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The Challenge: Make a radio from a saucepan

How a radio works

The simplest radio is a crystal set which makes use of the radio waves' electrical energy and so doesn't need a battery. It only works if you have a very large antenna and if the radio signals are strong. A crystal set radio is composed of five basic parts: an antenna, coil, tuning capacitor, crystal detector and earpiece (or headphone).

Any piece of wire (or indeed metal) can act as an antenna. As the radio waves pass across the antenna they create tiny voltages in the wire. In general, the longer the wire the larger the voltage produced. This voltage is actually a mixture of voltages from all the radio waves that are passing through the wire, perhaps from a hundred or more different stations! The antenna is connected to a coil and a device called a tuning capacitor. These act as a filter to select which of the radio-wave voltages should be picked out. As the name suggests, the coil is made up of many turns of wire. The capacitor is essentially two metal plates separated by a small gap. It's designed so that the extent to which the plates overlap can be varied. In this way, you can control the filtering — in other words, tune the radio. The voltages that make it through the coil and capacitor pass to the crystal detector, a device that converts them back into the original signal (as a tiny, varying voltage) which the earpiece converts into sound!