6B40.10 Variac and Light Bulb


Light bulb colour change with temperature


As the voltage is increased, the current increases which results in an increase in temperature of the filament. The increase in temperature affects the range of wavelengths emitted by the light bulb. Visually, the light bulb colour goes from an orange glow to a bright white glow as the voltage is increased.



  • [1] Light bulb
  • [1] Bulb socket
  • [1] Variac
  • [1] NEMA 1-15 to banana adaptor

Classroom Assembly

  1. Place the variac on a horizontal surface near a power outlet.
  2. Connect the light bulb to the variac by a wire.
  3. Start with voltage at zero volts to begin.

Important Notes

  • Increase the voltage carefully, be sure not to apply a voltage larger than 100 volts
  • If the banana plugs come loose while the variac is active, users may be exposed to dangerously high voltages


  1. Turn on the variac after making sure the light bulb is connected to the variac properly.
  2. Slowly turn the knob to increase the voltage, notice how the color of the filament changes.
  3. Return it to zero volts before turning off the variac.


Additional Resources


  • PIRA 6B40.10


  • Don't attempt this at home!

Last revised

  • 2017


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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.