This type of compass requires a cork, a sharp knife, a board, a steel pin, a magnet, a glass and some water. This method is often described on poorly researched websites, and as most of them describe it, their method does not work, but this one does.
The surface of water in a glass is always
curved. Here, the meniscus bulges up.
There is a fix which you won’t understand until you read something that will come later. (In coded form for older readers, the difficulty lies in surface tension.) Briefly, when you have a partly-filled glass of water with a bit of cork floating on it, the water near the sides of the glass is higher, which makes the outside edge of the surface curve upwards.
As a result, the cork
drifts across and sticks to the side, but if you over-fill the glass, you get a
bulging-up surface, and when you float a cork disc on this surface, it will
stay at the highest point, in the middle. The physics behind this movement is
simple and beautiful. Play with it!
First, read Making a weak magnet. Then magnetise the pin
by stroking it with the magnet, always moving the same end of the magnet the
same way along the pin. Cut a disc of cork, fill (and then over-fill) the glass
with water, and sit the disc of cork on the water. Put the magnetised pin on
the cork, and your compass is ready to go, though it isn’t very portable…
Note: as you stroke the pins, always in the same direction, some of the random magnetic domains in the pin are flipped over, making the whole of the pin a weak magnet.
Explanation: when you add more water to the glass, surface tension pulls all of the water molecules together, forming a sort of “skin”, and the cork will always float up to the highest point, thus letting as much dense water as possible flow down to the lowest point, making a more stable arrangement.
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