5B30.35: Point and ball with Van de Graff

5b3035point_and_ball_with_vdg_2 5b3035point_and_ball_with_vdg_3

Video: Watch this demo

Instructor’s video

Description: This demonstration shows how sharp points interact with high voltage (such as lightning rods).


  • Van de Graff machine
  • Metal discharge wand
  • DVM probe with a sharp point and plastic handle

Setup Procedure:

  1. Plug the Van de Graff machine into a table with a three prong plug (ground prong); else you risk getting shocked when turning it off.
  2. Connect both the metal discharge wand and DVM probe to the black grounding post on the Van de Graff machine.
  3. With the machine off, set the speed to about 1/2 speed.

Demonstration Procedure:

  1. Turn the Van de Graff machine on while holding the discharge wand in the other hand.
  2. Bring the discharge wand close to the metal dome and note that there are discharges between the dome and the discharge wand.
  3. Take the discharge wand away and bring the DVM probe next to the machine.
  4. Note that the dome no longer makes powerful discharges to the probe (even though the probe is grounded). This is because the sharp point bleeds off enough charge so that the dome can’t build up a high enough charge to make a large discharge.
  5. Bring the discharge wand near the dome again with the point still there and note that large discharges still don’t take place.

High Voltage
Due to the high voltage, keep away from sensitive electronics.