The telephone, a relatively simple yet essential part of our modern culture, relies on interactions between a permanent magnet and an electromagnet to translate electrical information into physical sound waves.
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A telephone sends sound waves as electrical current, which the receiving phone then re-converts into sound waves by causing a speaker membrane to vibrate, according to the Cornell Center for Materials Research.
The telephone's speaker contains a permanent magnet--one that retains its magnetism constantly. The "north" on this magnet points toward the speaker membrane.
The wire coil that carries electrical current to the speaker works as an electromagnet, becoming charged with magnetism only when current travels through it.
As the electrical current travels through the wire, the wire magnetizes and demagnetizes many times per second, interacting with the permanent magnet to push and pull the speaker membrane into vibration.
To reproduce natural sound frequencies, the speaker membrane must vibrate at a certain number of cycles per second, requiring the magnetic field to change direction at the same frequency. The pitch "A," for instance, requires 440 magnetic-field reversals per second.