## Sunday, March 11, 2012

### A Compass Puzzle

So I'm here for a week in Australia at the memorial conference for Alf van der Poorten.

I can tell it's really Australia because they are selling Kellogg's "Rice Bubbles" in the supermarket instead of Rice Krispies.

But there's another way to tell that it's the Southern Hemisphere, since I have a compass with me. Can you figure out what it is?

Dave Empey said...

By the magnetic inclination?

Anonymous said...

You're not stopped by airport security and given a full body search for carrying a compass?

Anonymous said...

In the Southern Hemisphere, compasses from the Northern Hemisphere point South.

Jeffrey Shallit said...

Anonymous is wrong and Dave Empey is right.

Many people don't realize that a compass needle that is free to move up and down actually dips into the Earth, with the dip depending on latitude. To counteract this, in most compasses the needle rests on a pin at a position slightly off center. If you buy a compass intended for use in the Northern Hemisphere, and then take it to the Southern Hemisphere, it will no longer be level, making it much harder to use.

My nice Southern Hemisphere Silva compass, bought on my last trip to Australia 20 years ago, finally works correctly again!

Wait, wait, I think I've heard this riddle before. Give the compass to the grocery superintendent in exchange for telling you which hemisphere you're in?

Miranda said...

You flush your compass down the toilet and see if it goes in the opposite direction.
(Yes, I'm just kidding.)

Luke Barnes said...

Strangers on the street say "mate, noice compass, mate."

I thought the answer was going to be that the sun is in the north at midday. Your answer is much more interesting.

Dave Empey said...

Interesting. I had the right physical principle, but the wrong implementation in mind. I would have suggested finding north with the compass and then turning it on its side to try to determine inclination. I didn't realize you had to weight the compass to compensate. I should have thought about it more.

Anonymous said...

Why don't you either buy a phone with a decent camera or stop posting s%it pictures?

Miranda FARTED said...

"Miranda said...
You flush your compass down the toilet and see if it goes in the opposite direction."

Why am I not surprised that you love a good toilet joke, Fart Queen?

Jeffrey Shallit said...

Anonymous:

Why don't you read another blog?

George said...

It is a matter or balance to the magnetic fields.

George said...

Gosh! "a matter OF balance" not "OR" sorry.