5H40.XX Drinking Cup Loudspeaker


Lorentz force


A drinking cup, a small coil, and a magnet are turned into a loudspeaker.

A cup with a hole and a second coil are provided to allow demonstrating the construction of the speaker.



  • [1] Waxed paper cup
  • [1] Small coil
  • [1] NdBFe magnet mounted on a rod
  • [1] Radio
  • [2] Banana plugs
  • [1] 3.5 mm plug adapter
  • [1] Camera

Important Notes

  • Please handle the 'speaker' with care. The coil is easily destroyed.
  • The magnet is very strong and can cause injury (by pinching, shattering, etc.) if not handled with great care.
  • The speaker action depends on the radial component of the field from the magnet. In a conventional speaker the magnet would sit in a permeable cup.
  • In large lecture halls, use a camera to show the details of the construction.


  1. Turn on the radio and tune in to a radio station.
  2. Connect the banana plugs to the 3.5 mm adapter and the 3.5 mm adapter to the headphone jack of the radio.
  3. Move the magnet into the coil.
  4. Move the magnet in and out of the coil, noting the variation in sound intensity as the position varies.


Additional Resources



  • Don't attempt this at home!

Last revised

  • 2020


  • The coil consists of approximately 50 turns of #32 wire, wound on masking tape. One AA battery was used as the form. The hollow coil is epoxied into a hole in the base of the cup. Banana plugs allow connecting the coil to the headphone jack of the radio/cassette player. The Nd-B-Fe magnet is on a steel rod for ease of handling.

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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.