Magnetic compass

The magnetic compass has been used for navigation for hundreds of years. At one time, it was the only reliable means of direction-finding on days when the sun and stars were not visible. Nowadays, sophisticated equipment is available that enables users to determine bearings accurately and to pinpoint locations to within a few metres. However, such equipment has not made the compass obsolete. It is still a very practical tool for navigation for many small craft and for people on foot. Even airplanes and ships equipped with more sophisticated equipment often carry compasses as backups.
Compasses come in a variety of shapes and sizes depending on their intended use. The type of compass used on a ship or aircraft is a complex electronic or mechanical device capable of compensating for both the motion of the craft and its metallic structure. At the other extreme are small pocket compasses of low precision intended for casual use.
Regardless of their intended purpose or the complexity of their construction, most mechanical compasses operate on the same basic principle. A small, elongated, permanently magnetized needle is placed on a pivot so that it may rotate freely in the horizontal plane. The Earth's magnetic field exerts a force on the compass needle, causing it to rotate until it comes to rest in the same horizontal direction as the magnetic field. Over much of the Earth, this direction is roughly true north, which accounts for the compass's importance for navigation.

Area of compass unreliability
The horizontal force of the magnetic field, responsible for the direction in which a compass needle is oriented, decreases in strength as one approaches the North Magnetic Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field. The compass starts to behave erratically when the horizontal force falls below 6000 nT, and eventually, as the horizontal force decreases to less than 3000 nT, the compass becomes unusable.
These definitions are only guidelines, and experience has shown that with care, high quality mechanical compasses can be used, even in areas where the horizontal force is less than 3000 nT. Electronic compasses will operate regardless of the horizontal field strength. However, both mechanical and electronic compasses are subject to another problem within the area of compass unreliability - daily fluctuations in magnetic declination become increasingly large as one approaches the North Magnetic Pole. Where the horizontal force is less than 3000 nT, daily fluctuations in excess of 5 degrees are normal. Therefore, even if you compass is functioning correctly it may not be pointing where you think it is.

Finding your way by compass
Navigating by compass requires determining bearings with respect to true or grid north (in the following we will assumegrid north) from a map sheet and converting them to magnetic bearings for use with a compass. One way of doing this is given in the following steps. 1. Place the compass on the map with the direction-of-travel arrow pointing along the desired line of travel.

2. Rotate the compass dial so that the parallel lines within the capsule line up with the grid lines on the map. Convert the grid bearing to a magnetic bearing using the information given on the map sheet. Adjust the dial to read the value of the magnetic bearing opposite the direction of travel arrow.

3. Now pick up and rotate the whole compass until the red end of the needle points to the north marker on the dial. The direction-of-travel arrow on the compass card will point to your destination. Choose a landmark in that direction and walk towards it.

Navigating by map and compass involves much more than simply converting bearings from true to magnetic. Anyone who plans to use a compass during a wilderness trip should read one of the many fine books that have been written on the subject. Some of these are listed in the references.

Gyro compass
A Gyro compass is a form of gyroscope, used widely on ships employing an electrically powered, fast-spinning gyroscope wheel and frictional forces among other factors utilizing the basic physical laws, influences of gravity and the Earth’s rotation to find the true north.
Construction
Gyro compass has become one indispensable instrument in almost all merchant ships or naval vesselsfor its ability to detect the direction of true north and not the magnetic north. It is comprised of the following units: * Master Compass: Discovers and maintains the true north reading with the help of gyroscope. * Repeater Compasses: Receive and indicate the true direction transmitted electrically from the Master Compass. * Course Recorder: Makes a continuous record of the manoeuvring on a moving strip of paper. * Control Panel: Governs the electrical operation of the system and ascertains the running condition by means of a suitable meter. * Voltage Regulator: Maintains constant supply of the ship to the motor-generator. * Alarm Unit: Indicates failure of the ship’s supply. * Amplifier Panel: Controls the follow-up system. * Motor Generator: Converts the ship’s DC supply to AC and energizes the Compass equipment.

Gyro compasses are linked to the repeater compasses via one transmission system. The fast-spinning rotor attached weighs from 1.25 pounds to 55 pounds. It is driven thousands of revolutions per minute by another electric motor. However, the most essential part in a Gyro compass system is the spinning wheel, which is known as the Gyroscope
Working
External magnetic fields which deflect normal compasses cannot affect Gyro compasses. When a ship alters its course the independently driven framework called ‘Phantom’ moves with it, but the rotor system continues to point northward. This lack of alignment enables it to send signal to the driving motor, which moves the phantom step in with the rotor system again in a path where the phantom may have crossed only a fraction of a degree or several degrees of the compass circle. As soon as they are aligned, electrical impulses are sent by the phantom to the repeater compasses for each degree it traverses.

The Gyroscope in the Gyrocompass is mounted in such a way so that it can move freely about three mutually perpendicular axes and is controlled as to enable its axis of spin settled parallel with the true meridian, influenced by the Earth’s rotation and gravity. The Gyro compass system applications are based upon two fundamental characteristics, which are: * Gyroscopic Inertia: The tendency of any revolving body to uphold its plane of rotation. * Precession: A property that causes the gyroscope to move, when a couple is applied. But instead of moving in the direction of the couple, it moves at right angles to the axis of the applied couple and also the spinning wheel.
These two properties and the utilization of the Earth’s two natural forces, rotation and gravity, enacts the Gyrocompass seek true north. Once settled on the true meridian the rotor indefinitely will remain there as long as the electrical supply of the ship remains constant and unaltered and unaffected by external forces.
Usage and Errors
Gyro compasses are pre-eminently used in most ships in order to detect true north, steer, and find positions and record courses. But due to the ship’s course, speed and latitude, there could appear some steaming errors. It has been found that on Northerly courses the Gyro compass north is slightly deflected to the West of the true meridian whereas on Southerly courses it is deflected to the East.
Modern ships use a GPS system or other navigational aids feed data to the Gyrocompass for correcting the error. An orthogonal triad of fibre optic design and also ring laser gyroscopes which apply the principles of optical path difference to determine rate of rotation, instead of depending upon mechanical parts, may help eliminate the flaws and detect true north.