Modern telephone instruments are largely electronic. Wire coils that performed multiple functions in older sets have been replaced by integrated circuits that are powered by the line voltage. Mechanical bell ringers have given way to electronic ringers. The carbon transmitter dating from Edison’s time has been replaced by electret microphones, in which sound waves cause a thin, metal-coated plastic diaphragm to vibrate, producing variations in an electric field across a tiny air gap between the diaphragm and an electrode. The telephone dial has given way to the keypad, which can usually be switched to generate either pulses similar to those of the dial mechanism or dual-tone signals as in AT&T’s Touch-Tone system. Finally, a number of other features have become available on the telephone instrument, including last-number recall and speed-dialing of multiple telephone numbers.