Capillary and bulb style thermostats use expansive liquid to open or close contacts in response to changes in temperature. No matter which style or model you choose, all capillary thermostats work with the same physics. A fluid is encapsulated in a metal tube (bulb and capillary) and when the temperature changes, the fluid expands or contracts. This fluid change moves the head, which triggers a snap-action switch, either opening or closing the circuit.