Reversing the polarity of a magnetic compass

Lock it and then run one pole of a permanent magnet along it a number of times. Not sure which way round though.

Oh yes.

Its dead easy.

Put compass in BIG hollow solenoid. Wind this out of almost anything wire shaped round a bot of drainpipe or wate pi[e depending on gow big it is.

Get car battery, and some 30 amp fuse wire. Make fuse in one bit of wire. Connect one end to battery and then apply other to other pole of battery. Fuse will blow, but not before compass is well and truly remagnetised. If it doesn't work, align the pipe in reverse direction and try again.

Be wary if your compass has correcting magnets. They will get remagged as well.

Whoops. The big secret is out....

You know every 7000 years the earth's magnetic poles flip and everything turns to ice...

Look outside. Amongst the snow you'll probably notice some goons closing in with walkie talkies....

Like the ones who have been outside my do

Vortex3 formulated on Friday :

My best guess would be that it has been sat for a long time with the needle pointing in the wrong direction, with the case tilted just enough to prevent the needle swinging and the earths field has managed to remagnetise it with the wrong polarity.

You need to tilt it enough to again jam the needle and then stroke the end of a strong magnet along the full length of the needle. It needs the strokes to be done in one direction - lift the magnet at the end of each stroke well away, then start fresh stroke. It might need some practise not to get the magnet so close that it makes the needle move.

On one write "Red = North"; on the other write "Red = South".

The correct direction is the one that works ;-)

Buy a new compass. The power needed to re-magnetise it reliably from outside the sealed vial is going to be considerably more than that needed to do a bare needle - I suspect impractically so.

What you need is a big hollow solenoid coil (lots of windings on a tube) and then a crude capacitor discharge system to put one hell of a wallop through that coil. As the coil is inductive, it's actually quite hard to do this! Big paper capacitors and a big old Dr Frankenstein knife switch are the usual way.

You're unlikely to do it with a battery, as you won't get enough current. You _might_ be able to do it with something like a short- circuited destructive use of a battery, if you have a crate of old laptops to dispose of... 8-)

How much current are you figuring on needing then ? The average car battery is good for at least 200 amps, and more if it's a big 'un ...

Arfa

Congratulations for noticing this before going out into the wilds. I had a similar compass with that problem. I fixed it by tilting it so the needle didn't move then waved a powerful magnet at it, which is probably how it got wrong in the first place.

I will try this approach first!

I am told hard disk heads contain very powerful "rare earth" magnets. Since I have an old crashed disk here I will begin by extracting its magnets later

This raises the question which is never answered by the 'survive on your own' books which helpfully explain how to magnetise a needle for compass purposes but never tell you how to work out the direction that the needle points. :-)

Richard

Exacltly and in fact we did this at achool with a big solenoid and AC. Its random as to which way it goes though.

Or put near a CRT TV and let the degaussers hit it when you switch it on..

A car battery and a fuse is what will supply the ommph. Magnetic field in the middle is ampere turns, so its not the amps alone. Have more turns. Just make the fuse go before the wire comprising the solenoid ...

The inducance of it will mean te current build up wuite loswly, reaches a peak, and then the fuse goes. It works.

Probably a 50 meters of single core bell wire would do the trick round a drainpipe.

I'm not so sure about hard disks having strong magnets in them. The head is a strong electromagnet but it only over a very small area!

If I wanted a strong magnet I'd aim for a magnetron in a microwave.

The motor that positions the head is based on a strong magnet. In my experience, fairly easy to open up and remove (might require a Torx driver).

Suggest you only do this to redundant drives as re-assembly is not the reverse of dis-assembly. (Haynes passim.) :-)

Those things are strong, and pinch flesh badly if mishandled.

Is there a legal maximum you are allowed to have in ownership least you accidently down planes, attract trains and dismantle automobiles?

In message , Vortex3 writes

They do indeed, I needed a magnet to hold a shower cubicle door open last week and had an old HD that was in need of permanent trashing so I opened it and took the magnets out. One of those glued to the door edge and a steel based penny to the wall worked fine. I let the two magnets hold on to each other and had a bit of a struggle to pull them apart!

The job is now done! Not difficult! No electricity needed.

I took a powerful magnet from an old HDD, and with a bit of careful "stroking" (oooer missus) the problem is solved.

Pix here:

This raises the question which is never answered by the 'survive on your own' books which helpfully explain how to magnetise a needle for compass purposes but never tell you how to work out the direction that the needle points. :-)

Richard

Well than ain't hard if you can see where the sun is, and you know which hemisphere you're in ...

Arfa