Principle of the centered rotary cap variable capacitor.
<a href="//commons.wikimedia.org/wiki/User:%D0%94.%D0%98%D0%BB%D1%8C%D0%B8%D0%BD" title="User:Д.Ильин">Д.Ильин</a>: translation
<a href="//commons.wikimedia.org/wiki/File:Cap-type-trimmer-en.svg" title="File:Cap-type-trimmer-en.svg">File:Cap-type-trimmer-en.svg</a> by Zátonyi Sándor, (ifj.) <a href="//commons.wikimedia.org/wiki/User:Fizped" title="User:Fizped">Fizped</a>