How Thermometers Work: The Mercury Thermometer


Silver, beautiful, shiny, dense and poisonous, no I’m not talking about the planet. I am talking about the ever-so-useful element on the periodic table – mercury! This blog isn’t just about the element mercury, but about the thermometers that use this gorgeously silver substance. Today I will break down how these thermometers work, why they are being phased out and why they are even being banned in some countries and states.

Mercury thermometers, remember that plastic tube your mother used to put in your mouth when you felt sick or had fever? Well that was likely a mercury thermometer. Mercury is a toxic substance, and using it to measure ones fever or cut of beef doesn’t seem to be the brightest move in human history, but we still used it. Technology has come into place during the last century and the phase out of mercury thermometers is on its way. Since 2001 the United States have lobbied to have mercury banned in the medical and food industry – 20 states have banned the use of mercury in food and fever thermometers completely. Countries such as Norway and Sweden have completely banned the use of mercury thermometers as well as Countries such as Denmark, Finland, Japan, Canada Netherlands and Hungary have implemented restrictions and plans to phase out the mercury thermometer.

The mercury filled thermometer is very simple design. A small, thin glass tube filled with a small amount of mercury. Mercury expands and increases when heated thus moving up the scale. Mercury have made terrific thermometers in the past. It gives quick and accurate reading; however (besides the poisonous features) this thermometer also has other restrictions to its function. The thermometer becomes useless when it reaches -39°C which is the freezing point of mercury. Once you reach that point and go beyond it you will have to get the thermometer repaired.

What about Mercury poisoning? Mercury poisoning can happen a number of different ways: Inhalation, absorption and ingestion which can cause harm to the brain, heart, kidneys and immune system. Unfortunately mercury is still used in some food thermometers. If you happen to come across one, proceed with caution when using these thermometers as this is a dangerous substance.


Next week we will continue our “How thermometers work” series by going over the classic Dial thermometers. In the meantime head over to CCI Canada or CCI USA to view the non-mercury filled candy thermometer! Til next time!

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