# Magnetism and compasses

1. May 11, 2005

### 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?

2. May 11, 2005

### Gokul43201

Staff Emeritus
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...

3. May 12, 2005

### 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 alot 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.

4. May 12, 2005

### DaveC426913

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

Yes would appreciate some corroboration of this effect.

5. May 12, 2005

### DaveC426913

Wow. That's a lot.

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

6. May 12, 2005

### 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.

7. May 12, 2005

### chroot

Staff Emeritus
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

8. May 12, 2005

### zoobyshoe

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

9. May 13, 2005

### Gokul43201

Staff Emeritus
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).