Silva Compass Has Inverted

[Wook]

I've not used my compasses in months, but I like to know they're reliable when I need them. Today I was explaining the principle of using an analogue watch to find south to my 10 year old daughter. I was telling my kids about how during BST you need to subtract an hour, but in winter you don't (today's clock change is how the subject came up). I got some of my compasses out to illustrate that this technique works. You can imagine my consternation today when I noticed my two Silvas no longer agree with one another.

The one on the left appears to be accurate. I don't have another compass against which to check it but that is certainly the approximate direction of south. I was scratching my head as to what caused this. My compasses live in "daddy's dangerous box" (where I keep my knives) and I wondered if the steel in the blades had done it. But I just realized that I'd bought some neodymium magnets for a project a while ago and I'd thrown them in there so little fingers wouldn't get trapped by them since they're pretty strong. I'd forgotten I'd put my compasses in there as well. B*gger!

Because a compass is a precision instrument I'm assuming there is nothing that can be done if they should become inaccurate. Given they've both been exposed I suppose it is quite likely they've both been affected and cannot be relied upon even if one still appears accurate.

New compass time I suppose?

 

[bopdude]

I have no idea but can they be retrained so to speak

Sent from my SM-G900F using Tapatalk

 

[Macaroon]

There is a way of getting them back, I read about it here a while back; can't find the linky at the moment but I'll give it a go in a while................

 

[lou1661]

Just swipe a magnet over the needle, should sort it out.

 

[Wook]

Just swipe a magnet over the needle, should sort it out.

Well drawing the same neodymium magnet that caused the problem in the first place over the needles of both compasses a few times seems to have at least made them agree. They're both pointing in exactly the same direction now, and it is certainly approximately north, although I need a way of calibrating them. I suppose I could get out my map and stand at a known point and then calibrate them by taking bearings.

 

[lou1661]

The needle will only point north south, move clear of anything magnetic, place compass on a sheet of plain paper, align north south, Mark a line along the baseplate. Remove compass and replace with the other, the baseplate of the second should be in line with the mark from the first, if that makes sense?

 

[Wook]

I took them a wee walk to the coast this afternoon. Both compasses returned the same bearing for several different distant objects. I struggled a wee bit to find something obvious on the map to aim for but eventually settled on some farm buildings which I was reasonably sure were the same ones marked on the map. Unfortunately such a target lacks the precision of (for example) a steeple. Using my protractor on the map I calculated that the farm should have been approximately 235 degrees from where I knew myself to be. The sighting compass returned a value of 231 degrees once magnetic drift is accounted for. The orienteering compass returned a value of 238 degrees, although since it isn't a sighting model is is harder to get exact readings with this one. I'm pretty rusty and I didn't have the best target to shoot for but I think that's about as accurate as I usually am with bearings. Obviously if I was unsure I'd take multiple readings so the errors would average down.

I think they're probably​ fixed.

 

[ged]

... I need a way of calibrating them ...

That's what stars were invented for.

Planets work too, and there's a particularly good one at the moment.

 

[ol smokey]

Can't give any advice on correcting your Compass, nut may be able to help others avoid this problem. I am led to believe
that this can be caused by leaving the compass in the glove compartment of your car, where the electrics may cause this reversal of the compass needle. I don't know but it is worth trying out the compass in its present state, and using the
South end of the needle to point North, I would think that this would work OK, but do not rely on this till you have tried it
out in an area that you know well, and against a compass that you know to be correct.

 

[XRV John]

I bought a cheap compass from China through ebay and it was 180 degrees out

It was my first and was for playing with rather than anything serious

Opened it up and turned the needle round 180 degrees and it's fine

Got a full refund as well !

Would this have been because it was for the southern hemisphere?

 

[lou1661]

I bought a cheap compass from China through ebay and it was 180 degrees out

It was my first and was for playing with rather than anything serious

Opened it up and turned the needle round 180 degrees and it's fine

Got a full refund as well !

Would this have been because it was for the southern hemisphere?

Nope, it's just where the polarity on the needle that has reversed, magnetic north is the same in both hemispheres.

 

[XRV John]

magnetic north is the same in both hemispheres.

of course it is !!!!

 

[Jaeger]

Aye Up,

That was brilliant - Magnetic South! I'd heard it was going to flip soon but didn't realise it already had!

Wook, your problem was a regular occurrence when young squaddies (not me honest!) left Sylvas and Suuntos on heavy metal objects (steel,not music).
We used to set them right by holding the south pole of a magnet a couple of mm above the compass at the (inverted) North end of the needle and moving slowly over the needle to the (inverted) South end, lifting the magnet up and then in an arc fashion bringing it back to the start point (so you are effectively making a vertical circular motion over the needle). They would usually flip back straight away.

I've had to do this recently with a lad who I was nav training. None of the students were carrying anything that might outwardly cause the issue.
One lad kept going well adrift on a simple ten legged circuit. The lad was bright and I was confident that it wasn't my teaching so I suspected a duff compass. Sure enough the needle was out. I loaned him my compass - a Sylva sighting compass (one of my treasures!). He still went adrift!
Worse still, when I checked my own compass that too was now well out?

I suspected that there might be some kind of magnetic or electrical anomaly on the offending leg of the route even though I hadn't experienced it during my initial proving of the course and neither had any of the other students as they'd gone around. Handing him a third compass with the intention of accompanying him around the course I suddenly spotted the problem - he was wearing mittens with fold-back finger sections - and guess what held those sections in place on the back of the mittens ? - You guessed it - magnets!
There then followed a lesson in kit selection and re-aligning compass needles!

 

[dave53]

Why aren't the two norths at the top?