compass

when in Sydney compasses came up in conversation. I asked if anyone new the
difference to northern and Southern hemisphere compasses. I was told by the
"I know it all in the group" north is always north its impossible for my
compass not to work". Below is cut from the web
but Im still not 100% happy with answer.

Correction.. you came UP to Australia ) Why compasses are different in
the different hemispheres has to do with spherical geometry. In the Northern
Hemisphere, a small counterweight is placed on the north seeking end [see
below -Pat] of the compass needle such that the needle will be parallel to
the earths surface and perpendicular to a line drawn from the centre of the
earth to the position of the observer on the earths surface. In the Southern
hemisphere the converse applies and a counterweight is positioned on the
south seeking end of the needle. That is why they only work in one
hemisphere.

Why do compasses not work ?

Regards David

> Why do compasses not work ?
here more
The bane of boyscouts, as they attempt to navigate with, or without, the
benefit of their trusty compasses, is the fact that this Pole chooses to
roam about in a 20-mile circle, and to shift its course between day and
night.
This 20-mile variance, however, is not one of global proportions. Modern
sea-farers compensate for the Northern Magnetic Pole's perpetual motion, by
using charts, and tools other than the compass. All things considered, 20
miles is a minor measure for distant travelers to take into account in
adjusting their travel agenda.

Thankfully, the Southern Magnetic Pole spares sailors the navigational
nightmare its Northern nemesis does. In the south, compass needles actually
do point true South, to the South Magnetic field.

david wrote:
> when in Sydney compasses came up in conversation. I asked if anyone new the
> difference to northern and Southern hemisphere compasses. I was told by the
> "I know it all in the group" north is always north its impossible for my
> compass not to work". Below is cut from the web
> but Im still not 100% happy with answer.
>
> Correction.. you came UP to Australia ) Why compasses are different in
> the different hemispheres has to do with spherical geometry. In the Northern
> Hemisphere, a small counterweight is placed on the north seeking end [see
> below -Pat] of the compass needle such that the needle will be parallel to
> the earths surface and perpendicular to a line drawn from the centre of the
> earth to the position of the observer on the earths surface. In the Southern
> hemisphere the converse applies and a counterweight is positioned on the
> south seeking end of the needle. That is why they only work in one
> hemisphere.
>
> Why do compasses not work?

<physicist hat on>

When you use a compass, you are basically allowing a magnitised needle
to line up with the magnetic field lines around it. The needle wants to
line up exactly, in three dimensions. If you put a magnet on a table top
then trace a compass around it, you get an impression of where the field
lines go.

You end up with three distinct sections [assuming a fairly long magnet]:

1. At the top magnetic field lines (direction the compass points) are
diverging (moving apart) away from the pole of the compass.

2. As the magnetic field lines come down the side of the magnet they
will be almost parallel to the walls of the magnet

3. At the bottom the lines will curve back into the pole.

The next thing to do is to imagine this bar magnet embedded in a tennis
ball. (you could just draw a circle on the table). If you look at the
middle of the ball (far from the ends of the magnet), the field lines
will be parallel to the surface. In this case were you to put your
magnetised needle (compass) near the surface, the needle would want to
line up parallel to the surface of the sphere.

Go to nearer the top of the magnet, and you find that the field lines
cut through the surface of the tennis ball. In this case, the magnetised
needle wishes to line up still - but in this case lining up means that
the needle over balances - if it is not allowed to tilt up and down it
may stick. Putting a tiny amount of mass at one end allows the needle to
balance, and swing freely - gravity (unbalanced mass) works against the
downward pull of the magnetic field.

When we go to the other pole, the unbalanced magnetic force is the other
way around - so the weight needs to be at the other end of the needle to
allow it to swing freely.

SO... To make a good NEEDLE compass you must balance the needle
according to the angle at which the earths magnetic field lines cut the
surface of the earth in the area which you intend to use the compass.
Compass manufactures know this, and sell different compasses for
different areas.

[You could also make the compass compartment deeper to allow the needle
to line up].

Here's a question for you:

Is this a problem for the spherical ball compasses you see on boats?

HTH,

Ben






--
Ben Panter, Edinburgh
My name (no spaces)@bigfoot which is a com.

david wrote:
>>Why do compasses not work ?
>
> here more
> The bane of boyscouts, as they attempt to navigate with, or without, the
> benefit of their trusty compasses, is the fact that this Pole chooses to
> roam about in a 20-mile circle, and to shift its course between day and
> night.
> This 20-mile variance, however, is not one of global proportions. Modern
> sea-farers compensate for the Northern Magnetic Pole's perpetual motion, by
> using charts, and tools other than the compass. All things considered, 20
> miles is a minor measure for distant travelers to take into account in
> adjusting their travel agenda.
>
> Thankfully, the Southern Magnetic Pole spares sailors the navigational
> nightmare its Northern nemesis does. In the south, compass needles actually
> do point true South, to the South Magnetic field.

I really wouldn't loose any sleep about this - I get the impression that
the source you took this from isn't that aware of what is going on.

The compass lines up along the field line running from north to south -
so pointing at the south pole is pretty much the same physics as
pointing at the north pole!

A 20mile variation from 5000miles away is such a small angle you would
have trouble recognising it using a compass - 20/5000 = 4/1000 = 0.004
radian, = 0.2 degrees.

HTH

Ben



--
Ben Panter, Edinburgh
My name (no spaces)@bigfoot which is a com.

"david" <im@fedup.com> wrote in message
news:bqa6mh$eb$1@hercules.btinternet.com...
> when in Sydney compasses came up in conversation. I asked if anyone new
the
> difference to northern and Southern hemisphere compasses. I was told by
the
> "I know it all in the group" north is always north its impossible for my
> compass not to work". Below is cut from the web
> but Im still not 100% happy with answer.
>
> Correction.. you came UP to Australia ) Why compasses are different in
> the different hemispheres has to do with spherical geometry. In the
Northern
> Hemisphere, a small counterweight is placed on the north seeking end [see
> below -Pat] of the compass needle such that the needle will be parallel to
> the earths surface and perpendicular to a line drawn from the centre of
the
> earth to the position of the observer on the earths surface. In the
Southern
> hemisphere the converse applies and a counterweight is positioned on the
> south seeking end of the needle. That is why they only work in one
> hemisphere.
>
> Why do compasses not work ?

The Earth's magnetic field is actually toroidal or doughnut shaped, with the
"hole" of the doughnut running from the north magnetic pole to the south,
hence the field lines do not run parallel to the Earth's surface but dip
into it. The amount of dipping or declination is large at the poles and
virtually zero at the equator so in theory your web expert is correct that
there is a tendency for the compass needle to dip down. In practice, of
course, the declination is insignificant except at the poles and can be
ignored for most practical purposes.

Bear in mind too, that a compass needle is not just an inert piece of steel
with a single, active, north seeking end; the whole needle is actually a
small magnet with both a north pole and a south pole, with both ends are
being attracted in opposite directions so the declination will tend to
balance out. As to counter balancing, this is achieved by having the needle
pivot set slightly above the plane of the needle so that as the needle dips,
its centre of gravity is shifted before or behind the pivot thus providing a
restoring force.

If this does not make sense, no doubt someone better qualified than I will
explain.

Cliff.

david wrote:
>> Why do compasses not work ?
> here more

Grief. Two posts on the subject and both show that you'll believe anything.
I must sell you something.

Yes, a compass in the north of the northern hemisphere does tend to tip down
due to the 'dip angle' and vice versa. Fortunately the people who make the
things know this and displace the pivot above the centre of mass of the card
and all is well. If they did it by weight the weight would need to be adjusted
by latitude so a Spanish compass would not work in Scotland and a ship
would be stuffed when away from home. Even if a dive compass was a bit
sensitive to the dip angle would it matter? You hold it in your hand so it turns
freely so you are compensating.

'Mag var', the error on a compass at a locality, is recorded on charts. It's never
much and since, when diving we are doing out-and-back or X degrees from Y
we really don't care how much it is.

Compasses don't work in wrecks, unless they are very old and made of wood
and at Wraysbury. Even in wooden wrecks you can find pig iron ballast making
them go round in circles and Wraysbury.... well I think that Richard just annoyed
the Gods of Diving somehow that they did it just to stop him running navigation
courses.

nigelH

"Nigel Hewitt" <nigelh@REMOVETHISnigelhewitt.net> wrote in message
news:bqacgg$17u$1@sparta.btinternet.com...
> david wrote:
> >> Why do compasses not work ?
> > here more
>
> Grief. Two posts on the subject and both show that you'll believe
anything.
> I must sell you something.



I was not losing a lot of sleep over it But did not want ask a question
with out looking first and the second you press send you find a better or
different answer. When you read it back off the net you see all of the
spelling mistakes and bad grammar.

Thanks to all.

david wrote:
> "Nigel Hewitt" wrote
>> david wrote:
>>>> Why do compasses not work ?
>>> here more
>>
>> Grief. Two posts on the subject and both show that you'll believe
>> anything. I must sell you something.
>
>
>
> I was not losing a lot of sleep over it But did not want ask a
> question with out looking first and the second you press send you
> find a better or different answer.

Thank you for the smilie.... I admit I'm a bit crunched after the flight
back from Hurgarda. I'll be nicer tomorrow but today I hate everybody
and everything.

nigelH

> The bane of boyscouts, as they attempt to navigate with, or without, the
> benefit of their trusty compasses, is the fact that this Pole chooses to
> roam about in a 20-mile circle, and to shift its course between day and
> night.

And occasionally (not in any of our lifetimes I don't suppose, can't
remember how regular it is - certainly the thousands of years) it decides to
flip completely, so N=S, and S=N, just to confuse people... apparently! No
idea where I got that from though!

David

"Nigel Hewitt" <nigelh@REMOVETHISnigelhewitt.net> wrote in message
news:bqacgg$17u$1@sparta.btinternet.com...
> Yes, a compass in the north of the northern hemisphere does tend to tip
down
> due to the 'dip angle' and vice versa. Fortunately the people who make the
> things know this and displace the pivot above the centre of mass of the
card
> and all is well. If they did it by weight the weight would need to be
adjusted
> by latitude so a Spanish compass would not work in Scotland and a ship
> would be stuffed when away from home. Even if a dive compass was a bit
> sensitive to the dip angle would it matter? You hold it in your hand so it
turns
> freely so you are compensating.

Nigel you are correct when talking about ships compasses and aircraft
compasses in which the suspension point can be placed well above the centre
of gravity of the card - which is why ships compasses are bowl shaped. A
ships compass will therefore work adequately anywhere below latitudes of
about 80 (at higher latitudes the dip can cause the compass to flip 180 if
the boat rolls beyond a certain amount on an E-W heading) though once you
get into the 7 microteslas region it becomes totally unuseable because the
"horizontal" component of the earth's magnetic field becomes too weak. But a
typical diving or orienteering compass is fairly flat and compact, so the
suspension point and CG of the card are virtually the same - so they do have
to be counterweighted to work when held "level" in a given hemisphere. A
typical diving or orienteering compass therefore only works well in the
hemisphere for which it is designed - though as you say you can get them to
sort of work in the opposite hemisphere by judicial use of the hand.

I've never used one, but I suspect those rather expensive flux-gate diving
compasses are able to cope with either hemisphere.