Makezine_COTM_Capacitor-BadgeInstructables user Jezan made an air variable capacitor:

I was building a crystal set for my son, but it came to a halt. When i found out that i have no variable capacitor in my pile of junk.
Scavenging one from an old radio was not an option. Since most of new radios uses analog tuning. And the ones with air variable capacitors are very rare, and are collectors item.

I have read an article once about building an air variable capacitor. So i decided to build my own from scraps of aluminum sheets and from things that are easily found around the house.

I love how Jezan built the capacitor out of scrap aluminum, using only basic household tools like scissors and a rubber mallet, along with salvaged scrap metal. [originally posted by SMR in 2011]