Electromagnets in Bells and Buzzers

Around 1850 John Mirand added a clapper and gong to Johann Philipp Wagner and Christian Ernst Neeff's old design to make the standard electric bell we know today. Electric bells are made up of one or more electromagnets, made of a coil of wire wrapped around an iron core, that attracts an iron armature with a clapper. When an electric current is sent through the coils, the electromagnet creates a magnetic field which pulls the armature towards it, causing the clapper to strike the bell. They were made to operate on very low amounts of voltage (five to twenty-four volts). This specific type of mechanism can be seen in doorbells, buzzers, telephones, and fire alarms. 

( older electric doorbell )

In an interrupter bell, the bell gets hit by a spring-loaded clapper ( an arm with a metal ball at the end of it) that is actuated by an electromagnet. The clapper is held away from the bell, when at rest, but when the switch closes it allows an electric current to flow from the battery, through the electromagnet. This creates a magnetic field, that attracts the metal of the clapper to the bell, resulting in a tap. This opens the electrical contacts (depicted as 'T'), which simultaneously interrupts the current to the electromagnet, resulting in the collapse of the magnetic field. This then closes the contact again and allows the current to flow back through the electromagnet, pulling the clapper back over to the bell, making another tap. It is this cycle that then repeats many times per second to make the traditional school bell sound. A buzzer uses the same mechanics as an interrupter bell, just on a smaller and quieter scale.

( interrupter bell )