Science & Technology in Action

3rd Edition

Magnetism and Magnetic Surveys


Variations in local and regional geomagnetism can be mapped and used to locate hidden structures.
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Lesson excerpt

Magnetised needles have been used to aid navigation for more than a thousand years. The use of magnetic compasses seems to have originated in China in the eighth century and by the twelfth century both floating and pivoted compass needles were used in the West.

What is a magnet?
A magnet is usually made of iron or steel. Individual atoms of iron contain magnetic properties. In an unmagnetised piece of iron these tiny magnets are not all aligned in the same direction and therefore there is no net magnetic effect. However, when the iron is placed in a strong magnetic field the atoms tend to align in one direction. In a sufficiently strong magnetic field all the atoms point the same way. When this occurs the material cannot be magnetised further and is said to be magnetically saturated. 

When the external magnetic field is removed some of the atoms may return to their previous orientation and the piece of iron loses some of its magnetism. Hard steel (e.g. a sewing needle) is not easily magnetised but it retains its magnetisation much better than soft iron (e.g. nails or pins). 

A few other metals (notably nickel and cobalt) exhibit the same kind of magnetism as iron; they are said to be ferromagnetic. Alnico magnets, which were first produced in the 1930s, are alloys of aluminium, nickel, cobalt and iron mainly.

Ceramic magnets are made of compressed mixtures of iron oxide (Fe2O3) and barium or strontium oxide or carbonate (BaO, SrO, BaCO3, SrCO3). They can be manufactured in a variety of shapes and magnetic field orientations and are commonly used, for example, in DC electric motors and as ‘fridge’ magnets. 

In recent decades stronger and more expensive magnets have been made from alloys of samarium or neodymium with cobalt and other elements.

Most metals are not ferromagnetic
Some people think that all metals are ferromagnetic; however this is not the case. Familiar examples of non-magnetic metals are aluminium, zinc, copper, brass (an alloy of copper and zinc), lead, silver and gold. 

Cupro-nickel coins are attracted by a magnet while brass or copper coins are not.

True or False?

  1. Compasses were invented by Michael Faraday false
  2. Lodestone is a naturally occurring magnetic mineral true
  3. Atoms of iron are magnetic true
  4. In magnetised steel the atoms are randomly oriented false
  5. The end of a magnet that points North is called a magnetic field false
  6. Copper and brass are ferromagnetic false
  7. Electricity can be generated without a battery true
  8. The Earth’s magnetic field strength is about 1 Wb/m2 false
  9. The unit of magnetic flux density is the tesla true
  10. There is a limit to the magnetisation of a piece of steel true
  11. An instrument that measures magnetic fields is called an ammeter false
  12. Ceramic magnets are made of compressed mixtures of iron oxide with some other materials. true