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 often made with a few handmade parts like an antenna wire,tuning coil of copper wire,crystal detector and earphones. A crystal radio receives programmes broadcast 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 electrictity 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.