Solenoid

A cylindrical coil, that is having a movable ferromagnetic core, is useful for several things. This is a solenoid, electrical relays, electric hammers, bell ringers, and other mechanical devices make use of principle of solenoid.

A ringer device

Figure given below is a simplified diagram of a bell ringer. The solenoid is an electromagnet, except that the core of it is not completely solid, but has a hole going along the axis of it. The coil has many layers, but the wire is always wound in same direction, such that the electromagnet is quite powerful. A movable steel rod runs through the hole in electromagnet core.

Figure--   A solenoid-coil bell ringer.

When no current is flowing in the coil, the steel rod is held down by force of gravity. But when the pulse of current passes through the coil, the rod is pulled upward so that it strikes the ringer plate. This plate is like one of the plates in xylophone. The current pulse is short, so that the steel rod falls back down again to the resting position, providing the plate to the reverberate Gonggg! Some office telephones are equipped with the ringers which produce this noise, rather than conventional ringing or electronic bleeping ernitted by most of the phone sets.