis will point to true North (to the North star), at which point the axis 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.
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 East-West direction.
The gyrocompass was patented in 1885 by the Dutch Martinus Gerardus van den Bos; however, his device never worked properly. In 1903, the German Herman Anschütz-Kaempfe constructed a working gyrocompass and obtained a patent on the design. In 1908, the American inventor Elmer Ambrose Sperry patented a gyrocompass in the US. When he 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 dampening method. Anschütz-Kaempfe won the case in 1915.