They could just have easily marked the south ends of compass
needles "North". They would then be "north-seeking" in both senses,
not just one.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Except the "north" end of compass needles and magnets had been determined, by convention, for centuries before physicists got involved. The north pole of a magnet was the one that pointed towards geographical north.
Then we discovered that the Earth had magnetic poles. We could either relabel all the magnets in existence, and seek out all navigators everywhere to explain the problem, or just accept that there is a south magnetic pole in the northern hemisphere of the Earth. I think the decision was probably an easy one to make.

Grant Hutchison

Compass needles were labeled "north" and "south", yes, but had
magnets in general been labeled that way?

No need to tell the navigators anything. They could continue
marking the end of the compass needle that points north the same
way as ever. They could continue to call it the north end of the
needle, since it would continue to be "north-seeking" for the same
reason it always had, plus the new reason that it would now be
seeking the magnetic north pole of the Earth.

Interesting side topic, but just a side topic. I need to know how
the direction of the magnetic field in electromagnetic radiation is
determined.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

They had to be, since soft iron compass needles lost their magnetism quite readily, and had to be periodically remagnetized by stroking with a chunk of lodestone. So navigators needed to know which way round to hold the lodestone to avoid reversing the field of their compass needle. When you bought a lodestone, you wanted the north end marked.

If only you had been there. Nowadays we could be explaining to children that the north end of a compass needle is a south magnetic pole, rather than explaining to them that the north end of the Earth is a south magnetic pole.

Grant Hutchison

Well put Grant. And don't forget that the navigators would have to be told to rub the compass point with the "N" on the lodestone end marked "S". That is certainly a possible solution to the whole thing, no one is disputing that there is indeed some logic in it, but it's still a question of changing one thing (the labels of Earth's magnetic poles) or everything else-- as you point out.

You are correct.

The old custom of putting the “N” on the north-seeking end of a compass face or needle was to clearly and unequivocally show the old uneducated sailors which direction geographical North was on the earth. That north-seeking end of the needle is actually the south pole of a compass-needle magnet, but that doesn’t matter, and sailors don’t need to know that.

What the sailors needed in the old days, and now too, is a compass that quickly points to the geographical North Pole of the earth, which is also the magnetic North Pole. They don’t care about the technicalities of magnetism.

It would serve no purpose whatsoever to try to teach sailors that the south pole of a compass needle points toward the North Pole of the earth, and the north pole of a compass needle points toward the South Pole of the earth, so it would have made no sense at all to put an “S” on the North Seeking end of the needle.

This is what I was referring to, albeit opaquely, when I wrote about having to "relabel all the magnets in existence, and seek out all navigators everywhere to explain the problem". Since labelled magnets were used to make compass needles, since about the only reason to own a magnet was to make compass needles, and since labelled magnets were bought and carried aboard ship for this purpose, one really would have to explain to navigators what was going on.
The alternative, I suppose, would be to maintain a trade in lodestones labelled according to the sailors' usage, and another small trade in lodestones labelled according to Jeff's convention, for strictly scientific purposes. The disaster-in-the-making is easily detected.

Anyway, if anyone is still wondering, it's easy enough to check which convention is in actual use, by repeating the old school experiments with a labelled permanent magnet and a compass needle. The north-seeking end of a compass needle demonstrably orientates itself towards south magnetic poles.

Grant Hutchison

Quite. I think Sam5 needs to reread Grant's posts above to understand the actual conventions chosen, and why they are perfectly natural conventions to choose. The only objection comes if you want a geographic north pole to also be a magnetic north pole, which is all well and good, except that it means that the compass needle ends with the N marking are now south magnetic poles and need to be rubbed on S ends of lodestones. I rather think that would have confused those sailors, even if they didn't "care about the technicalities", more than the relatively minor "problem" of having a south magnetic pole at our north geographic pole.

A related question is, when the Earth's magnetic polarity eventually reverses, and all the compass needles need to be changed to have the N switched to the other side, will it make more sense to swap our meaning of north magnetic polarity such that the Earth still has a south magnetic pole at the geographic north, but at least the N on the compass is a north magnetic pole, or will it make more sense to leave the polarity convention unchanged and just accept that the N on a compass needle is a south magnetic pole, and needs to be rubbed on the S end of the lodestone? Presumably the latter, so that "old" and "new" magnets are the same, but people needing to rub the "N" against the "S" will cause a lot of head-scratching I'm sure.

And this will tell you which way it's "poynting"?




(I'm sorry. Feel free to hate me.)

__________________
Gimme a minute to read through Jay's latest observations...

On the convention regarding magnetic poles:

The reason for calling magnetic poles "north" and "south" is that the ends
of a free-moving magnet point toward Earth's north and south magnetic
poles, and they do that because the Earth is itself a magnet. The north
pole of one free-moving magnet points toward the south pole of another
magnet. So logically, the end of a compass needle which points toward
Earth's north pole should be called the needle's "south pole". However,
since people marked the north-pointing end of compass needles for the
direction they point rather than for their polarity, the north-pointing end
of the needle came to be called its north pole instead of correctly being
called its south pole. So compass needles point toward the opposite pole
of all magnets except the one which established the convention, which is
the only case where the convention doesn't apply.

Is Jupiter's north magnetic pole defined by its magnetic polarity or by its
proximity to the planet's rotational pole? How about the Sun's magnetic
north pole? This year or next?

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Don,

You are an idiot if you think anyone would hate you for expressing the
stupid pun that everyone thinks and probably almost everyone blurts out,
given an opportunity. I'm just a little bit miffed that you didn't give me a
clear and authoritative explanation of the direction of the magnetic field
relative to the electric field in electromagnetic radiation!

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

John Henry Poynting is rolling over in his grave, but that is indeed a curious coincidence in words that has baffled many a young physics student!

__________________
Physics doesn't predict the future, it predicts the past that hasn't happened yet.

There are two kinds of delusions, the obvious kind that clearly don't work, and the insidious kind that clearly do.

I think Grant was trying to explain why this is not at all the "logically" required conclusion, and indeed is not the convention used, because you made the implicit and incorrect assumption that a magnetic pole at Earth's geographic north should be a "north magnetic pole." Instead, the meaning of a magnetic "north pole" is "that pole of a magnet that points north in the Earth's field". That is a perfectly "logical" convention, no inconsistency there whatsoever. It also has the side effect of meaning that a copy of the magnetic pole at the Earth's north geographic pole would point south, and is therefore a "south magnetic pole". That is only an inconsistency if you think a pole at the north has to be a "north magnetic pole", but as I said, the actual meaning of a "north magnetic pole" is a pole that points north. There is no problem with that convention, and no aspect of it needs "fixing".

The convention that electrons have a negative charge, in contrast, is indeed a poor convention, because of our desire to think in terms of positive, rather than negative, numbers. Hence the fact that the electron is the mobile charge carrier implies that it should have been given a positive charge, to avoid the inconvenience of dealing either with currents that are negative numbers, or which flow in the opposite direction as the charge carrier.

__________________
Physics doesn't predict the future, it predicts the past that hasn't happened yet.

There are two kinds of delusions, the obvious kind that clearly don't work, and the insidious kind that clearly do.

Oh, I forgot in my post just above about the convention on magnetic pole
naming to say that I find it hard to believe that there was a gap between
the realization that one end of a free-moving magnet points north, and the
realization that opposite poles of magnets attract. If anything, I would
expect that the fact that opposite poles attract would have been noticed
long, long before anyone noticed that one of those poles always pointed
north. That is why I think there was no excuse for making the convention
backwards from the way it should have been.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

I was getting confused with all the pole shifting in the thread (no surprise there), so I took a look at Wikipedia. Here's how they explain the issue:

But then they wouldn't point north anymore...

__________________
"All your bias are belong to us." Ara Pacis
"A witty saying proves nothing." Voltaire

Disinfo Agent's quote explains this nicely. The meaning of a "north magnetic pole" was taken for simplicity to mean a pole that "points north". It is only if one implicitly requires that the magnetic pole that resides in the north be a north magnetic pole that one arrives at your "logic", that's the entirety of your argument. I see that as one perfectly acceptable possibility, with advantages and disadvantages, just as I see the actual convention of "a pole that points north" as a perfectly acceptable meaning for a "north magnetic pole". I see no "should" coming from these simple conventions, for the simple reason that yes, it might be nice to have a north magnetic pole in the north, but not at the expense of having to explain to generation after generation that "north magnetic poles actually point south". Sure it makes sense since opposite poles attract, but conventions are for people who use them, not necessarily physicists, and most people just want to use a magnet to find north. For such people, it makes more sense to just let them use the north pole of a magnet to find north, exactly as it is currently done.

__________________
Physics doesn't predict the future, it predicts the past that hasn't happened yet.

There are two kinds of delusions, the obvious kind that clearly don't work, and the insidious kind that clearly do.

Ken, they always have been the south magnetic poles of the compass needle ends that point in the direction of NORTH on earth.

The whole “N” labeling concept started when Europeans began calling the North Pole of the earth, “NORTH”, and when they began making compasses.

The earth direction called "North" in English is called Norté in Spanish, Norté in Portuguese, Nord in German, Nord in French, Nord in Italian, and Noorden in Dutch. I think you can see why it became a tradition to stamp a big “N” on the end of a compass needle that points NORTH at the surface of the earth.

The ends of all compass needles that seek North on the earth should be marked “N”, meaning “This needle points in the direction of North on earth”. It is the bar magnets that should be labeled “S", along with this additional note on a label: "(Note: This is the south pole of this magnet, which seeks and points toward the North Pole of the earth if suspended from a string or floated in water).”

With all the new government requirements for labels on products today, maybe the manufacturers can add this kind of label to all bar magnets that are sold to schools and universities.

See this:

Britannica, “Great Books of the Western World”, Volume 28:

“On the Lodestone and Magnetic Bodies”, William Gilbert, AD 1600:

On floating a lodestone in a vessel placed in a bowl of water:

“...there the stone, as if in a boat floating in the middle of an unruffled surface of still water, will straightway set itself, and the vessel containing it in motion, and will turn in a circle till it’s south pole shall face north and its north pole, south.”
http://i37.tinypic.com/2nr0mm9.jpg