Van de Graaff Generator

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Van de Graaff Generator

• A Van de Graaff generator is made by a belt of a
  flexible dielectric material, running over two
  rollers (wheels), one of which is surrounded by a
  hollow metal ball.
• 1. Power Supply that pulls off electrons.
• 2, 4. Rollers.
• 3. Positively charged belt (lost some electrons)
• Metal comb that drops electrons from the ball
  onto the belt.
• Metal ball that gives up electrons to the belt,
  then it is charged!
• Negatively charged belt (gained some electrons
  from the ball).
• 8. Grounding wand, it can give or take electrons
  to make everything even again.
• Discharge A spark, a flow of electrons through
  the air.
• The bigger the ball, and farther from the ground
  it is lets you build more charge up on it.

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History

• The fundamental idea of friction machine as high
  voltage supply traces back to 17th Century.
• Generator invented to supply the high energy
  needed for early particle accelerators.
• The first model was demonstrated in October 1929,
  using a silk ribbon bought at a five and dime
  store as the charge transport belt.
• Originally used as a research tool in early
  atom-smashing and high energy X-ray experiments.

1931, Robert J. Van de Graaff (left) poses with
his electrostatic generator and Karl T. Compton,
MIT president
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Recent Developments

• Tandem Van de Graaff accelerator
• Contains one or more Van de Graaff generators.
• Negatively charged ions are accelerated through a
  potential difference then stripped of electrons
  and accelerated again.
• Pelletron
• Belt is replaced by a chain of short conductive
  rods connected by insulating links.
• Air-ionizing electrodes are replaced by a
  grounded roller and inductive charging electrode.
• Can be operated at much higher velocity than a
  belt, so both the voltage and currents attainable
  are higher than a conventional Van de Graaff
  machine.

The largest VDG generator, on display at the
Boston Museum of Science