This question relates to a Silva Type 4/54 Expedition Compass which has given 2 years faithfull Service. It is stored in a cupboard with other D of E gear. No obvious magnetic or radiation sources. Yesterday whilst training Scouts, one of them clearly didn't know his left from his right - so I took the compass from him to explain the basics. After a moments confusion, it dawned on me that the Compass needle was now pointing due South! Further investigation showed it to be working fine except that it was exactly 180 degrees out.
Any explanations? It worked fine last week. Other compasses stored in the same cupboard all function normally. Why exactly 180 degrees?

Hi IanPS,
Very strange! I am going to get in touch with some contacts who are currently running some free navigation clinics in selected stores and see if they can answer this for you...
Michael

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Michael

Thanks Michael, I'll be interested in their replies.

I asked the same question of Silva and they were most helpful in their response, which I've summarised below:

“The only thing that can cause a compass to reverse its polarity is a magnetic field of enough strength. There is no way to say how strong the magnetic field has to be since it is basically down to bad luck if the needle turns 180 degrees or not but if you send the compass to us we will be happy to fix it for you! In a worst case scenario a small compass can be turned by a large compass since every compass emits a magnetic field (the larger the compass the stronger magnetic field it generates), however this is very rare.
The reason for the 180 degree turn is because the magnetism can only reverse itself. It will allways align itself according to the strongest magnetic field in the area and therefore it will point north or south in earths magnetic field.”

They also kindly sent me a PowerPoint presentation about magnetism and compasses. Top marks to Silva!

Updated 06 April 2010 at 12:17 PM

Hi,
I also had a compass reversal, with a SILVA compass. I have a faint possibility of a cause: I went to have a look at the Large Hadron Collider at CERN, in Switzerland, last year. I had not used the compass since then, and just noticed the anomaly on a trip to France last week.
Cheers, Chris-O

This month's TRAIL magazine has a helpful section on this issue (on page 113). It lists a possible cause as keeping your compass and mobile phone close together, as the phone speaker contains a tiny magnetic induction coil, which given a short time can reverse the polarity.

Silva are reported to offer a re-polarising service.

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DM
Store Supervisor
Cotswold Reading