Is the ring magnetised?
Practical Activity for 14-16
This experiment raises an interesting question. Can something be magnetised and not show poles?
Apparatus and Materials
- Steel rings, magnetized, 2
- Sheets of paper, 2
Health & Safety and Technical Notes
Eye protection must be worn.
The rings are small, flat rings, about 0.25 mm thick, 1 cm internal diameter and 2 cm external diameter. They are stamped out of thin steel and hardened by heating them to cherry red and then plunging them into cold water. The rings should be
glass hard and easily broken in the fingers.
The rings need to be magnetised with circular magnetism so that no poles are seen. For that, the circular magnetic field produced by a current in a straight wire is used. There needs to be a wire carrying a current of 100 amps going through the centre of the rings. Multiple wires (e.g. a wire carrying 10 amps threaded 10 times through the rings) may need to be used. To ensure even magnetisation with no poles, pass the wire through the centres of the rings; the rings could be placed on a wooden dowel which has a hole down the centre for the current-carrying wire.
- Test the ring with iron filings. Does it have magnetic poles?
- Snap the ring in two with your fingers and test the pieces with filings.
You might introduce this experiment by saying:
"Can a ring of steel be magnetised even though it shows no sign of poles? Here is a ring of steel. I believe that I have magnetised it yet I can find no poles. You see no clumps of iron filings hanging on the ring when I dip it into iron filings. I find no magnetic field near it. When I put this compass needle nearby it is not affected. So I find no poles, no field and yet I thought I had magnetised it."
"How does the theory help to explain the appearance of the poles at the break? (The "basic magnets" are nose to tail around the ring and are not exposed until the ring is broken.)
This experiment was safety-tested in December 2004