Making a permanent magnet
Using a current-carrying coil of wire to make a permanent magnet from a steel rod.
Apparatus and materials
For each student group
Copper wire, PVC-covered, 150 cm with bare ends
Rod, hard steel
Tintacks or paper clips, supply of
De-magnetizing coil (300 turns or 2400 turns)
Transformer or low-voltage AC supply
Health & Safety and technical notes
The steel rod may be a knitting needle or a piece of clock spring. As a poor substitute, short pieces of thick piano wire can be used.
Make sure the hard steel samples are not magnetized. If any are, de-magnetize them by passing them slowly through a coil carrying AC: for the 300-turn coil, use about 6 V AC: for the 2,400-turn coil, use about 20 V AC.
a Use iron filings or a plotting compass to check that the steel rod is not magnetised before proceeding
b Wind a few dozen turns of insulated wire around the steel rod. (Leave enough wire free at either end to make connections to the power supply.)
c Connect the ends of the wire to the low-voltage DC power supply, so that a large current flows round the coil.
d Switch off the current. Test the steel rod again to see if it has become magnetised.
e Determine where the rod's magnetic poles are.
f Devise a method for magnetising the rod in the other direction.
4 How Science Works Extension: Students can make a magnet (by this method, or by the stroking method) and then test its strength. This requires them to devise and evaluate an approach to measuring the strength of a magnet. Here are some suggestions:
- Find how many pins, tacks or paper clips will hang end-to-end from the magnet.
- Lay a pin on the table. Gradually bring the magnet towards it. Measure the distance at which the pin starts to move.
- Place a plotting compass on the table. Bring the magnet towards it from the side (east or west). Measure the distance at which the compass needle points at 45° to its original direction.
Students should be able to think of other ideas. By trying out several, they can evaluate the sensitivity of each.
This experiment was safety-checked in July 2007