This device consists of flexible bellows connected to a hollow bulb via a
capillary tube. Increasing temperature causes the gas/liquid trapped in the
bulb to expand and the bellows to deflect. This movement may be used, via
linkages, to indicate temperature on a calibrated scale, or, to operate electrical
contacts. A cam may be used to change the temperature at which the contacts
separate.
Capillary bulb thermostats function in the range -130 to +540°C, though the
lower limit is usually 30°C. They are used in domestic ovens and industrial
furnaces that operate within this range.
This experiment illustrates the use of a capillary bulb thermostat to control
temperature.