What is Capillary Thermostat working principle?

The capillary thermostat operates on the principle of fluid expansion. A temperature change in a fluid filled sensing system consisting of probe (also called bulb), capillary and diaphragm capsule, produces a volume change.

The resulting movement of the diaphragm acts through a mechanism to operate the micro switch. i.e a single pole, single throw normally closed circuit breaks (opens) on rise of temperature.