Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

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

Plotting compass

Cardboard, rectangle


Plotting compass (optional)

Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)

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.


Magnetic field due to a long close-wound coil

a 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. 

b Lay the coil on the cardboard and sprinkle iron filings onto the board.
c Switch on the current, tap the board, and observe the pattern.
d Try using a plotting compass after you have tried the iron filings. Investigate what happens if the connections are reversed.


Teaching notes

1 The long, closely wound coil behaves just like a bar magnet. 

2 The direction of the magnetic field reverses when the current is reversed.

This experiment was safety-checked in April 2006


Related experiments

Magnetic field due to a coil carrying a current

Magnetic field due to an electric current in a wire

Magnetic field inside an open coil 


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