The Ship’s Compass – A History

The Earth has a core of molten iron which provides it with a strong magnetic field. This is not true of all planets, and it transpires that this phenomenon is vital to life. The magnetic field shields us from much of the sun’s more harmful radiation, without which our world would soon be reduced to a dry, rocky wilderness. It also helps many animals, such as migrating birds and whales to navigate, and has long been used by humans to help them find their way.

Early navigators principally used the stars and sun for direction, but this was only possible when the sky was clear. Particularly in temperate regions the sky is often cloudy, requiring a reliable alternative that could be used in poor weather. Work on a solution centred on lodestone, a naturally occurring magnetized form of the mineral magnetite. One of the earliest known references to lodestone's magnetic properties was made by the Ancient Greek philosopher Thales of Miletus in the 6th century BC, who noted that a fragment of the mineral will turn towards the north when suspended from a string. He also observed that a small piece of iron, when exposed to a lodestone, displayed the same property. Similar references to lodestones occur in Han Chinese texts, although their interest in the material was for divination and the propitious orientation of buildings rather than seafaring.

The first magnetic compasses were very crude, and would only have served as a backup for a sea captain unable to navigate in any other way. A needle that had been magnetised by rubbing the tip with a lodestone was thrust into a piece of cork or straw and floated on a bowl of water. The needle would turn on the surface to point towards the north. This method was fine when all else failed, but by the medieval period ship-borne commerce was becoming increasingly important, and a more permanent solution was required. In the thirteenth century the first French and Italian descriptions of a new type of compass appear in the historical record. The general design was of a bowl, usually made of wood, with a nail driven up through the base. Instead of resting on the surface of a liquid, the compass pointer was attached to a hollow cone of brass which rested over the point of the nail. A round card with the cardinal points was attached to the needle and the compass was encased in a protective box. By the fifteenth century such compasses were in common usage by navigators. Columbus writes of taking bearings with one in his accounts of his voyages, and they started to appear in contemporary portraits of seafarers, such as that of Lord Clinton and Saye, who was Lord High Admiral of England in 1562.

Once compasses were reliable, they became the main method of determining direction on board ships. Larger and more robust versions were mounted prominently in a binnacle where the captain and helmsmen could easily refer to them. Various improvements continued to be made. The compass bowl was fitted between gimbals to keep it flat, and the binnacle was lit by oil lamps at night. Now that it was such an important tool, some navigators noticed that the magnetic north pointed to by a compass was a few degrees adrift from the true north shown on their charts. The solution was to offset the compass card a little to correct for this. But as ship’s compasses continued to develop, more worrying inaccuracies in their performance began to appear.

Mathew Flinders noticed the inaccuracy of his ship’s compass on his mapping expedition around the Australian coast in 1802. He studied the variation with care, and concluded that it was caused by the local attractive effect from the iron fittings in his ship. He made a careful study of the phenomenon, and realised that since it was constant for a given ship, it was predictable. He concluded that he should be able to correct for this effect, and after some experimentation he found the solution. What became known as the Flinder’s bar was an unmagnetized rod of iron positioned vertically near the compass. It was an innovation that was quickly adopted.

The Flinder’s bar worked well to correct for the modest effect of the relatively few iron fittings to be found on a wooden ship, but the advent of vessels made entirely from iron and steel in the 19th century led to a further dangerous drop in compass accuracy. Something more sophisticated was going to be required to allow all-metal ships to navigate safely. The problem was solved in 1838 by Sir George Airy who corrected the compass on the steamer Rainbow by placing small magnets around the binnacle to compensate for the ship’s magnetic field. This tuning process is still carried out today when a magnetic compass is fitted to a ship. Since no two ships have the same magnetic field, the correct placement of the corrective magnets is unique to each one.

There was a further change to the ship’s compass that coincided with the move from sail to steam power. The amount of vibration produced by a ship’s engines when compared with sail power meant that compass cards suspended in air became difficult to read accurately. The solution to this problem was a return to an idea from the first compasses, with the compass bowl being filled with liquid. This effectively dampened down any vibration, and all commercial compasses are now of this type.

The final development was a move away from magnetism with the introduction of the gyroscopic compass. They were developed at the start of the twentieth century independently in Germany, the US and Britain. A gyroscopic compass uses the innate stability of a rapidly spinning wheel. Once this is aligned with true north, so long as the wheel is kept spinning, it will continue to point in the same direction irrespective of any course changes made by the ship. These compasses have the advantage of being aligned with true north, and of not being influenced by magnetic changes in the environment, although they do require constant power to remain accurate. For this reason, many ships still retain a traditional magnetic compass as a backup – just in case.