[Wikipedia excerpt: ‘….An electric ‘B’ is a mechanical or electronic ‘B’ that functions by means of an electromagnet. When an electric current is applied, it produces a repetitive buzzing, clanging or ringing sound. Electromechanical ‘B’ have been widely used at railroad crossings, in telephones, fire and burglar alarms, as school ‘B’ , door’B’ , and alarms in industrial plants, since the late 1800s, but they are now being widely replaced with electronic sounders. An electric ‘B’ consists of one or more electromagnets, made of a coil of insulated wire around an iron bar, which attract an iron strip armature with a clapper. When an electric current flows through the coils, the electromagnet creates a magnetic field which pulls the armature towards it, causing the hammer to strike the ‘B’ .….’ ].