Why do compasses always point in the wrong direction?

[ulrichburke]

Homework Statement

Not sure what to put here - it's not really homework, just something that's always puzzled me, didn't know where else to put it (anatomical details not allowed!)

Relevant Equations

It's a question about compasses....

Dear Anyone.

We're told compasses point to magnetic North because they're attracted to magnetic North, that's why there's an N where the arrow's pointing BUT - if you get two magnets and want them to attract eachother, you have to turn them so their opposite poles are going towards eachother because opposite poles attract, right? So surely the NORTH pole can't be attracting magnets NORTHwards, because opposite poles attract it MUST be the SOUTH pole attracting magnets NORTHwards. So surely the label should be S for South, not N for North as opposite poles attract.

I must be wrong in the above because someone else would've seen it by now if I wasn't! But I don't see where I'm wrong.

And yup - this IS the klutz who posted a 'perpetual motion machine' question without realizing it was one - please forgive me for that and let this question get answered!

Yours hopefully,

Chris.

 

[Ibix]

Originally, a north pole on a magnet was called a north seeking pole, because it turned towards the north. That's why it's labelled as north - it points to the north and labelling it otherwise would be confusing for the most common case where people care about magnetic polarity.

But you are correct that opposite magnetic poles attract. So what kind of magnetic pole must be at the north geographic pole?

Remember that this terminology pre-dates an understanding of magnetism by centuries.

 

[PeroK]

The geographical North Pole represents the south pole of the Earth's magnetic field.

 

[Steve4Physics]

To add to what has already been said, it's as if the earth contains a magnet like this:

From https://www.dreamstime.com/diagram-...field-earth-showing-north-pole-image257473063

 

[kuruman]

To add to what has already been said, it's as if the earth contains a magnet like this:
View attachment 358472
From https://www.dreamstime.com/diagram-...field-earth-showing-north-pole-image257473063

To complete the picture, one must add that a compass needle in a magnetic field aligns so that its "south to north" direction is parallel to the arrow representing the local magnetic field of the Earth. This explains why compasses point (generally) towards the north geographic pole in the northern hemisphere and away from it in the southern hemisphere.

Look at the figure in post #4. Imagine two people with compass needles at the north and south poles. The two needles point in the same direction relative to the Earth. Just by looking at the needle, what direction would each person conclude is "north"?

 

[Steve4Physics]

Look at the figure in post #4. Imagine two people with compass needles at the north and south poles. The two needles point in the same direction relative to the Earth. Just by looking at the needle, what direction would each person conclude is "north"?

It might be worth noting that the answer depends on how the people use their compasses. Compass-users almost invariably hold is their compasses 'horizontally' (i.e. axis of rotation vertical). In this case, at the (magnetic) poles a typical compass-user might conclude that their compass is faulty because it appears not to point in any particular direction.

Burt of course holding a compass 'vertically' provides additional information. From the vertical and horizontal readings (in the absence of local/regional anomalies) it’s even possible to estimate your latitude. But I prefer GPS.

 

[PeroK]

It might be worth noting that the answer depends on how the people use their compasses. Compass-users almost invariably hold is their compasses 'horizontally' (i.e. axis of rotation vertical).

I've spent many happy hours tramping across the Scottish Hills following a compass bearing. It was always reliable.

Back in the 1990s, magnetic north was 5 degrees west of grid north on the OS (Ordnance Survey) maps. So, you had to add 5 degrees going from map to compass. This reduced by about 1 degree every eight years and now the two are approximately aligned.

I still use the compass, although I have a GPS as well. When I get a new phone I'll look at getting the OS mapping app, which is about £30 per year.