How does the presence of ferrous material affect a compass's accuracy?

[DaveC426913]

As a diver, I'm constantly struggling with accurate compass readings.

How much ferrous material at what distance would affect a compass? For example, would fifty pounds of steel two feet away affect the needle?

If the needle of a compass is magnetic, will the needles on two compii affect each other? At what range? Inches?

 

[Gokul43201]

How much ferrous material at what distance would affect a compass? For example, would fifty pounds of steel two feet away affect the needle?

It depends on the bearing of the steel object with respect to you. If it is due north or south, the effect will be smaller, and will possibly go unnoticed by the human eye. Will do a rough calculation if I find more time...

 

[zoobyshoe]

I have a pair of common needle nose pliers which I can see affects my compass visibly starting at about a foot away.
Safe bet, then, that 50 pounds of steel would throw a compass off a lot at two feet.

I also have two compasses. Placed next to each other east to west, I see no deflection. The more south or north one or the other is, the more they both vere from magnetic north and try to align with each other.

 

[DaveC426913]

It depends on the bearing of the steel object with respect to you. If it is due north or south, the effect will be smaller, and will possibly go unnoticed by the human eye. Will do a rough calculation if I find more time...

The point though is that it will render any readings unreliable.

Yes would appreciate some corroboration of this effect.

 

[DaveC426913]

I have a pair of common needle nose pliers which I can see affects my compass visibly starting at about a foot away.
Safe bet, then, that 50 pounds of steel would throw a compass off a lot at two feet.

Wow. That's a lot.

I also have two compasses. Placed next to each other east to west, I see no deflection. The more south or north one or the other is, the more they both vere from magnetic north and try to align with each other.

Also wow. I assume when you say next to each other, you mean < ~3".

 

[DaveC426913]

There is no way to mitigate this effect is there? (I mean, other than the obvious: remove the offending metal, or at least balancing it.)

So divers using a compass are fighting a losing battle. I guess using an aluminum tank would be a definite advantage.

 

[chroot]

The compass is indeed not very accurate for a diver swimming around on a wreck. Divers typically only swim a short distance on a compass heading, though, so even a fairly large error won't put a diver more than a few feet from the intended destination. The compass is still an indispensable tool for a diver who needs to do nav.

- Warren

 

[zoobyshoe]

Also wow. I assume when you say next to each other, you mean < ~3".

I mean the compass housings are touching. One compass is two inches in diameter, the other, an inch and a half.

 

[Gokul43201]

With excellent experimental results (thanks zoob), why do a calculation, eh ?

Going purely on intuition (when you can't calculate, you intuit ), zoob's results look reasonable : a tiny compass needle, a couple of inches away will have much less effect than 50 lbs of steel a couple of feet away if the needle weighs 10 grams or less (this from the r^{-3} approximation of a dipole field).