In spite of the precision of the GPS (Global Positioning System), the simplest way to determine direction and location is still by a magnetic (mariner’s) compass or gyro compass. Nowadays there are also electronic compasses. These compasses have no magnetic needle, but a magnetic sensor that is based on the Hall effect. Such a sensor needs only a few components to show the directions north (N), north-east (NE), east (E), south-east (SE), south (S), south-west (SW), west (W), north-west (NW) on a compass card.

The Swiss firm Pewatron sells two kinds of compass sensor, both of which react with great sensitivity to the earth’s magnetic field. These sensors facilitate the construction of an electronic compass for various applications. However, the analogue type, with its very high resolution, requires a quite complex electronic circuit, whereas the digital type (as used in the present design) only needs a few standard components and a standard logic IC to construct a compass with 45° resolution. The direction is indicated by a number of LEDs built into the compass card. The digital sensor is primarily intended for a hand-held compass, which may also be used in a car, boat or light aircraft.

Not many people know how to handle a compass correctly. It is only when you have learnt to fly, sail a yacht across the sea, or have been on a ‘really wild’ holiday that you appreciate that the compass needle does not normally point to the north. This has several causes. First, the geographical (true) north and magnetic north are not at the same location. This means that the lines of force surrounding the earth magnet are not parallel to the geographical meridians. Moreover, lines of force do not flow in a constant direction from the south magnetic pole to the north magnetic pole. Their direction fluctuates considerably, and for this reason the magnetic meridian cannot be defined as ‘the arc of a great circle joining the north and south magnetic poles’. Instead, it is defined as ‘the direction that a compass needle will take up when under the influence of the earth’s magnetic field only’. The angle between the magnetic meridian and the true meridian at any place is called the magnetic variation or declination. The north magnetic pole is moving slowly all the time: it makes a circle round the true north pole once in about a thousand years. In Britain, the westerly declination is decreasing by about 10 minutes of arc annually. The foregoing makes it clear that the resulting error in compass reading is greater near the true north pole than at the equator.

Another cause of error is the earth’s magnetic field, which is the space around it occupied by its lines of force. Any freely suspended magnetic needle placed in this field will align itself with the lines of force of the field. The direction of this field is horizontal at the magnetic equator, but as you travel northwards, the lines of force begin to dip, until, in Britain, they are inclined at an angle of about 60° to the horizontal. In a modern magnetic compass, the magnetic needle is not allowed to dip. This is achieved by having more than one needle (usually four to eight) so arranged that their common centre of gravity is below the point of suspension of the compass card.