The magnetic compass is generally ranked next to printing, paper and gunpowder as one of the greatest gifts ancient China gave to the West. Before this life-changing invention, mariners had to rely solely on the position of the sun, moon and stars to determine the direction they needed to travel in.
How Does It Work?
Think of a typical north-south bar magnet. Magnetic lines run through it in one direction, from “south” to “north”. When they leave the magnet at the “north” pole, they return to the “south” pole in an elliptical arc.
A magnetic compass works because the Earth itself functions exactly like an enormous bar magnet. The huge arcs of magnetic force created by returning magnetic lines are referred to as the geomagnetic field, which surrounds our entire planet. It is generally believed that the source of the Earth’s magnetism lies in the powerful electric currents running through a central core of molten iron deep in the Earth’s center.
In the same way that a magnet causes all the domains in a magnetic metal to align with it, so a bar magnet placed in magnetic field will slowly rotate until it lines up with direction of that field. The same principle applies when a magnet, suspended from a piece of string, moves until it is pointing in a north-south direction. This is the phenomenon that the Chinese noticed as far back as the first Century BC – even though they were then unaware of what was causing this to happen.
This discovery ultimately led to the invention of the magnetic compass. This simple instrument typically comprises a magnetized iron needle, attached to a pivoting mechanism and suspended over a card bearing the four cardinal directions – N, S, E and W.
Magnetic North vs. True North
The fly in the ointment of this otherwise world-changing invention, is that magnetic north is not the same as true north. In other words, if you followed a compass to its most northerly conclusion, you would end up not at the North Pole, but at the Queen Elizabeth Islands off far Northern Canada. This is caused by the center of the Earth’s magnetic field being the Equator, whereas the North and South Poles are 11° off the planet’s axis.
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