Compass - I can't figure this out!

[stuartmac5]

hi Lithril,

a neodymium magnet is amazingly powerful and kids in classrooms can have a bit of a prank, whearabouts they can use one to change a normal classroom bar magnet from n-s into s-n. They are that powerful.
maybe some of the kids with you were taking out that day were having a bit of a prank with you?
did you let that compass out of your grip?

Im obviously a suspicious person

 

[Voivode]

And on the topic of neodymium magnets, I use a 10mm diameter disc magnet to realign my compasses when they spend too much time close to emf sources, like the wiring in the walls, televisions and such. I found in my apartment that over time my compasses (I have 2) would gradually become demagnetized and just spin freely. A single application of the rare earth magnet pops them right back into shape.

I don't see why your compass would fail in such a manner, as it requires a magnetic field to realign the field of the pointer. There must have been an outside source that influenced your unit...

 

[PhilParry]

So what you're not saying is that you walked in the wrong direction for a whole day?!?

Ok I've been using compasses for a while and I understand the principles on how they work (have to teach it to 11-16 year olds...) but I'm stumped at this one.

Usually use Silva compasses as they're basic and fairly robust, we give them to the kids for DoE to use. I went out last weekend and there were 3 compasses in the group, the kids had 2 and I had 1, all identicle and made by silva. I put my compass in my top pocket (nothing else in there), next time I took it out to check a bearing, walked about 200 meters and the compass swithced so that north points almost to south and vica verca. Only happeded with this compass and its still like it, so a permenant change.

Any ideas?

Matt

 

[quarterstaff]

I have a small marbles compass and at first it was ok,then one day I used it just to check my direction and it was way off,mate had his gps,bit miffed,tried a few days later spot on again,next time way off,it was then that I realized that I had got a new copper arthritis braclet with magnets in and I had been using the compass in the left hand-ok,right hand-way-off .
cheers

 

[Pete E]

I had the same 180 degree swap happen on a Silva Type 27 last year..No idea what caused it as I am careful to store my compasses away from any magnets or stong electrical fields...

Anybody any suggestions as to the best way to store a compass to prevent this happening???

 

[Voivode]

A box made from a ferromagnetic material (iron, steel, whatever) will create a Faraday Cage effect and shield the object contained within from external magnetic fields; Your compass won't work inside the box, but it should be immune to demagnetization.

 

[Pete E]

A box made from a ferromagnetic material (iron, steel, whatever) will create a Faraday Cage effect and shield the object contained within from external magnetic fields; Your compass won't work inside the box, but it should be immune to demagnetization.

Voivode,

I wondered about the same idea... I agree that such a "cage" would isolate the needle from the effects of the earths magnetic field, or anyother external source,but storing a magnetic needle that close to a ferro substance, even if it does surround it, surely can't be good??

Regards,

Pete

 

[Voivode]

Voivode,

I wondered about the same idea... I agree that such a "cage" would isolate the needle from the effects of the earths magnetic field, or anyother external source,but storing a magnetic needle that close to a ferro substance, even if it does surround it, surely can't be good??

Regards,

Pete

"ferromagnetic" refers to the properties of the metal, in that it responds to magnetic fields. Imposing a magnetic field on them does not (necessarily) make them themselves magnetic, but merely acts as a barrier. A paramagnetic material (aluminum) might be superior, but not necessary.

 

[gregorach]

Sorry guys, but a Faraday cage will not provide protection against static magnetic fields - unless it's made from a superconducting material. It'll block only RF, EM, or electrostatic fields.

http://en.wikipedia.org/wiki/Electromagnetic_shielding

 

[Voivode]

Sorry guys, but a Faraday cage will not provide protection against static magnetic fields - unless it's made from a superconducting material. It'll block only RF, EM, or electrostatic fields.

http://en.wikipedia.org/wiki/Electromagnetic_shielding

Fair enough, but I wasn't meaning to specifically advocate building a Faraday Cage, just providing an effect like it. The shielding effect of the Cage also requires a ground connection of some sort to work properly and certainly doesn't need to be solid to work, or it would be the Faraday Box.

Everybody go get your compass, a piece of metal that reacts to a magnet (like a butter knife) and a magnet. Bring the magnet close to the compass and it reacts, aligning with the magnetic field of the magnet. Now, interpose the metal between the compass and the magnet. The field should be blocked, interfering with the response of the compass to the magnet. Therefore, a box made of butter knives should protect your compass from magnetic fields. Even better, a box made from the material of butter knives rather than knives themselves.

This webpage gives a moderately technical discussion of magnetic shielding as it relates to non-changing fields. What it boils down to is this: When you put a magnet in a steel box, the field (flux lines) is contained inside the metal of that box rather than radiating out into space. The reverse is true as well; if you wrap yourself in steel, magnetic fields can't get to you. Here is a simple physics experiment where they do just what I'm advocating; placing a compass inside a soup can to protect it from a magnetic field. Now, they've refined the concept so that the compass doesn't touch the sides, is centred in the can and the can opening isn't pointed at the magnetic source; It's still proof of concept.

If I were to build a box, I would put little rubber or foam spacers to hold and cushion my compass; keeps it off the walls and in the area of greatest effect.

Or, I'll pull it out of the camping bin, check it out and give it a stroke with the neodymium magnet to tune it up if it's not acting properly. Whatever you find easier.

 

[Emma]

Bit worrying this.The accepted wisdom is "always trust your compass".

So if you are in unfamiliar territory and you trust a dodgy compass,you could land up in trouble.

If you're in unfamiliar territory and you don't trust a dodgy compass, you could land up in trouble.

 

[Lithril]

If you're in unfamiliar territory and you don't trust a dodgy compass, you could land up in trouble.

I've normally at least one other compass but normally I wouldn't use it for micro nav just as a check, only in the New Forest so the compass on my watch was more than accurate enough to check the route that the kids were taking (they also had their own compasses).

It has made me rethink my kit list slightly though and plan to carry a spare.

 

[demographic]

A box made from a ferromagnetic material (iron, steel, whatever) will create a Faraday Cage effect and shield the object contained within from external magnetic fields; Your compass won't work inside the box, but it should be immune to demagnetization.

A Faraday cage doesn't have to be made from a ferromagnetic material as the MRI room at the local hospitals farraday cage made from copper sheet.

 

[Voivode]

A Faraday cage doesn't have to be made from a ferromagnetic material as the MRI room at the local hospitals farraday cage made from copper sheet.

Right. No problem. I wasn't actually talking about making a Faraday Cage for blocking EMF; I was talking about making a metal box for storing your compass that would produce an analogous effect to a Faraday Cage, but for an unchanging magnetic field. This metal box would indeed have to be constructed of a material that responds to magnetic flux better than air, and those are ferromagnetic substances.

 

[S8AN]

I have an oil based compass and a normal one and have had the same problem with both! The only thing that i noticed is if the compass was put near metal for an extended period (i stupidly carried them in a little metal tin) and both then ended up pointing in diff directions. I suppose the small magnetic field that all metal objects carry can cause a prob