Building a simple crystal radio. ("HOW TO")
"...A crystal radio is the distilled essence of a radio. It has very few parts, it needs no batteries or other power source, and it can be built in a short time out of things you can find around the house.
The reason a crystal radio does not need any batteries is the amazing capabilities of the human ear. The ear is extremely sensitive to very faint sounds. The crystal radio uses only the energy of the radio waves sent by radio transmitters. These radio transmitters send out enormous amounts of energy (tens of thousands of watts). However, because they are usually far away, and we have at most a few hundred feet of wire for an antenna, the amount of energy we receive with the crystal radio is measured in billionths of a watt. The human ear can detect sounds that are less than a millionth of even that.
We are going to launch right into this chapter by building a working radio using parts that we buy at stores like Radio Shack or through mail order. We will try to use common household objects when we can, but our emphasis will be to quickly put together a radio that works.
Later we will learn more about radios by looking at even simpler versions that might not work as well as our first radio, but can show the important radio concepts more easily, because they have fewer parts.
Then we will improve our radio, making it louder, making it receive more stations, and making it look real nice.
Lastly, we will build each part of the radio from scratch, using things we find around the house. This will take a lot longer than our first radio, but it can be done by replacing store-bought parts one at a time, so we always have a working radio..."
(from: http://sci-toys.com)
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