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What's a Capacitor?

Everyday Einstein: Quick and Dirty Tips for Making Sense of Science
Everyday Einstein



Quick & Dirty Tips

Scientific American presents Everyday Einstein by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.

If you were to take apart pretty much any electronic device in your home, mixed in with the various bits and bobs of circuitry you would find that one device appears over and over again. Capacitors come in a variety of shapes and sizes, but they all have the same job: to store up electricity. Let’s look at some of the science behind how they work.

A Long and Noble History

Way back in the 1700s, people knew about electricity, but still didn’t understand exactly what it was. For some time, scientists had been able to produce static electricity easily, but couldn’t really do anything useful with it. Many scientists believed that electricity was some kind of fluid, so they thought it might be possible to store it in a jar, just like any other fluid.

So one day, at the University of Leiden in the Netherlands, some scientists invented just such a jar, which came to be called a Leiden jar (though usually spelled with a “y” as Leyden jar). These special jars consisted of a glass jar with most of the inside lined with a thin metal foil. The outside and bottom of the jar were also lined with foil. The jar was sealed with some kind of insulating lid, such as wood or cork, and a brass rod was inserted inside, with a chain leading from the rod to the inner foil. Finally, the outside of the jar was “grounded,” which means it was connected to the earth by some form of conductor.

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