Magnetic compass question

In this threadon another board, there is a (tangent) discussion about how accurate cheap compasses are. One poster claiming he has three cheap ones and they "they miss magnetic north by 3, 4 & 7º". Can this really be true? It seems a magnet will point to north regardless of how much it costs.

I can think of three ways it could be off. The printed North direction could be misaligned WRT the actual North of the magnet. The case could have a magnetization that is affecting the magnet’s alignment with North (although the case looks plastic, which wouldn’t have that problem). Finally, the friction between the magnet and the case could keep the magnet from swinging all the way to North.

I am pretty sure that wherever North is painted doesn’t matter since you have to orient the compass.

He’s thinking of one where the compass card itself moves.

They can also be out because of ferrous metals in the area, buildings, iron deposits etc.

Magnetic compasses are a useful device that can get you to roughly the right place but don’t expect them to be accurate to within a degree.

interfering magnetic and electric fields might interfere, but i think the friction between the needle and the swivel would be the prime culprit. those things can’t be magnetized but so much, and relying on the earth’s field to compensate for the friction while getting direction within a degree is probably asking for too much.

The other reason, and an important one is that they may have been manufactured for different lattitudes. The magnetic field lines do not run parallel to the ground. They exit the ground at an angle, and that angle depends upon your magnetic lattitude. Compasses are magnetised so that they allow for this (the term is the “dip angle”). Minimally you need to worry about North versus Southern hemipheres, but a good compass needs better than this. If these cheap compasses have varying dip angles a local purtubation of the magnetic fields lines may mean that they all point slightly differently.

You sure about that? Dip will cause the north end of the needle to point toward the ground in the northern hemisphere and toward the sky in the southern hemisphere but it’s still pointing to magnetic north laterally.

The problem with dip is that it causes the compass needle’s centre of gravity to be out of line with its pivot point which results in errors when accelerating and decelerating. It is an issue in aircraft and is most prominent when heading westerly/easterly. It also causes errors when turning onto a heading. But once stabilised in heading and speed the compass should read as accurately as can be expected given the amount of electronic equipment in an aircraft.

I think you are right. I was thinking of a mixture of effects - including a local purtubation of the field, but the final result still doesn’t make them point differently. I should have worked out the vectors properly, not relied on trying it in my head.