View Full Version : Compass - I can't figure this out!
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
I thought that only happened to me :D
No idea, though my destruction of compasses has been blamed on walking too near to an electricity sub station, passing under pylons, thunderstorm and my rucksack containing the compass being left too near to a fire. I can't see any of them really being responsible but I'd love to know what reason you finally come up with :rolleyes: :)
Cheers,
Mary
It happened to me too. I noticed that the compass showed 180 degrees wrong. For that time i worked in a shop. We had a strong magnet mounted on the desk in the shop to demagnetize the pricetags (prevent shoplifting). By putting the compass on top of the magnet it became "set at zero" and was OK again. I donīt know if a cellular phone or anything else with a strong magnetic field are to be blamed. /Swede
Roving Rich
05-02-2007, 23:51
Prolly too close to your pacemaker Lith ;)
Cheers
Rich
magnets can go a bit funny if they have a bang even a small one, maybe a small bang and its lost some strength allowing it to affected by something small you have on you, watch or something else metal??
oldsoldier
05-02-2007, 23:59
I think the poles just changed for you!!
The Joker
06-02-2007, 00:03
I think the poles just changed for you!!
On some tv prog the other day i was watching they said we're suposed to be nearing a time when the poles swap :confused:
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
Saw the same thing in an episode of the X-Files :eek:
You need to re magnetise the needle.
Get a small bar magnet use only the North pole end and place it on the glass dial and then slide it along the glass towards the tip of the north (RED) end of the needle, do this about ten times and it should bring it back to life.
Make sure that you only do it to the red half and not the white half, we used to have a really dopy scout leader and I magnetised the white (South) end of the needle and then gave the compass to him on a hike, it took him about ten minutes to realise that there was something wrong and he never really switched on to the fact that it was a windeup, quite worrying that someone so stupid could be allowed to be responsible for our kids. :)
Hi Lith,
You can send the compass back to Silva and they will realign it for you - also will take bubbles out of the capsule. Take it in to your local Cotswold and get them to send it off for you.
gregorach
06-02-2007, 10:35
I've had the same thing happen to me. Darned if I could figure out what had caused it...
Hi Lith,
You can send the compass back to Silva and they will realign it for you - also will take bubbles out of the capsule. Take it in to your local Cotswold and get them to send it off for you.
Fantastic, cheers Rod, I didn't realise they would do that, I was going to bin it.
Thanks for the responses.
Matt
BorderReiver
06-02-2007, 12:55
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. :eek:
Just as well to carry a GPS as backup. :D
anthonyyy
06-02-2007, 13:28
I can understand exposure to a magnetic field altering the compass and putting it a bit "off" but an 180 degree change is wierd.
Would it have been near a phone ? Or anything else with a speaker in it ?
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. :eek:
Just as well to carry a GPS as backup. :D
which is why you should always carry 3
Fantastic, cheers Rod, I didn't realise they would do that, I was going to bin it.
Thanks for the responses.
Matt
When i asked Silva direct if they could remove bubbles they said that it could not be done, and why go to the tropuble of returning the compass when you can fix it yourself in two minutes. :)
When i asked Silva direct if they could remove bubbles they said that it could not be done, and why go to the tropuble of returning the compass when you can fix it yourself in two minutes. :)
The trouble with trying to induce a magnet by this method is that they're more prone to losing magnetism through bumps and over time. Permenant magnets are normally electrically induced from what I can gather.
The trouble with trying to induce a magnet by this method is that they're more prone to losing magnetism through bumps and over time. Permenant magnets are normally electrically induced from what I can gather.
Go with your own choice, but about three years ago I remagnetised two of my own Silva compasses and have done several for the scouts and they are still working fine. :)
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. :eek:
Always worth checking, with the Sun, to make sure your compass hasn't done a pole swap.
stuartmac5
07-02-2007, 00:24
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 :D
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
07-02-2007, 18:35
:o So what you're not saying is that you walked in the wrong direction for a whole day?!? :o :o :lmao: :lmao: :D :D
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
07-02-2007, 19:58
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
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???
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 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,
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
08-02-2007, 10:52
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
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 (http://www.acornnmr.com/appnotes/shielding.htm) 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 (http://www.physics.ubc.ca/~outreach/phys420/p420_97/james/emprotec.htm) 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. :D
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. :eek:
If you're in unfamiliar territory and you don't trust a dodgy compass, you could land up in trouble. :eek:
If you're in unfamiliar territory and you don't trust a dodgy compass, you could land up in trouble. :eek:
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
11-02-2007, 17:39
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.
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.
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