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An electrostatic generator is a device which generates high voltages by gradually accumulating a charge through a low-power current, which can then be stored for later use. These devices convert mechanical action into an electric charge, producing what is known as static electricity, like that generated when you pet a cat or rub a balloon on your hair, but in much greater amounts. The first electrostatic generator is believed to have been invented by Otto von Guericke in the late 17th century.
Devices of this type generally fall into two categories: triboelectric or friction machines and electrostatic induction machines. Both types of machine generate the same type of electric current but in different ways. The first electrostatic generator was a friction machine, but later improvements introduced more advanced machines of the electrostatic-induction–type.
Friction-type electrostatic generators rely on the triboelectric effect, which is the generation of a small electric current when two materials are rubbed against one another. The first machine had a rotating globe of sulfur that accumulated an electrostatic charge by rubbing one's hand against it as it turned. Later research showed that other materials, like glass, could also be used.
Electrostatic induction machines, also called influence machines, operate on the principle of charge by induction. Through this process, a charged object can impart an opposite charge to another object simply by being placed near the second object. This process is coupled with some type of mechanical operation, like a rotating disk of glass, to continually add charge to the storage object.
A Van de Graff generator is an advanced electrostatic generator of the induction type. Some of the largest electrostatic generator machines can produce charges of up to 10 million volts. First introduced by researchers at MIT in 1929, this type of electrostatic generator is the most well-known, having appeared in many movies and television shows as a popular component in "mad scientist" laboratories. These devices have great legitimate scientific value, however.
Machines like the Van de Graff generator and other electrostatic generators are common in teaching and demonstration settings, like physics labs or museums. They are commonly used to demonstrate the properties of static electricity and phenomena associated with high-voltage electric current. They have also found uses in x-ray tubes and other medical applications as well as in physics research, industry and food processing.