The magnetic compass

The most underrated tool on a boat, the compass quietly points to magnetic north without consuming electricity or diesel or demanding constant maintenance.

The Chinese were the first to incorporate the natural magnetic qualities of lodestone into a workable compass around 1100 A.D. It was a simple affair; lodestone on a piece of wood floating in a bowl of water. By the 16th century a card marked with cardinal points was added and the bowl of water was suspended in gimbals to allow it remain level at sea. In 1745, the English inventor Gowin Knight developed a method for magnetizing steel for long periods of time, allowing needles of magnetized steel to replace needles of lodestone. In 1862, improvements in the design of liquid compasses quickly made the dry-card compass obsolete for marine use.

THE MANUFACTURING PROCESS: MAKING THE NEEDLE

The core of the marine compass is the magnetic needle which is manufactured out of pig iron. This is made of iron ore, coke, and limestone which are heated in a blast furnace by hot pressurized air.

The result is 90 percent iron, five percent carbon, and five percent impurities. The iron is further purified by mixing it with oxygen and then re-heating it while it is rolled between grooved rollers to form slabs which are then cut into needles by a die machine.

The needle is then exposed to a high-powered electronic magnetizer which gives it magnetic qualities in the correct orientation.

MAKING THE HOUSING

The housing of the compass needs to hold this delicate arrangement together, make it readable, yet not influence the magnetic qualities of the needle. Plastic is the most common form of housing as it is easily shaped and remains firmly non-ferrous.

“The needle pivots on a jewel bearing”