Blueprint - compass

Classic Projects: magnetic compass

We get our bearings on the history of the magnetic compass.

The magnetic compass is one of the simplest instruments ever invented and has been around ever since we understood that magnetised iron-bearing rock had north-seeking properties. It has been crucial to our exploration of the globe, playing pivotal roles in transoceanic, circumnavigational and polar expeditions. The magnetic compass was the key breakthrough in freeing us from short-range exploration based on the stars or geophysical landmarks. The compass meant that a mariner could go out to sea to a point with no visual references and still find north.

Depending on who you consult you will get varying estimations as to how long we have had this knowledge, but there seems to be a consensus of the technology emerging three millennia ago in China. Early compasses consisted of a lodestone suspended over a board that displayed the cardinal navigational points: north, east, south and west. Over the millennia the design would evolve to the point where today a simple magnetic compass of reasonable quality will consist of a dial or needle that rotates on a pivot with jewelled bearings in a sealed circular container. The compass may be air filled, but others contain liquid (usually alcohol) to help dampen vibrations on the card, while the amount of geographical data printed on or surrounding the dial will vary. Because a compass needle will always point north, navigation with nothing other than a compass and a map over short distances can be extremely reliable once the principles are understood.

According to the Royal Museums Greenwich it wasn't until the 10th Century that the idea of magnetic navigation came to Europe. Within a few hundred years magnetic compasses were in common use in the Mediterranean, despite being not very reliable with little known about how they worked.

The problem for navigators ''and this was starting to be understood by the Renaissance - is that a magnetic compass does not point 'true north', that is along a line to the Geographical or Terrestrial North Pole where the Earth's rotational axis meets the surface. Instead, the pointer makes an angle with that bearing. And rather confusingly it doesn't even point to the Geomagnetic North Pole. But the compass's rose or needle will align itself to the Magnetic North Pole, which is helpful for navigation but not particularly accurate because this particular pole wanders significantly due to variations in the Earth core magnetism. The closer you get to either the north or south magnetic poles the less reliable your magnetic compass will be. These differences were investigated by the English astronomer Edmond Halley (of Halley's comet renown) who introduced isogonic lines, which define areas of equal magnetic deviation on a map.

In the Great Age of Sail mariners found that their compasses responded badly to the motion of their wooden ships and so they fitted their compasses with gimbals, which allowed them to remain level irrespective of their ship's movements. This was an enormous help, but could hardy disguise the biggest drawback of the magnetic compass, which is that it is prone to interference from external influences. As wooden ships made way for a new generation of iron leviathans, the instrument became of virtually no use. Since the Victorian age the seafaring version of the magnetic compass has been replaced by gyroscopic technologies and in the digital age, GPS and smartphone apps.

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