magnetic compass

wailedcentipede · Posted: Tue Feb 13, 2007 12:20 pm Post subject: magnetic compass

on a magnetic compass which end of the needle is pointing north ... other than the pointy end . wc

Sea Wolf · Posted: Tue Feb 13, 2007 12:37 pm Post subject:

wailedcentipede-

In magnets, opposites attract, so it is actually the South end of the needle that points toward the North Magnetic Pole.

How much did you win on the bet?

Joe

SEA3PO · Posted: Tue Feb 13, 2007 1:49 pm Post subject:

So yer saying the pointy end is the negative polarity ??

Joel
SEA3PO

Captains Cat · Posted: Tue Feb 13, 2007 2:07 pm Post subject:

Yep, just to put both + ends of two magnets together. They repel each other! They put the "arrow" on the negative end. You can call it the south end if you like but it's the one that points toward the magnetic north pole!

Charlie

Sea Wolf · Posted: Tue Feb 13, 2007 2:17 pm Post subject:

Joel-

Yup, the pointy end is the South Pole of the needle, the other, of course, the North Pole.

Since opposites attract in magnetic fields, the pointy end South Pole points toward the Earth's Magnetic North Pole out there between Greenland and Canada.

Since this magnetic pole is not on top of the Earths North Pole, the difference is Compass Deviation, 17 degrees in Northern California (Bay Area).

Electronic compasses are set for the local user to correct for the deviation to give bearings in what we call True North. These compasses detect the Earth's magnetic field by using coils set at right angles to each other to find (calculate, really) the bearing from the difference in signals set back from the coils.

With a conventional magnetic compass, you have to use the local charts to get the Magnetic Variation for where you are, and then add or subtract this from the magnetic bearing.

You see the Compass Rose on charts, with the round Compass Rose pointing True North, and a drawn compass needle inside pointing to the Magnetic North Pole, and the deviation indicated thereby and noted in writing.

GPS systems calculate True North as well as position from the signals recieved from satellites.

Probably more than you wanted to hear, but oh well...........! (Still get locked in the "teacher mode" w/o thinking sometimes!)

Joe.

MOOSE · Posted: Tue Feb 13, 2007 6:00 pm Post subject:

wailedcentipede · Posted: Tue Feb 13, 2007 6:38 pm Post subject:

to darn many teachers on this site ... Joe hit it right on ... trying to snag some one in Kansas they all drive tractors and they don't come with a magetic compass .... the pointy end was my answer .. lol ..

thataway · Posted: Tue Feb 13, 2007 7:02 pm Post subject:

But don't forget that every few thousands of years the poles switch polatirty. (Estimates from 3000 to a million years--generally accepted number is 7000 years) The magnetic poles are moving, and that there is enough dip error, that a compass built for most of our United States Lattitudes will not work well in other areas. As said the compass tends to follow magnetic lines-but there is also the dip error--that is that the pole is not on the surface of the earth. As one goes further South in Lattitude, a different magnetic compass must be used. There are 6 zones for compasses for the world. The US, Europe, and Asia are all in Zone 1. Panama is in Zone 2. Most of the time a compass can be used with reasonable accuracy in the adjacent zone. But a Norther American compass, will not be reliable in New Zealand. A fluxgate compass somewhat bypasses the lattitude rules, but not entirely the dip error. Magnet Compasses are built for our North lattitudes will not work properly in New Zealand. Danforth has recently developed a balanced compass which can be utalized in all parts of the world.

cmdnav · Posted: Tue Feb 13, 2007 10:22 pm Post subject:

Sea Wolf wrote:

"Since this magnetic pole is not on top of the Earths North Pole, the difference is Compass Deviation, 17 degrees in Northern California (Bay Area)."

Just to clarify, the difference between magnetic and true direction is variation. This can be read on the chart from the center of the compass rose along with any annual change.

Deviation is the difference between magnetic and ship's compass direction on a particular vessel at a particular heading. It is caused by all of the magnetic "stuff" onboard including speakers, electronics, alternators etc. and their relative relationship to magnetic north. This could be different even among the same model of boat. To determine deviation one would "swing ship" and create a deviation card. This is explained in Chapmans. Or they could hire a compass adjuster to do this for them.

You may remember the "True Virgins Make Dull Companions" memory aid when discussing the computation from true to compass direction.

To see a good example of unwanted compass error, place your cell phone near the compass. Many phones or even car keys can offset the compass 30 degrees or more.

Chris

Sea Wolf · Posted: Tue Feb 13, 2007 10:52 pm Post subject:

Holy magnetotron!

Thanks for all the great details!

And I was working hard not to get too detailed and give a "snow job" lecture!

Chris-Thanks for correcting my error on Variation and Deviation. I shouldn't have lapsed and confused the two, having lived so close to that phenemon in San Francisco. Believe me, I know the difference between the two!

"You may remember the "True Virgins Make Dull Companions" memory aid ............. etc." NOW YOU'RE MAKING SOME SENSE!!!

Y'all keep your magnetic shield deflectors up!!!

Joe.