Before the doorbell, there was the door knocker. A visitor would lift an iron or brass knob on the door and strike a metal plate to signify his presence. Later, a small bell was installed inside the home; the visitor would pull the string to ring the bell.
The electric doorbell is by far the most common doorbell style. Joseph Henry, an American scientist and first secretary of the Smithsonian Institution, invented the electric doorbell in 1831.
The electric doorbell is a simple device. When the door button is pushed, electrical current flows to a transformer, which activates a noise signaler. The noise signal may be a buzzer, a bell or a chime. Manufacturers have developed many styles and sounds from which consumers may choose.
The first doorbell was a loud electric buzzer. In the early 1930s, musical chimes with attractive tunes such as "Westminster Quarters" became available. Modern wireless doorbells utilize radio waves to sound the doorbell. The deaf use visual devices such as lights to signify a doorbell alarm.