How does a mercury thermometer work?

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There are two types of mercury thermometers. The first type of thermometer either measures the temperature, indoors or out. Used in medicine, the second type of thermometer measures body temperature.


Mercury, also called quicksilver, is one of 92 naturally occurring elements that become liquid at room temperature. Though it expands and contracts evenly with the temperature, Mercury is a highly toxic poison that attacks the central nervous system.


An Islamic philosopher is credited with inventing the first simple thermometer in approximately 1000 A.D. Notables, such as Drebbel and Galileo, followed with their own interpretations of devices that measured changing temperatures. What we would consider a true thermometer wasn't invented until 1714, when Daniel Gabriel Fahrenheit created the mercury thermometer and an accurate measurement scale.

Bulb Thermometers

Acting on the principle that certain liquids will expand and contract with heat and cold, a long tube with a liquid filled bulb at the end is exposed to heat. The liquid expands and forces its way up the tube.

Mercury Thermometers

Because water freezes at 32ºF, it is not an accurate measure of temperature. Sealing mercury within the tube allows for expansion and contraction without the risk of breakage.

Medical Application

Maximum Thermometers used by a doctor works on the same principle as a regular thermometer, except a constriction exists between the bulb and the tube. Once the mercury rises it breaks off making it easier for medical personnel to read. When finished, shaking the thermometer will force the mercury back into the bulb.

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