An electric bell has:

An electric bell consist of one or more electromagnets, made up of a coil of insulated wire around an iron bar which attract, iron strip armature with a clapper. 

Bell works with the principle of working of electromagnetism. When current passes through the coils in the bell, magnetic field is generated.

When the switch is on, circuit gets completed and the current starts flowing through U- shaped electromagnet, creating a magnetic field in the core.

Which further attracts the iron armature.