4.11 the Magnetic Compass

4.11 THE MAGNETIC COMPASS

As you have already learnt, when a magnet is free to swing, one end always points towards the north. This property of magnets is used in the magnetic compass.

A simple magnetic compass has a magnetised pointer that swings on a pivot at the centre of a Magnetic variation circular card. The card is marked with the directions North, East, South and West. These A magnetic compass does not point exactly are called the cardinal points of the compass. north. The difference between true north and The compass may also be marked with degrees the magnetic north on a compass is only a few from 0o to 360o. The degrees corresponding to degrees. This angle is called is called the each direction are shown in the table below. To magnetic variation. Magnetic variation varies use the compass, the North point on the card slightly from place to place. It also changes must always be lined up with the pointer. slowly from year to year.

In a ship's compass, magnetic needles are fixed to the underside of the compass card. The card rests on a pivot at the centre, and floats in a liquid in a sealed container with a glass cover. The liquid keeps the compass card level and steady, even when the ship is thrown about in a rough sea. There is a line on the glass cover called the lubber line. The lubber line points in the same direction as the ship. The reading of the compass card against the lubber line shows the direction in which the ship is travelling.

Direction Degrees North 0o East 90o South 180o West 270o A pocket compass (The compass must be rotated so the ‘N’ on the compass card is under the compass Magnetic compasses needle that points north) A ship’s compass are used by sailors, explorers and map makers. The compass tells them in what direction they are travelling. It can also tell them the direction, or bearing, of important landmarks such headlands or high mountains. Compasses are still used in this way, but now anyone with the right kind of radio can Natural magnets find exactly where they are with satellite navigation systems. Magnets were used in China more than 2000 years ago and in North Africa soon after that. The magnets they used were lodestones, which occur naturally. We now know that lodestones are made of a substance called magnetic iron oxide, or magnetite. The Chinese and the North Africans knew that, when a lodestone was free to swing, one end always pointed to the north. They used lodestones to make simple magnetic compasses to help them steer their ships far out of sight of land.

 1. The person in the sketch above is taking a bearing  2. Give each of these directions in degrees: (i) North- of a church on a hill. (i) What does taking a bearing east, (ii) South, (iii) South-west, (iv) North-west. mean? (ii) Why will the bearing have to be corrected  3. Look at the ship's compass illustrated above. In for variation? what direction was the ship travelling?

4 - 11  4. The case of a magnetic compass is made of metals like copper, zinc or aluminium. Why are iron and steel not used?

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