A basic telephone has both a speaker– transmitter and a receiver (Fig. 20.33). Until the advent of digital phones in the 1990s, the transmitter had a diaphragm coupled to a carbon chamber (called the button), which contained loosely packed granules of carbon. As the diaphragm vibrated because of incident sound waves, the pressure on the granules varied, causing them to be more or less closely packed. As a result, the resistance of the button changed. The receiver converted these electrical impulses to sound. Applying the principles of electricity and magnetism that you have learned, explain the basic operation of this type of telephone.
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