The bulb thermometer is the common glass temperature sensor for which you might be grown up. This thermometer is filled with a liquid, typically mercury. Bulb thermometers work on the simple premise that it volume of the liquid varies in proportion to its volume.
It is a glass tube that is pre full with a fluid such as a hydrogen or liquor and is sealed at both sides. The fluid in the glass tube rises as the mercury inside the thermometer's bulb increases. When the temperature gauge is warm, the liquid inside flows up in the tube.
