Physics Dept. Demonstration Manual

DEM 04.1 - Van de Graaff Generator/Bat Activator


To show charge generation using a Van de Graaff Generator.

To show the repulsive effects of charge accumulation on body where the "body" is a piece of artificial fur cut in the pattern of a bat.

The "bat" used in this experiment replaces the need to have a student volunteer as a charged object (see DEM 4). Shows the same principles, will catch the class interest and is a lot less of a fuss. You choose.

  • Welch model 1913B Van de Graaff-type generator
  • Welch model 1911B insulated sphere
  • interconnecting lead
  • model bat


Set up the generator and insulated sphere with a separation 15 to 20 cm. Connect a grounding lead between the insulated sphere and the base of the generator.


  1. The insulated sphere is used to increase the capacity of the system. An increased capacity will increase the amount of charge accumulated and therefore enhance the effects one wishes to demonstrate.

  2. One should avoid touching the spheres or their lucite supports. Contamination resulting from such touching will cause unacceptable electrical leakage during charging.

Place the bat on the sphere of the generator, with the head towards the class. Switch on the generator and adjust the speed. Within a few seconds the "hair" of the bat will stand on end, then the head and tail will lift and finally the bat will fly off the sphere. To achieve flight the humidity needs to be fairly low and the sphere separation just right. If the spheres are to close together arcing will occur and if too far apart the capacitance too low.

More Notes:

  1. During the charging the operator is cautioned not to stand too close. Generator to operator arcing has been known to occur. I can verify that an arc, generator to metal frame eyeglasses is an experience to be avoided. But you need not take my word for it, just TRY IT in true scientific tradition. The class will love it. Could be the only idea they will remember by next week.

  2. When done, switch off the power to the generator motor. If power is left on with the speed control set at a minimum, the motor does not rotate but it still draws current. In this situation the motor can overheat.