A mercury thermometer is a device used to measure temperature. It consists of a glass tube filled with liquid mercury, which expands and rises in the tube as the temperature increases. The scale on the thermometer allows users to read the temperature accurately. These thermometers are commonly used in laboratories and medical settings, although their use has declined due to safety concerns regarding mercury, a toxic substance. Alternatives like digital thermometers are now more popular, but mercury thermometers are still valued for their precision in certain applications.