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What is a Mercury Thermometer?

By Summer Banks
Updated Mar 03, 2024
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A mercury thermometer is a tool used to measure temperature. There are a number of different model types, sizes, and specifications, but the general operating principle is about the same in all cases. The thermometer usually has a small reserve of liquid mercury in its base, which is most often pressurized; above it is usually a glass or synthetic tube with a calibrated temperature scale. Mercury, which is a chemical element, rises and falls in response to temperature changes, and watching where it stops along the scale provides a reading of how hot or cold a particular substance is. Many of these tools are used in medicine and around the home to gauge body temperature, though they can also be used outside and in more professional meteorological situations. In some instances they’re also used in kitchens, usually as a way of monitoring warm liquids, but most modern chefs prefer more food-specific gauges. There are usually a number of different options and models available depending on the intended use.


The basic instrumentality is usually pretty simple. Mercury, which is also sometimes known as quicksilver, is a silvery liquid that responds very well to atmospheric changes, particularly temperature and barometric pressure. The German physicist Gabriel Fahrenheit is credited with inventing the first mercury thermometer in 1714, which he made with a glass tube attached to a small well of mercury. That mercury was secured in a metal tip. Metal is usually regarded as a good conductor of heat, and Fahrenheit noticed that when that tip was placed into liquids of varying temperatures, the mercury rose and fell in the glass chamber. He then developed a system of calibration to create a scale for easy reading. Most modern models work this way, and though advancements have been made in terms of presentation the basics are usually about the same.

Mechanics and How They Work

Most thermometers hold between 0.02 ounces (0.5 g) and 0.1 ounces (3 g) of mercury. Thick glass generally encases the temperature-sensitive liquid metal. When the tool is placed outdoors or in the body, the metal tip heats and mercury stretches along the length of hollow space within the tube. Higher temperatures cause mercury to grow longer, while low temperatures cause it to fall or shrink back down. These thermometers are usually marked with temperature readings on the Fahrenheit or Celsius scale.

Most Common Uses

These sorts of thermometers are most commonly used in medicine and meteorology. Medical doctors often use a version known as the maximum mercury thermometer to measure human body temperature. This type can be used either orally, which is to say slipped under the tongue, or rectally.

A maximum mercury-style thermometer works in much the same way as the standard version, but the mercury does not shrink back into the tip when removed from a heat source. This allows temperature readings to be kept until mercury is forced back into the tip by centrifugal force, or shaking the thermometer with the head pointed toward the floor. Nitrogen gas is what usually holds the mercury in place, and usually has to be inserted during the manufacturing process.

Mercury-driven thermometers may also be used by meteorologists to measure air temperature. Standard mercury freezes at -37.89°F (-38.83°C). In order to measure temperatures below this point, meteorologists will often mix mercury with thallium alloy. Thallium reduces the freezing point to -78°F (-61.1°C). If mercury freezes, nitrogen can be caught behind the metal causing the thermometer to stop working.

Safety Concerns

Mercury can be toxic if consumed, and it can also cause skin and eye damage if touched or respiratory distress if inhaled. This has led some people to question the wisdom of having mercury so readily accessible in homes. Within the confines of the thermometer there isn’t usually any safety risk, but should the glass break, exposure is likely. Spilled mercury can be very challenging to clean, since the loose liquid often forms rounded “balls” that can bounce and scatter across a floor or room.

Most modern thermometers are made of much stronger glass then their original counterparts, and the amount of mercury contained in each is quite low in any event. Just the same, due to the dangers associated with mercury poisoning, many countries around the world, most notably in Europe, have banned mercury-based thermometers for medical use. It is also difficult to purchase mercury thermometers for home use in many of these countries.

The United States has not banned mercury in medical settings, though many advocacy groups have recommended a number of safety precautions. The American Academy of Pediatricians and the United States Environmental Protection Agency, for instance, have each suggested alternative temperature-measuring devices for personal use in order to reduce the risk of accidental exposure.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By Sara007 — On Jul 12, 2011

If you have children and they are scared of having their temperature taken in a traditional method you can buy nifty plastic strip thermometers. These thermometers work exactly like bandages and you stick it to your child's skin for a few moments and it gives you a temperature reading. They are painless to use and disposable so you don’t have to worry about cleaning anything.

I really loved the idea of plastic strip thermometers because with mercury thermometers I was always worried about my children dropping the glass vial and breaking it. With the strips you stick on you no longer have to worry about any of that.

By lonelygod — On Jul 12, 2011

I have always preferred mercury thermometers because of how easy they are to use. I think as long as you are careful there is no need to drop $50 or more on a digital thermometer.

I have found that the digital thermometers that you press against your forehead are absolutely terrible. I am not sure if it was just the brand that was being used on us at work, but it never got a reading correct without multiple tries, and even that final reading was questionable.

If you do have a mercury thermometer just make sure to wipe it off with a new alcohol swab after each use so you can disinfect it.

By miriam98 — On Jul 11, 2011

@allenJo - Ask your doctor what medical thermometer they use. I know the hospitals use some digital ear thermometers that are supposed to be rated very high for accuracy, and as a consumer you can buy these models yourself.

I think you can get them for under $50. Some of them will deliver temperature readings within two to three seconds rather than forcing you to wait for almost a minute as with other consumer grade digital thermometers.

By allenJo — On Jul 10, 2011

I know that the ear thermometer has become the instrument of choice as physicians got away from the more invasive rectal thermometer, but I wonder if the they are as effective as they claim to be.

At home I’ve taken different temperature readings when using the ear thermometer and got different results each time, usually off by a degree or two – sometimes within the same hour. Maybe I’m not using it properly, I don’t know.

The ear thermometer is cleaner and less invasive than an oral thermometer, which is what it has going for it, but if I can’t be sure that the temperature is accurate it’s not much use.

By Azuza — On Jul 10, 2011

@SZapper - I remember when the digital thermometers for at home use came out too.

I know mercury thermometers are still alive and well in science classes! I remember using them in every lab science class I ever took in high school and college. I remember they did always remind us to be careful and not to drop them.

By SZapper — On Jul 10, 2011

I'm pretty sure I remember when they started phasing out mercury thermometers for at home use. I remember when I was very little we used a mercury thermometer and my mom always warned us to be very careful with them.

Then when I got a little bit older digital thermometers came out and we replaced our mercury thermometer. Like everything else electronic I remember digital thermometers used to be a little bit more expensive than they are currently!

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