Why Mercury is Used in Thermometers (and Modern Alternatives)
While modern thermometers are usually filled with dyed alcohol, mercury is a better liquid to use. Well, as long as you don’t count mercury’s health hazards.
Nowadays, mercury thermometers are rare. But why was mercury so common before people cared about the health effects?
Mercury was used in thermometers primarily because of its liquid temperature range, its constant coefficient of thermal expansion, and its high thermal conductivity. However, nowadays, mercury is usually banned because of its health hazards.
Outline
Why Mercury is a Great Thermometer Filling
Mercury is an ideal thermometer filling for 5 reasons:
1. Mercury is a liquid from -39 ºC to 357 ºC.
Few materials can be liquid in a similar range–and mercury is the only pure metal that is liquid at room temperature. Many of the other benefits I list would apply to most metals, but being a solid is a dealbreaker!
Mercury’s wide temperature range makes it much more versatile–you can use the same thermometer to measure a person’s body temperature, check how cold the weather is, and even measure the temperature of cooked food.
In certain alloys (such as with Thallium), the freezing point of mercury can be dropped even further.
2. Mercury has a large, linear coefficient of thermal expansion.
Although you probably learned in high school that thermal expansion is always linear, that’s not always true. Thermal expansion is actually approximated with polynomials, and the linear approximation is the simplest.
The large thermal expansion is good because it allows the thermometer reading to be determined more easily. The linear approximation is less important because it would be possible to draw a thermometer with a non-linear scale, but that’s a lot of effort.
As you can see, mercury’s coefficient of thermal expansion is not only linear, it barely changes with respect to temperature. Ethanol (current replacement of mercury) changes linearly with temperature, and water changes nonlinearly.
3. Mercury is easy to see because it is reflective.
Yes, you can take other materials and dye them, but the dye may affect thermal expansion. It’s not a bad solution (and it’s the most common current solution), but mercury’s natural reflectivity is a point in its favor.
4. Mercury Conducts Heat Well.
Good thermal conductivity means that the thermometer will have the correct temperature, faster.
5. Mercury doesn’t “Wet” Glass
Wetting refers to the way that a liquid sticks to a solid. You can see how much a liquid wets a solid by putting a drop of the liquid on a flat surface of the solid.
Since mercury is non-wetting to glass, it sticks to itself more than the glass. Water sticks to the glass a bit more.
In a thermometer, if the liquid wets the glass, then falling temperature will cause the liquid to stick to the glass, showing a higher measurement than reality.
Summary Chart
Among the obvious liquids to fill a thermometer, mercury has the best combination of properties. Its thermal expansion barely changes over temperature, which makes measurement very accurate. Mercury is liquid under all temperatures where it is safe for humans, and it has a high-enough thermal conductivity to enable quick temperature measurements.
| Element/Compound | Liquid Range (ºC ) | Thermal Expansion (1/ºC) | Thermal Conductivity (W/(m-K)) |
| Mercury | -39- 357 | Low and linear | 8.4 |
| Water | 0 – 100 | High and nonlinear | 0.609 |
| Ethanol | -114 – 78 | Medium and linear | 0.171 |
| Gallium | 30 – 2400 | — | 40.6 |
Gallium is an interesting replacement for mercury, but since it’s solid below 30ºC, it’s not ideal for measuring the weather. It is possible to make alloys with a lower melting point than gallium, such as gallinstan.
However, colored ethanol is the most-common material in liquid thermometers today, because it’s cheap. If you wanted a “better” thermometer, your money is better spent buying a non-liquid thermometer, rather than using an expensive fusible alloy. But if you are interested in other applications for low-melting point metals, you might want to check out my article about gallinstan and other fusible alloys.
Why Mercury is No Longer Used in Thermometers
Well, perhaps I should say they should no longer be used in thermometers. Government regulations around the world have put restrictions to stop this practice. In many places, mercury thermometers are completely forbidden.
Mercury is extremely toxic. It’s so toxic, in fact, pollutants (of all kinds) can build up trace amounts in fish, and humans who eat too much of this fish can even suffer mercury poisoning.
If mercury builds up inside animals (including humans), the animals cannot get rid of the heavy metal. Scientists have disputed whether amalgamous dental fillings pose a risk of mercury poisoning, but broken mercury-filled thermometers definitely pose a risk of mercury poisoning.
Not only is mercury poisoning from broken thermometers dangerous because of accidents, mercury is mesmerizing to play with and many children have intentionally broken thermometers to play with the raw mercury inside.
Surprisingly, inhaling vapor from the mercury is actually more dangerous than swallowing it. So don’t try playing with mercury; even a careful adult who knows not to swallow mercury may accidentally inhale it.
For these safety reasons, mercury is no longer used in thermometers. There are still applications that require mercury (and also face increasing regulation), but thermometers are especially dangerous because they are a common household object which can easily release pure liquid mercury.
Alternatives to Mercury-based Thermometers
There are many ways of determining the temperature, besides mercury-filled thermometers.
Colored Water
Honestly this is a bad choice. Water wets the glass and is not useful for reading temperatures below 0 ºC or above 100 ºC. It also has a very nonlinear coefficient of thermal expansion. Water-based thermometers could be used for reading body temperature, but that’s about it.
Colored Ethanol
Colored ethanol is the standard liquid thermometer filling these days. Ethanol is cheap and has a very low freezing point, but it boils at a lower temperature than water. It also wets glass. Ethanol thermometers are good for reading body temperature or the weather, but not for cooking.
Gallinstan
One liquid metal alloy that is similar to mercury is gallinstan, which is a eutectic alloy of gallium, indium, and tin. The eutectic allows gallinstan to reach a lower melting point than any of its constituent pure elements. Gallinstan still has a higher freezing point than mercury, however, so in most cases that require a more expensive thermometer, it’s better to go with a non-liquid option.
Springs
Meat thermometers often use a (solid) metal spring to determine temperature. Since these are used exclusively at high temperature and accuracy is not that important, the comparatively low thermal expansion of a solid metal is not a big issue.
Bimetallic Strips
Bimetallic strips are mostly used in thermostats. These have two metals connected together with different thermal expansions. As the temperature changes, one piece of metal will expand/contract more than the other, causing the strip to bend. Depending on the direction that it bends, the thermostat will trigger cooling or heating in your home.
Thermocouple
If you are a scientist, you’ve probably used a thermocouple. These are among the most accurate devices for reading temperature, and rely on measuring the resistivity change of a metal as an electric current goes through it (it’s a bit more complicated, but that’s the simple version).
Digital thermometers usually use a thermocouple.
Infrared Sensor
These devices are usually not super accurate, but they have many advantages (you can use them from a distance, and they are very fast). All objects with thermal energy radiate that energy–which typically falls in the infrared spectrum. Infrared sensors detect this infrared spectrum, compare it to the surrounding background spectrum, and calculate the temperature.
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