[en] A tuning fork is disposed with one prong arranged horizontally within a vessel containing a medium, such as a foam, and the other prong outside the vessel. Excitement of the prong outside the vessel causes sympathetic excitement of the prong within the vessel. If the vibration of the outer prong is stopped suddenly the vibration of the inner prong dies away with a decrement which depends on whether the inner prong is immersed in the medium or is above the surface of the medium. It is therefore possible to determine when the level of the medium within the vessel rises or falls above or below the inner prong. By utilising two tuning forks it is possible to determine upper and lower level limits for the medium within the vessel. (author)