In this type of switch, a metal tuning fork structure is used to detect the presence of a liquid or solid (powder or granules) in a vessel as shown in the below photograph.
An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. The resonant frequency of the structure dramatically decreases when the prongs of the fork contact anything with substantial mass. The circuit detects this change and indicates the presence of material contacting the fork.
Instrumentation Basics 