The moving-coil microphone (Fig. 43.24) has a diaphragm with a light coil attached. The coil is situated in the strong magnetic field of a cylindrical pot magnet. Sound waves cause the diaphragm to vibrate, and as the coil moves back and forth in the magnetic field an alternating e.m.f. is induced in it. This is fed into an amplifier by a light flexible lead.
It is of interest to compare the construction of the moving-coil microphone with that of the moving-coil loudspeaker in Fig. 38.9. In one, energy is transferred from electric current into mechanical energy of vibration in a cone and thence to sound waves. In the other the reverse occurs. Energy in sound waves is transferred to mechanical energy in a vibrating diaphragm and thence to electric energy.