The moving-coil microphone

Figure 13.22 shows a diagram of a moving-coil magnetic microphone.

When sound is directed at the microphone's diaphragm, the diaphragm vibrates in and out. The light coil attached to it vibrates in step. Because the coil vibrates in a magnetic field, the magnetic flux linking the coil changes, and an emf is induced in the coil. The coil moves back and forth, so the direction of the emf also keeps reversing. The microphone therefore produces a small alternating current (one that flows both ways) that varies in step with the sound.

Why it looks like a loudspeaker

Put figure 13.22 next to the loudspeaker of figure 13.9: a magnet, a light coil in its field, and something light fixed to the coil to move the air. They are the same machine, run in opposite directions. In the loudspeaker a current pushes the coil; in the microphone the coil is pushed and makes a current. The microphone's current is tiny, so in a public-address system it is first made much bigger by an amplifier, and then fed to a loudspeaker, where the motor effect turns it back into sound — louder than it went in.