The solenoid

A cylindrical coil, having a movable ferromagnetic core, can be useful for various things. This is a solenoid. Electrical relays, bell ringers, electric “hammers,” and other mechanical devices make use of the principle of the solenoid.

A ringer device

Figure 1 is a simplified diagram of a bell ringer. Its solenoid is an electromagnet, except that the core is not completely solid, but has a hole going along its axis. The coil has several layers, but the wire is always wound in the same direction, so that the electromagnet is quite powerful. A movable steel rod runs through the hole in the electromagnet core.

Figure 1: A solenoid-coil bell ringer.

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