Usually used for orienteering, it is also a good sensor to detect the presence of a strong magnetic field.

The Earth behaves like a big magnet, generating a magnetic field. The compass is a tool, which comprises a magnetized needle that is free to rotate and that feells the attraction of the Earth’s magnetic field. The needle, alignsing in the direction of the field lines connectings the magnetic North and South poles.

How does a compass work?

The compass is a device that takes advantage of the terrestrial magnetic field to identify the four Cardinal points (North, South, West, East). The Earth’s magnetic field can be understood as that of a huge magnet, as such it attracts objects made of iron. In a compass, a magnetic needle fixed in the center is free to rotate on a horizontal plane and, according to the Earth’s magnetic field, the needle will position itself in the direction of imaginary lines linking the South and North poles: the LINE FIELDS. The compass is a perfect device for orientation. Nevertheless, using it, you should take into account that if nearby there is an object emitting a strong magnetic field, it could interfere with the correct detection of the North Pole. Furthermore, the magnetic poles of the Earth do not correspond exactly with the geographic poles, but are tilted with respect to the latter by about 11 degrees.

Fig.1 Picture of a compass

How to build a homemade compass

If you want to build a compass, you simply need to bring together a needle, a piece of polystyrene (or a cork), a magnet and a pot full of water. Now cut the cork or the piece of polystyrene to obtain a thin circle (washer) that can float on water. Draw the symbols for the 4 Cardinal points (each at 90 degrees) on the washer. Magnetize the needle by rubbing its tip in the same direction about fifty times on a magnet; insert the needle into the cork washer with the magnetized tip pointing out from where you have drawn the North symbol. Put the washer on the surface of the water so that it floats. If you have followed the instructions correctly, the needle will automatically align along the direction of the Earth's magnetic field, pointing North. Tip: the needle tends to demagnetize after a while and should therefore be magnetized again by rubbing it on the magnet every time you want to reuse the compass.

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