Components
The ringer of a typical rotary telephone can be comprised of a small hammer and one or two bells attached to a mount. The small hammer, which is linked to an electromagnet, rests near the bells without touching them.
Mechanics
Dialing a number sequence on a rotary telephone, according to the Macao Museum of Communications, sends a series of pulses or interruptions in the flow of electric current to telephone company routers. The routers send a ringing signal that activates the electromagnet in the receiving telephone and causes the hammer to strike the bells.
Fun fact
Thomas Watson, an assistant to telephone inventor Alexander Graham Bell, received a patent for a "polarized ringer" in 1878, according to the Telecommunications Virtual Museum. The caller would spin a crank to create an alternating current on the line. The current would magnetize an electromagnet on the receiver's end and send the hammer into action, striking the bells.