english: That's a wire running down the middle, and not only is it a wire, if you take that piano circuit, you can thumbtack into the middle of a pencil, and you can lay out wire on the page, too, and get electrical current to run through it.
non_english: To je žica koja prolazi kroz sredinu i ne samo da je žica, ako uzmete klavirsko kolo, možete ga čiodom prikačiti za sredinu olovke, pa onda i žicu položite na papir i dobijete strujni protok unutar toga.