One of the installations in Grizedale Forest involves a key that goes through a tree. You have to turn the key 10 times and then music comes out of a speaker in the ground a few metres away. The key clearly turns a generator in the brass disc on the other side of the tree but why 10 turns before you get music? My guess is that you are charging a capacitor from the generator and that when it is charged enough, it can power the speaker circuit. Further evidence for this is that it plays for a while after the key stops turning - presumably whilst the charge runs down.
