Total attraction

7th September 2001 at 01:00
Ray Oliver offers a selection of activities for classroom investigations into magnetism

Magnets are fun, especially big ones. We are standing on the biggest magnet of all, the Earth. The credit for this insight goes to William Gilbert of Colchester (1540-1603). As well as working as royal physician to both Queen Elizabeth I and King James I, Gilbert had a passion for the mysteries of magnetism. Nobody had thought of the planet in this way until he published his book On the Great Magnet the Earth in London in 1600. The Earth's magnetic field is similar to that of a simple bar magnet, but its magnetic field extends 80,000km into space.

Any material that forms a magnetic field allowing it to attract iron (or nickel or cobalt) is a magnet. There are always two magnetic poles, conventionally labelled north (north-seeking) and south (south-seeking). Floppy discs are just one example of magnetic data storage. Here are some ways for pupils to start investigating magnetism.

Find the poles 1

Enclose a bar magnet completely in a large ball of Plasticine to represent a model of the Earth. The challenge is to locate the two hidden magnetic poles without touching the Plasticine planet. The quickest way is to use a small compass, a plotting compass. Since the compass needle will be attracted by the two poles at the ends of the bar magnet, a systematic survey of the planet's surface will locate the poles. A messy alternative is simply to sprinkle iron filings all over the planet and look for the largest clusters near the magnetic poles.

Find the poles 2

Not everyone believes the Earth is round. For the flat-Earthers in the class try this alternative. Hide the bar magnet underneath a flat cardboard Earth. Place the compass on the Earth and mark an arrow to show the direction indicated by the compass needle. Move the compass and repeat until a pattern begins to appear. The arrows will point towards, or away from, the two magnetic poles. Check whether the poles have been correctly located by removing the card Earth.

The magnetic rope trick and the orb of virtue

Pupils will realise that the influence of the magnet must be able to travel through other materials; air, Plasticine and cardboard for certain. Every magnet, including the Earth, is surrounded by what Gilbert called an "invisible orb of virtue". By the late 18th Century writers were still calling it the magnetic virtue and described the "effluvia emitted by the magnet". The prosaic name of "magnetic field" seems devoid of mystery by comparison.

Investigate which materials can block a magnetic field. A cotton thread attached to a steel paper clip can be held vertically by a magnetic field. It should be arranged to leave a narrow gap between the clip and the magnet, just enough to test the "orb of virtue" (the magnetic field).

Pupils can place materials in the gap to investigate which can block the magnetic field. Try plastics, wood, copper or aluminium foil and thin steel, for example a tin lid. Any material capable of blocking the field will cause the "rope" to collapse. But then, so does a shaky hand, demonstrating the perils of operator error in experimental science.

The magnetic balance trick

Magnetic games have a long history. William Hooper's Rational Recreations published in 1774 gives plenty of examples. Even allowing for problems in obtaining the apparatus - the list includes ivory tubes, ebony rods and pewter - we can try to replicate one of his magnetic games.

A simple beam balance can be made using a ruler and two plastic cups suspended by threads from each end. A strong bar magnet is concealed on the table, directly under one of the cups. Pupils check the balance for accuracy by placing identical non-magnetic objects in the cups, for example plastic cubes. When they repeat the check using two identical new 2p pieces, one end of the balance goes down. If they swap over the coins, to put the "heavier" one at the other end, the balance still goes down as before. The coins cannot both be heavier than each other. Check that they remember that copper is unaffected by magnets and that these are copper coins.

The explanation owes as much to economics as to science. The price of copper makes it uneconomic to mint low value copper coins. Modern British copper coins have a heart of steel. The cheaper steel is coated in copper to reduce production costs. Since steel (iron) is magnetic, whichever coin is above the concealed magnet will be attracted downwards, so producing the movement of the balance.

Get sorted

Use a magnetic coin sorter to solve a mystery: in which year did the Mint start putting a steel core into copper coins?

A magnetic coin sorter will allow pupils to find the year that the change was made. Set up a cardboard slope with a strong bar magnet fixed to the lower surface. Using a collection of old and new copper coins, slide them slowly down the slope over the magnet. True copper coins will be unaffected by the magnet. Those with a steel core will move differently over the magnet. The may slow down, be deviated or stop altogether. By checking the dates on the coins pupils can solve the mystery.

A maze yourself

Magnetic football was once a very popular game. The model players each had a base containing a magnet. By moving a second magnet below the playing surface, individual players could be moved around. It is easy to apply this idea to construct a magnetic maze. The maze is drawn out on card and a steel ball-bearing must be timed as the player moves it through to the centre, using a magnet below the card. The quickest player is the winner.

Plastic magnets?

Both iron metal and magnetite, one of its oxides, are magnetic materials. When fine particles or iron oxide are incorporated in plastics we get a very useful composite material with magnetic properties. Audio and video tapes use this technology, as do the door seals on fridges and the magnetic pads of fridge magnets. Fridge magnets can be used to investigate the shape and strength of unusual magnetic fields. Place a selection of fridge magnets below a rigid plastic sheet. A combination of sprinkling iron filings on the sheet and gentle tapping will produce a range of field patterns. Pupils can sketch the field patterns and compare them with bar magnets.

Magnetic rocks

Legends surround the discovery of naturally magnetic rocks and minerals. A shepherd on Mount Ida was supposed to have discovered magnetite, also called lodestone, when the iron handle of his crook stuck firmly to a cliff. When pieces of this mineral are freely suspended, they point to the magnetic north, like a simple compass. This kind of compass was first mentioned in English records, although written in Latin, by Alexander Neckham, who was born in St Albans in 1157.

Looking for the big dip

Explain to pupils that the problem with using a simple compass to reach the North Pole is that there are two of them: the geographical North Pole is fixed but the magnetic North Pole moves from year to year. This is caused by variations in the Earth's magnetic field. The magnetic North and South Poles have exchanged positions many times over geological history. The change is unpredictable and not fully understood. Nobody knows when the next magnetic flip will begin - hopefully not when you are in a plane over the Atlantic.

Magnetic lines of force curve right around the Earth. At the magnetic poles they are vertical changing to horizontal at the magnetic equator. Children can measure the angle of magnetic dip where they live. All they need is a compass needle that pivots in a vertical plane, unlike the ordinary horizontal compass.

Push a thin steel knitting needle through the centre of a bottle cork to act as the dipping needle. Push another needle at right angles to act as the pivot, supporting the device on the rims of two cups. Adjust the needle so that it rests horizontally. Magnetise the dip needle by stroking it repeatedly with a strong bar magnet. When it is replaced on the supports, the dip needle will align with the local magnetic field and point down towards the Earth.

Animal migration and magnetic fields

There are still competing theories about the way animals align themselves with the Earth's field. Some animals have tiny crystals of the mineral magnetite within their bodies. It is not clear whether these act as a simple compass or whether the magnetic crystals register local changes in the Earth's magnetism. Since the dip angle of the Earth's magnetic field lines varies with location, birds may also get navigational clues from this. Birds may have two magnetoreceptors, one for local orientation and the other to detect the direction of the magnetic poles. During magnetic storms, often associated with sunspot activity, pigeons can become disoriented.

The solar wind

The Earth's magnetic field extends far into space, forming the magnetosphere around the planet. This protects the Earth from the stream of charged particles released from the Sun's atmosphere. Luckily the magnetosphere deflects most of this "solar wind" around the planet and off into space. Since the Earth's field is strongest near the poles, it is here that the particles from the Sun sometimes break through and enter the atmosphere. They cause the shimmering effects known as the aurora or Northern Lights.

Plotting the field

Just as the Earth's magnetic field extends far from its core, so does the field around a bar magnet. Place a bar magnet in the centre of a table and see how far away a sensitive compass can register the field. Sketch the shape of the field and then see if it extends in three dimensions by placing the compass both above and below the magnet. Unlike the Earth's field, which is distorted by the solar wind, the field around a bar magnet is symmetrical.

Ray Oliver is a former teacher

MORE READING

'On the Loadstone and Magnetic Bodies'. William Gilbert, in Encyclopaedia Britannica Inc, University of Chicago. 1987.

'The Earth's Magnetic Field'. New Scientist Inside Science supplement no 26. 1989.

Hidden Attraction, The Mystery and History of Magnetism. G L Verschuur. Oxford Paperback. 1993.

Our Solar Connection, leaflet from PPARC, tel: 01793 442123.

Physics in Everyday Life (The World of Science series), Equinox Publishing. 1989.

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