What is crystal radio ?

A crystal radio is the basic form of a radio,which can detect radio signals without a power supply. It has very few parts and it can be built in a short time with commonly available items.It works best if there is a transmitter within 40 km of the set.Simple crystal radios are ofen made with a few handmade parts like an antenna wire,tuning coil of copper wire,crystal detector and earphones.A crystal radio receives programmes braodcast from radio stations which convert sound into radio waves and send out the signals everywhere.The crystal radio antenna, a very long wire,picks up the signals and passes them through the set as an electronic current.It uses a crystal detector to convert this radio wave electricity to sound electricity.The detector can be made from a special rock of galena in a holder.It uses earphones to convert the sound electricity to the sound you can hear.