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How a Van De Graaff Generator Works

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The Van De Graaff Generator, invented in the 1930s by the American scientist Robert J. Van De Graaff, is a type of electrostatic machine that can generate very high voltages (millions of volts) used to produce artificial lightning for demonstrations and experiments. Many science museums have large models that can generate more than six million volts and produce sparks more than five feet long! Small desktop Van De Graaff models commonly used in classrooms and laboratories can generate between 200,000 and 500,000 volts yielding sparks up to 12 inches long.

Components

The VDG is a fairly simple machine comprised of three primary components:

  1. Dome or spherical terminal: A hollow metal sphere ('A' in schematic above), sometimes molded into two pieces to allow easy access inside.
     
  2. Belt and roller assembly: A rubber or fabric belt (F) moving on a set of two rollers, one made of metal (G) and the other of an insulating material (D) such as plastic or resin.
     
  3. Charging combs or sprayers: Two sets of sharp metal points, one located at each roller (C, J). The charging combs are aimed at the belt at the point where it rounds each roller. The top set of combs are connected by wire to the inside of the dome (P), the bottom set of combs are connected to earth ground.

Construction

The dome (A) rests atop an insulating support column (B). The upper (metal) roller and charging comb is located inside of the dome (C), while the lower (insulating) roller and comb (J) is located at the base of the machine (Q). The belt is moved on the rollers by means of either an electric motor (I) or a hand crank mechanism.

Operation

The belt is moved on the rollers by means of the motor (I) or hand crank. Charge separation occurs at the point where the belt 'peels away' from the lower (insulating) roller, causing the roller to acquire a charge. This charge then begins attracting opposing charge from the grounded comb points. The comb's points 'spray' charge onto the belt, which carries the charge upward. When the charge reaches the upper roller inside the dome, the upper combs 'pull' the charge off the belt and transport it to the dome. The charge spreads out over the surface of the dome until the voltage is great enough for electrical breakdown of the air to occur, initiating the process of a discharge or spark (see our VDG gallery for photos of Van De Graaff sparks). This process is very similar to that of a lightning discharge, only on a much smaller scale.

Safety

The static charge and sparks produced by small Van De Graaff generators are safe to touch. While the voltages produced by a VDG are very high, the degree and duration of the current in a spark is too brief to be hazardous. The sparks are just like those created by scuffing feet on carpet on a winter day, only more powerful. A few spark discharges can sting somewhat if they contact skin, especially if they are allowed to strike the same spot over and over. Although there have been no known cases of injury due to a small Van De Graaff machine's spark in its nearly 80 years of existence, persons with pacemakers or other serious medical conditions should steer clear of a Van De Graaff machine. Just like a lightning strike, Van De Graaff sparks will permanently damage or destroy electronics such as cell phones, computers and watches. A Van De Graaff generator should not be operated near computer equipment, and any electronic devices should be kept a good distance away. The use of a Leyden Jar with a Van De Graaff generator should be approached with caution. Leyden jars are a type of high-voltage capacitor, capable of storing potentially lethal amounts of charge when used with a Van De Graaff machine.

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