How Compasses Work

How Compasses Work


Compasses have to be balanced for the area in which they're used.


How Compasses Work 

How Compasses Work



The Earth has a magnetic field around it, with the 'top' end being Magnetic North


      
The magnetic field consists of 'parallel' lines that radiate and converge at the magnetic poles. See Fig 1.

A compass consists of a magnetic needle, which tries always to point towards magnetic north by positioning itself parallel with the magnetic field. See Fig 2.


Inclination

To work accurately the compass needle must be able to turn freely inside its liquid filled capsule, else it may drag against the compass housing and give an incorrect reading. This is why it’s always important to hold a compass horizontally when in use.

However, the magnetic field does not work equally around the globe and compasses needles tend to dip downwards the further north (or south) you go, causing drag, with the potential for inaccurate readings.




At the "magnetic equator" (see Fig 3) the needle is perfectly level, but in Northern Europe the needle dips, and this is called inclination. To overcome this situation, which could cause compass inaccuracy, manufacturers balance their compasses to allow for inclination.


Balancing


   

Adding an appropriate weight to one side of the compass balances it for a particular area, or zone of the Earth's magnetic field. Not all parts of the world are balances in the same way. Silva use the following balancing zones.


Silva's Balancing Zones

  

Silva's zones are Magnetic North (MN), Magnetic Equator (ME) and Magnetic South (MS).

For example, if you bought a Silva compass in the UK it may have on the base plate the letters ‘MN’, meaning ‘Magnetic North’ (although sometimes there are no such letters). That compass will be fine for using in the UK, Europe, most of Asia and North America. However, if you were to use the same compass in New Zealand, it would be inaccurate as it would not be balanced for the Southern Hemisphere, as you would need a compass balances to Magnetic South, or MS. Likewise, if you bought a compass elsewhere, accuracy in the UK could be compromised.

Suunto manufacture a globally balanced compass. Not all manufacturers are the same, so check when you buy.


Summary

Basically, there should be no issue using a compass in the UK if it was bought in the UK. Be mindful of this issue if travelling to equatorial or Southern Hemisphere locations, where you'll need a different compass.


Acknowledgement

Thanks to Silva for assistance with information and images.


Shop

  • Compasses in the shop (quote discount code SVAC13 for a £5 discount from quoted price of Silva compasses)