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Electricity & Magnetism

AM Radio

A long white antenna wire drops down from the roof of the Exploratorium and leads visitors to the AM Radio exhibit. The wire connects to a large coil of copper wire and an air-core capacitor. Visitors can turn a knob and adjust the position of the plates of the capacitor this adjusts the resonant frequency this "tuner" to that of a local AM radio station. A switch allows visitors to connect this "tuner" to either a commercial diode or to a diode formed by a metal "cats-whisker" probe touching the surface of a crystal of Galena - the crystal of a crystal radio. The diode detects the amplitude modulation of the radio signal. The output signal from the diode is then connected to an earphone. A visitor listening to the earphone will hear AM radio stations. Visitors are often surprised when they notice that the power for the sound is picked up by the antenna from the power broadcast by the radio station. The visitor can also attach an amplifier to the output of the diode so that other people standing around the exhibit can hear the sound.

This exhibit shows the basic elements of an AM radio, Antenna, tuner, rectifier, and speaker.

The visitor selects stations by adjusting the resonance frequency of the tuner to match that of a radio station.

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