Reversing the polarity of a magnetic compass

[Rod]

I was thinking that - but remembered that thing called 'night', especially 'moonless night with no stars'.

[Harry Bloomfield]

Arfa Daily expressed precisely :

Or can see which side the moss is growing on the trees, again assuming you know which hemisphere you are in.

[ARWadsworth]

Can you not just empty a sink to decide which hemisphere you are in?

Adam

[PeterC]

yebbut, then you need to know which way up you are.

Not of use in 'wilderness', but TV dishes tend to point towards the Equator; in the UK they're about 20 - 30 deg. E of S.

[Graham.]

No, paradoxically the strongest magnets you are likely to encounter are from the head actuator of a hard-drive. The ring magnets from oven magnetrons aren't in the same league. Trust me on this, I have a pair of each on the table as I type this!

[Andy Dingley]

It's not good for 200A into an inductive load though.

It's not ampere turns, as suggested, unless you have a true solenoid (or good approximation). That means a _long_ solenoid, which means yet more inductance. You're better with a coil that's only 3 - 4 times the length (if I felt like the maths, I'd work it out properly) as that's the maximum of efficient magnetic behaviour (i.e. most useful flux through the needle) vs. fall off in achievable current through the circuit.

[dennis]

A battery is DC! After a few milliseconds 200A into a coil is easy.

[Matty F]

It's easy for me. At night I just look for the Southern Cross. That tells me accurately where South is. I don't need a compass :)

[The Natural Philosopher]

200A is 200A, no matter waht its into.

Its ampere turns whatever you have.

The "*constant*" varies with the length of the solenoid, that's all.

[Fred]

I forgot about that part!! Floppy disc drives have a similar same open type motor as presumably do CD drives.

[Fred]

The effect of any inductance is soon lost when driven with a DC voltage, where after a short time any coil looks like a resistance!

[Mike Clarke]

All the floppy drives I've pulled apart use a little stepper motor and screwed shaft to drag the head slowly across the disk.

[geoff]

Then you should stay indoors in case the ghoulies get you

[Adrian C]

Er, Nope. This is not the motor that spins the disc. In a hard disk, it's the actuator for positioning the head.

Top left hand bit of the second picture on

a look at the pictures that David eventually put up, or take a dead thing to bits yourself. :-)

[ARWadsworth]

I can't help thinking is must have been a good party if you wake up in the morning with no idea on which hemispere you are on and you need to find your way north.

Adam

[Rod]

I couldn't help wondering, what colour was the bear?

[PeterC]

the bear what?

[Matty F]

on

> Have a look at the pictures that David eventually put up, or take a dead

Just use a compass to find the magnet. I believe the Silva brand is good for that :)

[Arfa Daily]

What, so a starter motor is not an inductive load then ? Of course a car battery can drive 200 amps through an inductive load. It will take a few mS for the current to ramp up to that level, but providing that everything holds out for long enough, and the DC resistance of the winding is low enough to allow it, it *will* get there.

Arfa

[Man at B&Q]

A compass won't be much use if you can't see where you are going. Wait until sunrise.

MBQ