A doorbell is a signaling device typically placed near an entry door to a building. When a visitor presses a button the bell rings inside the building, alerting the occupant to the presence of the visitor. Although the first doorbells were mechanical, activated by pulling a cord, modern doorbells are electric — they are actuated by an electric switch. A precursor to the electric doorbell, specifically a bell that could be rung at a distance via an electric wire was invented by Joseph Henry around 1831. An early patent for an electric door bell, patent 237,455, was obtained in 1881 by Edwin Swan of Indiana for an "illuminated electric push button".
Read more about Doorbell: Wired Doorbells, Wireless Doorbells, Musical and Continuous Power Doorbells