5J10.20-Inductance Spark


Video: Watch this demo

Instructor’s video

Description: This demonstration shows that the current stored in an inductor will take the path of least resistance,
in this case the small air gap between the SPST switch when it is opened.


  • Large DC power supply
  • Variable Iron core inductor
  • SPST knife switch
  • C clamp
  • 3 Banana plug cables
  • Spark fuse

Setup Procedure:

  1. Connect the equipment as shown in the schematic.
  2. Clamp the knife switch down to the table using the C clamp.
  3. Put the iron core inside the inductor.
  4. Turn on the power supply and set it to 40VDC.
  5. Test to make sure everything is working.
  6. Include a spare fuse.

Demonstration Procedure:

  1. With the knife switch open, turn on the power supply and set it to 40VDC.
  2. Close the knife switch, wait a half second (much longer and the fuse may blow).
  3. Open the knife switch quickly. The quicker, the larger the spark.
  4. Note that a large greenish spark is produced due to the back EMF of the circuit.
  5. When finished, turn off the power supply.

Note: The power supply in this picture works best, due to the high DC voltage.
Avoid going over 45VDC.