The principle of Wagner’s hammer is used in horns to generate sound. In this quite simple construction, pressing a button (a) closes the circuit in which an electromagnet (b) is located. The magnetic field now created moves another switch (c), which immediately breaks the circuit again (d). However, this also leads to an immediate deactivation of the electromagnet (e), which is now moved back to its original position by a return spring (f). This closes the switch again (g). However, this also closes the circuit again so that the process can start anew.
So as long as “the horn is pressed”, the magnet and the return spring create a rapid sequence of closing and opening of the circuit, i.e. an oscillating movement, which is picked up by a diaphragm (h) and converted into a sound signal.