5H15.10 Magnetic Field of a Wire Current


Magnetic field from line current


Compasses show the direction of the magnetic field around a current in a wire. Iron filings align with the field lines.



  • [1] Mounted copper wire
  • [1] 40 A power supply with leads
  • [3] Compass
  • [1] Shaker of iron filings
  • [1] Paper sheet
  • [1] Power bar
  • [1] Document camera or overhead projector

Classroom Assembly

  1. Place wire and compasses, if desired, under document camera or on projector. If using document camera, place sheet of paper underneath for visibility. If using an overhead projector, focus it on the Plexiglas platform.
  2. Place power supply nearby, plugged into a power bar that is plugged into mains power. Make sure the power bar is off.
  3. Attach power supply leads to wire.

Important Notes

  • Do not turn on for longer than necessary since the current carrying conductor becomes hot over time and the power button on the power supply can weld open. If it does, turn it off using the power bar.
  • The contacts are critical. If the demo is not working properly, loosen the contacts on the power supply and retighten.
  • The Plexiglas platform will not focus on all overhead projectors. Ensure the demo will work on your classroom overhead projector in advance.
  • Compass polarities can be reversed. Bring spare compasses. Undamped compasses work best for this demo.



  1. Observe undeflected compass direction.
  2. Press the power supply button to activate current. Observe deflected compass needles.
  3. Release the button.
  4. Turn off the power bar.

Iron filings:

  1. Sprinkle iron filings evenly around the wire.
  2. Hold down the power supply button to activate current.
  3. Tap the Plexiglas frame with your knuckles, screwdriver, or other tool to help the filings 'jump' to the magnetic field lines.
  4. Release the button. Discuss the pattern of the iron filings.
  5. Turn off power bar.
  6. Dump iron filings onto paper sheet and put them back into the shaker.


Additional Resources


  • PIRA 5H15.10


  • Don't attempt this at home!

Last revised

  • 2020


  • Original construction: purchased.
  • This demo works worse with a 20 A power supply.

Related AV

Related demos

  • Magnetic field of a solenoid current


If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.