Physics Dept. Demonstration Manual

DEM51 - Vibrating Filament Demonstration


To show that the filament is carrying an alternating current.

  • vibrating filament assembly (2 lamps)
  • bar magnet
  • custom AC & DC power supply
  • isolation transformer


The filament being powered by AC will vibrate 60 times per soncond when a magnet is brought near it. The DC powered filament will not appreciatively respond to the magnet.

The alternating magnetic field produced by the AC current is responsible for the vibrations in the presence of an external magnetic field.