## Sunday, 16 June 2013

### Science: Make a compass

This is the simplest way to make a compass that I've found...

All you will need is...

• A washing up bowl
• water
• sticky tape
• a circle of Styrofoam, we used the one from pizza packaging (such a good excuse for pizza)
• a bar magnet with the north and south poles marked on it.

All you have to do is stick the magnet onto your Styrofoam circle then fill your bowl with water and let the circle of Styrofoam float on top as in the photo.  You will notice that the circle will turn one way then the other until it settles.  Once it has settled you will find your poles.

Here is another way to make a compass...

This time you will need:
• A plastic bottle top (or a small circle of Styrofoam or cork will work too)
• A needle
• A magnet
• A bowl
• water
• Double-sided sticky tape

Don't ask me why there's a permanent marker in the photo above, you do not need it.

First you put a bit of your double-sided sticky tape on the bottle cap like so...

Then take off the backing.

Then you need to make your needle into a magnet.  You do this by rubbing your magnet across your needle in one direction, from the eye of the needle to the point of the needle (never back again) like this...

Then take your pin and stick it to the sticky tape on the plastic cap and place it in the water...

Then let it go...

It will spin about for a bit but eventually point you in the direction of North!

Why does this work?

No matter where you stand on Earth, you can hold a compass in your hand and it will point toward the North Pole. How completely cool is that!  Say you are the middle of the sea, you are looking all around you and all you can see is water, to top that, due to a overcast weather, you can't see the sun.  How in the world would you know which way to go?   You use a compass!

A compass is an very simple device.  A magnetic compass which is the sort we have made here consists of a small, lightweight magnet balanced on a nearly frictionless pivot point. The magnet is generally called a needle. One end of the needle is often marked "N," for north, or coloured in some way to indicate that it points toward north. On the surface, that's all there is to a compass.

The reason why a compass works is more interesting. It turns out that you can think of the Earth as having a gigantic bar magnet buried inside. In order for the north end of the compass to point toward the North Pole, you have to assume that the buried bar magnet has its south end at the North Pole, as shown in the diagram above.

If we think of the world this way, then the opposites attract rule of magnets would still apply and therefore cause the north end of the compass needle to point towards the North, reaching out to the south end of the bar magnet within the earth.  This is what makes our compass needle point toward the North Pole.

Another interesting fact about how a compass works is that the bar magnet inside the earth doesn't run along the Earth's rotational axis as we see in the diagram.  This off-linement with the rotational axis is called the declination, a line that changes a little depending on where you are on the planet.

The magnetic field of the Earth is rather weak on the surface. this is because the planet Earth is 7918 miles in diameter, so the magnetic field has to travel a long way before it has an affect your compass. This is why the compass needs a lighweight needle with a frictionless bearing (hence why we use water in a bowl).

If you want to learn more about magnets or try some other experiments you can check out my science page here.

#### 1 comment:

1. Magnetic compass adjusting and plotting of deviation card.
Magnetic compass surveys are carried out and large errors are corrected on site, with remnant errors, a deviation card is issued which needs t be displayed on board vessel.