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Modern ships are built from metal alloys and so have the potential to become magnetised. In fact the simple acts of hammering, welding and any vibrations during construction or repairs whilst the ship is sitting in the Earth’s magnetic field, causes the boat to become permanently magnetised. The ship itself behaves like a big bar magnet!
In addition to the ship having permanent magnetism, further magnetism is induced just by the ship being in the Earth’s magnetic field. This induced magnetism may add to or subtract from the permanent magnetism of the ship depending on how the ship is aligned with the Earth’s magnetic field.
If you have a magnetic compass near a permanent magnet, the magnetic field around the magnet is likely to be much stronger than the Earth’s magnetic field and therefore will control where the compass points. As the ship is a permanent magnet it will therefore have an effect on the ship’s magnetic compass causing it to point in the wrong direction!
Ship’s compasses therefore have to be corrected so they will point the right way. This is done by placing them near other smaller magnets which will pull the compass needle back into line. Remember that a magnetic field is stronger when the magnetic field lines are closer together, and this happens near the poles of a magnet. Therefore small magnets close to the compass will have a greater effect than large magnets (such as the ship) further away.
There are many ways of correcting a ship’s magnetic compass, and making it point in the required direction, but the main two are: a small magnetic rod called a ‘Flinder’s Rod’ which hangs vertically underneath the compass and helps to correct variation and ‘Kelvin’s Balls’ which are iron balls that sit either side of the compass which can reduce most other compass errors. You can see Kelvin’s Balls in the photo of the Aurora Australis’ magnet compass binnacle (compass stand and casing) found on the ship’s monkey island.
To demonstrate how permanent magnets affect a magnetic compass, take a small magnetic compass and place a bar magnet near it. Students will be able to observe (and draw if required) the deflection in the direction the compass points. Moving the bar magnet will change the direction the compass points in.
To simulate the use of Flinders Rod and Kelvin’s Balls get a second permanent magnet and ask students to place it around the original compass and magnet to make the compass point north again. This activity can also be used to demonstrate to students how distance from a magnet affects the strength of the magnetic field.
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