Liquid-in-glass thermometers, also known as glass thermometers, are commonly used in laboratory and testing applications. They are constructed of a glass tube that’s filled with a dyed liquid that expands or contracts at a known rate corresponding with the temperature. The liquid level can then be read, and the temperature recorded, using the graduation marks on the outside of the thermometer. Compact or pocket liquid-in-glass thermometers have a shorter stem, may come with a protective sheath, and are used to record general temperature readings.