|MadSci Network: Other|
Congratulations! You have created a crystal radio. It may be a dangerous crystal radio, though: do you know that "ring voltage" is much higher than the 40 volts you have on the line now? (Ring voltage is the alternating voltage that the phone company sends down your line to ring your phone when someone calls you. You don't want your body in the circuit when this happens.)
A traditional crystal radio consists of a long antenna (wire), a frequency filter, and a rectifier (the crystal). Radio waves oscillate back and forth around a million times per second (AM), and average out to zero, which we can't perceive. The rectifier, however, effectively chops off one half of each cycle (say, the negative-going half), and what is left no longer averages to zero, so it is detectable by slow things like piezoelectric speakers and human ears.
In the radio you have created, the phone line is acting as the antenna, and the piezoelectric speaker is probably converting the electrical energy to sound. I'm not sure what is acting as the rectifier, though: it could be the speaker, or sweaty contacts between your body and the rest of the circuit. If you want to experiment to find out, please start by trying to get the same effect using a long wire (say, 100 feet, strung through the air) instead of the phone line.
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