Magnetic field due to a long close-wound coil

Class practical

Iron filings show that a long, closely wound current-carrying coil behaves just like a bar magnet.

Apparatus and Materials

• Copper wire, PVC-covered, 100 cm with bare ends
• Iron filings
• Cardboard, rectangle
• Pencil
• Plotting compass (optional)
• Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)

Power supplies

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance

Procedure

1. Make a coil of twenty or thirty turns by winding the wire around a pencil. (Leave enough wire free at either end to make connections to the power supply.) The coils should be wound firmly and closely on the pencil.
2. Lay the coil on the cardboard and sprinkle iron filings onto the board.
3. Switch on the current, tap the board, and observe the pattern.
4. Try using a plotting compass after you have tried the iron filings. Investigate what happens if the connections are reversed.

Teaching Notes

• The long, closely wound coil behaves just like a bar magnet.
• The direction of the magnetic field reverses when the current is reversed.

This experiment was safety-tested in April 2006