Gerald Haigh explains the fascination of polar opposites and looks at magnets designed for schools
At Birmingham Airport it used to be possible to see a genuine levitation act. The airport was linked to the nearby railway station by a train which floated a clear few millimetres above the rails without any wheels. It was all done by magnets.
Hold a magnet an inch or so above a paper clip and the clip will jump the intervening space, propelled by an invisible force. Both these effects - repulsion and attraction - are described as "a scandal of reason" by Dr Stephen Johnston of the Museum of the History of Science at Oxford University.
Throughout history, people have wondered what this force was. In the middle ages, apparently, philosophers decided that magnetic material had a soul and saw a clear link with other forms of attraction, including that between men and women. Even now, echoes of earlier explanations remain in the way that the words "magnetism" and "magnetic" have attached themselves to human personality. It was not until Victorian times, and the work of Michael Faraday, that there emerged anything like a convincing account of magnetism and of its connection with electricity.
The point here, as Dr Johnston points out, is that magnetism is useful as well as interesting. "It is not a recondite phenomenon, seen only in a laboratory. For many hundreds of years natural lodestones - naturally magnetic iron ore - were used as compasses."
Then, of course, there is the fridge magnet. And the magnets in the MRI (Magnetic Resonance Imaging) machine that scans the inside of your head and body in the hospital, measuring the tiny magnetic field variations that betray lurking problems.
Magnetism does not have a high profile in the national curriculum. At primary level, the requirement is for pupils simply to know what the forces are; in key stage 3 this extends to knowing about magnetic fields, and to some understanding of the link between electricity and magnetism.
However, because children are intrigued, it is not surprising that teachers often spend more time on magnetism than the national curriculum demands. Even so, there are many misunderstandings even among teachers. A science adviser in the Midlands recently found on a course that few teachers knew which metals were magnetic.
In the words of freelance science lecturer Dr Bryson Gore, who gives presentations at the Royal Institution: "It is because magnetism is action at a distance. You can do a simple experiment with a magnet you keep in your pocket, and what it does is mysterious. It was mysterious 200 years ago, it's mysterious today and it will still be mysterious 2,000 years in the future."
So what do you need to explore magnetism? As Dr Gore hinted, the starting point is simply a magnet you can keep in your pocket, and it is no exaggeration to say that at a pinch you could cover at least the key stage 2 requirements with two bar magnets and a nail.
Bar magnets come in various sizes and strengths, but Stan Davies, head of physics at Finham Park School, recommends that a secondary school should have class sets of these and a primary school at least enough for pupils to work in pairs. Pack of 2 pound;5.25 HORSESHOE MAGNETS
find their best use with younger children who are finding out what magnetism does. For them, one or two big chunky horseshoe magnets are very useful. NES Arnold has two examples - a fat one, 10 cm high (good value at pound;4.85) and a huge one, 21 cm (pound;7.95). However, plastic coated magnets might confuse because children are learning that plastic is non-magnetic. Some teachers will prefer a horseshoe magnet which is obviously made of steel. Pack of three in different sizes up to 12.5 cm, pound;7.49 FLOATING MAGNETS
Floating magnets are small washer-shaped magnets threaded on a wooden rod. The dowel stands upright, and the magnets repel each other so they seem to float. Demonstrate magnetic levitation and look very striking. Set of five with wooden rod pound;4.50.
A few magnets of other shapes "button" magnets, fridge magnets from home - will demonstrate, especially to younger pupils, that magnets do not have to be a particular shape. A pack of three different sized button magnets costs pound;4.99.
Alnico is an alloy that makes very powerful magnets. They are more expensive than steel magnets, and are perhaps most useful in secondary schools because they produce clearly defined magnetic fields. These will not replace ordinary bar magnets but a few sets of two would be useful. Alnico bar magnets, set of two pound;7.59.
It is necessary to demonstrate electro- magnetism at secondary level. If you put a coil of wire round a core of magnetic material, such as iron, and run a current through the wire, the core will become magnetised. Such electro- magnets have lots of applications in the home and in industry because you can switch the magnetism on and off. NES Arnold's version is a very professional-looking magnet for use with 4v battery pound;10.95 SUPPORTING EQUIPMENT
To "see" an invisible magnetic field you need something visible that responds to it. Traditionally, teachers have used iron filings. This method is now frowned on in schools because children have been known to rub iron filings into their eyes from their fingers. The solution is to encase the iron filings in transparent plastic. NES Arnold has iron filings in a plastic bubble for pound;1.19. A larger magnetic field pattern fits on an overhead projector for pound;19.95.
Stan Davies at Finham Park makes his own pattern demonstrators with filings between sheets of plastic. You can also demonstrate and draw magnetic fields using small plotting compasses. Stan Davies likes the ones you can see right through, because you can use them on the OHP, pound;1.30 each.
The best way for children to find out what materials are magnetic is to try lots of ordinary objects. To reinforce this, a set of small labelled samples of magnetic and non-magnetic metals is useful. A set of eight metal discs costs pound;9.95.
KITS amp; COMPENDIUMS
A typical one will contain a range of different magnets, with plotting compasses and other materials. Good for a non-specialist teacher. NES Arnold's Magnetic Compendium for pound;99.95 has samples of just about everything you need, including an electro-magnet.
Its Electricity and Magnetism Kit for pound;59.95, is angled more to the primary school and includes, as well as magnets, equipment for making electrical circuits. There is a teachers' book and a set of 20 workcards, all referenced to the EnglishWelsh curriculum and the Scottish 5-14 guidelines.
Also suitable for key stage 2 is the Magnets Science Pack at pound;48.95, with assorted magnets and magnetic materials. I am not so sure about this. With care and for the same money you could buy individual items to suit your own needs.
For younger children (six plus) who will learn much from just playing with magnets, there is Magnasticks, a construction toy in which the pieces are held together by magnetism, for pound;14.99.
TRAPS, TRIPS AND MISUNDERSTANDINGS.
lYoung children, typically, see magnetism in terms of things "sticking" to magnets rather than as a force. One answer is to do lots of little experiments showing that magnets act at a distance. Work on repulsion, too, with children using two magnets, or the floating magnets.
* Children (and some teachers) are confused about what materials are magnetic (that is, capable of being attracted by a magnet): iron is; steel is usually, but not always. Stainless steel is not. Nickel is magnetic and so is cobalt. But no other common metals are.
* Children think bigger magnets are stronger than smaller ones. If you have suitable magnets, they can see that this is not necessarily so. At key stage 3 children need to be taught the link between magnetism and electricity. An electro-magnet shows this.
* Electro-magnets present two problems: they drain batteries quickly and if you show the principle by making your own simple electro-magnet with a core and coil, always use a soft iron core. Other materials - a nail, for example - may remain magnetised when the power is switched off, thus confusing the experiment.
* According to Stan Davies, the fact that magnetic fields are conventionally drawn as lines, and iron filings settle down into lines, leads pupils, and some teachers, to believe that the magnetic field actually consists somehow of lines when it is just what it says it is - a field.
* "Pupils will ask why, if like poles repel, the north pole of a magnet points to the north pole of the Earth," says Davies. The answer is that the north pole of a magnet is properly a "north-seeking" pole.
* Magnetism does not "drain away" from a magnet. However, a magnet can be demagnetised by being knocked and dropped, or by being stored in the wrong way. So never leave magnets in a jumble in a drawer. Store bar magnets side by side with unlike poles together, preferably with something non-magnetic keeping them apart and a magnetic "keeper" at each end. Use keepers on the ends of horseshoe magnets.
These products are from the catalogue of NES Arnold (0115 971 7700). The same, or similar, will be available from other suppliers, and it is worth comparing prices. Another supplier of magnet kits, each at about pound;12.99 with activity sheets or books as well, for primary and lower secondary, is Dowling Magnets, Ashley Road, London N17 9LN. The Consortium of Local Education Authorities for the Provision of Science Services (CLEAPSS) has a pamphlet, Magnets for Primary Schools, so good it is worth going to some trouble to obtain. It is available to local authorities which are members of CLEAPSS (ask your science adviser).