A magnetic attraction: the Earth’s Magnetic Field

Better understand how a compass works and the Earth’s Magnetic Field.

A compass will provide you with the direction of the four cardinal points – North, East, West, and South.  By understanding how a compass works and knowing the numerous recognizable signs that indicate direction, you will rarely be lost.  So, it is important to understand the Earth’s magnetic field and how to make a temporary compass.  The compass, like gunpowder, was developed by the Chinese who used a chunk of magnetic iron ore suspended by a piece of lace to help them navigate on land and sea.  Today, the simplest compass is a magnetised needle mounted on a pivot so that it rotates freely.

To make your own temporary compass, stroke a sewing needle in a single direction with a piece of silk or with a pocket magnet.  If you stroke the needle form the head to the tip, the head will point North.  Press you thumb and forefinger over your nose to collect some oil and rub this bodily secretion onto the magnetised needle.  Create two loops from thin bits of grass or other fibre and use them to suspend your needle.  Lower the magnetised needle into still water such as tiny pool trapped in a banana plant, a tree stump or a rock.  The surface tension on the water will hold the needle and, once freed, the floating needle will turn until it is aligned with the North-South magnetic poles.

By understanding that a compass is basically a magnetised needle that, when allowed to spin freely, will line itself up with the Earth’s magnetic field, you can effectively create your own direction-finding device very simply.  But, the compass’ needle only points to magnetic North and not “true” North.  Hence, the term “Variation” is the angle measured between magnetic North and “true” North and varies depending on where you are on the Earth’s surface.  The importance of Variation is well-known to navigators, but was forgotten by the British military when soldiers were sent to relieve the Argentinean-occupied Falkland Islands in 1982.  Because these British soldiers were using compasses issued for use in the Northern hemisphere, several map reading and navigational mistakes were made, especially when on patrol.  As a result of poorly configured compasses and a misunderstanding of variation, several “blue-on-blue” incidents occurred which could have been prevented.

So, how is the Earth’s magnetic field generated?

The Earth’s magnetic field is generated within its molten core through a combination of electrical forces within the core, thermal movement and the Earth’s rotation.  When the molten iron within the Earth’s core rotates, convects and generates electricity, a dynamo effect is created which sustains a magnetic field.  This magnetic field is similar to that of a bar magnet slightly inclined to a line that links the North and South geographic poles.

The magnetic field extends approximately 63 700 kilometres from the Earth on its Sun side and 384 000 kilometres on its Moon side.  Every several hundred thousands years, the magnetic poles swap as the magnetic lines loop around each other from the northern polarity (South Pole) to the southern polarity (North Pole).  Solar winds (electrical particles streaming from the Sun) also distort the geomagnetic field lines, flattening them on the Sun side and stretching them on the Moon side.  The Earth’s magnetic field moves over time with shifts in the dynamo at the Earth’s core.

By understanding the forces that create the Earth’s magnetic field you will be able to plot the direction and strength of the magnetic field at the Earth’s surface using your compass

In our H.E.A.T. course, we provide you with the necessary knowledge to read a a map.  This skill will be of paramount importance if you find yourself lost in a Hostile Environment.