liquid filled temperature switch which comprises a brass bulb filled with a chemical fluid (sometimes gas).
It includes a small tube which hooks up the bulb to a pressure sensing mechanism consisting of bellows, bourdon tube or diaphragm.
When temperature inside the bulb increases, it gets hot and causes the liquid or gas inside the bulb to spread out.
This expansion of liquid increases the pressure inside the bulb.
This rise in pressure results in the activation of the pressure switch connected to the bulb.