Liquid-in-glass thermometers make use of the differential expansion of a pure liquid with respect to its glass container to indicate the temperature. The stem is a tube having a fine bore attached to the main bulb; the volume of liquid in the thermometer is such that the bulb is filled completely but the stem is only partially filled at all temperatures to be measured. The changes in volume of the liquid with respect to its container are indicated by changes in the liquid column; by calibration with respect to a standard thermometer, a scale of temperature can be marked on the stem, or on a separate scale tightly attached to the stem.
The Hong Kong Observatory uses the sheathed type mercury-in-glass thermometer. Its thermometer stem, marked with divisions and figures, is fused to an outer glass sheath, which seals it so that no condensation can occur. A rubber ring inside the sheath supports the far end of the stem.