A gyrocompass is similar to a gyroscope. It is a compass that can find true north by using an electrically powered, fast-spinning gyroscope wheel and frictional or other forces in order to exploit basic physical laws and the rotation of the Earth. Gyrocompasses are widely used on ships. Marine gyrocompasses have two main advantages over magnetic compasses:
- they find true north, i.e., the point of the Earth’s rotational axis on the Earth’s surface, as opposed to magnetic north, –an extremely important aspect in navigation, and
- they are unaffected by external magnetic fields which deflect normal compasses, such as those created by ferrous metals in a ship’s hull.
A gyrocompass is essentially a gyroscope, a spinning wheel mounted on gimbals so that the wheel’s axis is free to orient itself in any way. When it is spun up to speed with its axis pointing in some direction other than the celestial pole, due to the law of conservation of angular momentum, such a wheel will normally maintain its original orientation to a fixed point in outer space (not to a fixed point on Earth). Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope’s axis is completing a full rotation once every 24 hours. Such a rotating gyroscope cannot ordinarily be used for marine navigation. The crucial additional ingredient needed for a gyrocompass to seek out true north is some mechanism that results in an 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 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) towards the north celestial pole (approximately toward the North Star). Once the axis points toward the celestial pole, it will appear to be stationary and won’t experience any more frictional 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 axis orientation 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’s automatic north-seeking function depends on the rotation of the Earth to deflect the compass via gyroscopic precession, it will not orient itself correctly to true north if the platform it’s mounted on is moving very fast in an east to west direction, thus negating the Earth’s rotation. However, many aviation models, called heading indicators or directional gyros, can be quickly aligned manually to north as is commonly done on aircraft flights.“
Born in Cortland, New York, in 1860, Elmer Ambrose Sperry was educated at the local State Normal Training School. By 1890, he had founded two companies. In that year, G.M. Hopkins invented the first electrically driven gyroscope. A gyroscope is a disk mounted on a base in such a way that the disk can spin freely on its X- and Y-axes; that is, the disk will remain in a fixed position in whatever directions the base is moved. Hopkins’ modification, as Sperry and others saw, made practical the possibility that the gyroscope, once a mere curiosity, could be turned into a reliable reference device in steel ships, where a standard magnetic compass was unreliable.
After years of work, Sperry produced a workable gyrocompass system (1908: patent #1,242,065), and founded the Sperry Gyroscope Company. The unit was adopted by the U.S. Navy (1911), and played a major role in World War I. The Navy also began using Sperry’s “Metal Mike”: the first gyroscope-guided autopilot steering system. In the following decades, these and other Sperry devices were adopted by steamships such as the RMS Queen Mary, airplanes, and the warships of World War II. In fact, after his death in 1930, the Navy named the USS Sperry after him.
Along the way, Sperry had invented and patented a wide range of devices, including electric trolley cars, high-intensity searchlights, dynamos, and railroad safety devices. After his death, Sperry’s company expanded into electronics –a move he would have appreciated. Still, Sperry himself will best be remembered as the father of modern navigation technology.
Before that success in the US, several attempts had been made in Europe. By 1880, William Thomson (lord Kelvin) tried to propose a gyrostat (tope) to the British Navy. In 1889, Arthur Krebs adapted an electric motor to the Dumoulin-Froment marine gyroscope, for the French Navy. Giving the Gymnote submarine the ability to keep a straight line under water during several hours, it allowed her to force a naval block in 1890.
The gyrocompass can be subject to certain errors. These include steaming error, where rapid changes in course, speed and latitude cause deviation before the gyro can adjust itself. On most modern ships the GPS or other navigational aids feed data to the gyrocompass allowing a small computer to apply a correction. Alternatively a design based on an orthogonal triad of fibre optic or ring laser gyroscopes will eliminate these errors, as they depend upon no mechanical parts, instead using the principles of optical path difference to determine rate of rotation.