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Humans have long known that rubbing substances together can produce static electricity. The ancient Greeks used balls of amber on spindles that they rubbed to generate sparks. Electrical effects puzzled early scientists, but they lacked a means of generating electricity in order to study it.
Then, in the mid 1600s, German physicist Otto von Guericke came up with a very early version of a machine that did just that: the electrostatic generator. In the decades that followed, other inventors, including English experimentalist Francis Hauksbee, improved on this design. One version of the machine, dating from 1785, is pictured below; it generates a high voltage charge at very low current that could be stored in a Leyden jar for later use.
The tutorial illustrates how this machine works. A hand-turned crank causes the glass cylinder to revolve. Leather pressing on the glass produces an electric charge: The friction causes positive ions to collect on the leather, and negative particles (the electrons depicted here in yellow) to stick to the glass before being collected by the comb-shaped metal collector. At the end of the collector is a metal ball. With sufficient buildup, the charge can jump as a spark from the generator’s metal ball to the metal ball on top of the nearby Leyden jar, where the charge can be stored. The first capacitor, the Leyden jar was a glass jar coated inside and out with metal. The inner coating was connected to a rod that passed through the lid and ended in a metal ball. As you can see by moving the Leyden jar separation slider above, the farther the jar is from the generator, the greater the build-up of charge required in the generator for it to discharge to the jar.
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