Pump for applying an increased or reduced pressure to a container. The
pump is provided with a plate-like part which reacts to the pressure
difference between the container and the environment. This part can
preferably adopt two stable positions, and a change is signalled. This
change can be brought about, for example, by the sound generated as a
result of the plate-like part turning over. The pitch and the like of the
sound can be influenced by arranging the plate-like part in a hollow
piston and providing the latter with openings leading to the environment.