Why is mercury used as a thermometer?

Hits:Updated:2020-04-11 16:04:48【Print】

In principle, any substance whose properties vary significantly with temperature can be used to measure temperature. We can also use a substance whose color changes with temperature. When the substance emits blue light, people can recognize that the temperature is high, and when it emits red light, it can recognize that the temperature is low.
Similarly, when the temperature rises or falls, the water in the small glass tube will rise or fall. This is the working principle of every sealed, liquid-filled thermometer. However, if something rich and cheap like water can be so effective, why do we insist on using something extremely rare and expensive like mercury?
In the centuries after the invention of the first thermometer, humans learned the thermodynamic temperature. Therefore, the thermometer initially defined the temperature. However, the first thermometer is not a temperature measuring device or a calorimeter, but a temperature measuring range device, which only indicates whether the temperature is high or low. These devices are not calibrated to a standard range, they can only make a rough or vague assessment.
The invention of the first thermometer is due to Hero of Alexandria. He is an intellectually eager engineer and considered to be the greatest experimenter in ancient times. The device he designed consisted of an air-filled tube whose end was submerged in a small bowl of water. When the thermometer touches a hot or cold surface, the air expands or contracts, causing the air-water interface to fluctuate.
Even Galileo's invention followed the same principle. However, these inventions not only do not have any scale, but they are also sensitive to the pressure of air. In order to design a device that only responds to heat, Galileo student Joseph Delmedigo invented the first sealed liquid built-in glass thermometer. This is the first thermometer because it is marked. However, the liquid he sealed was not water, but alcohol.
Coefficient of expansion
At a constant pressure, matter expands when heated, because heat increases the kinetic energy of atoms, causing them to move violently and separate from each other. The increase in volume is evident in solids such as metal rails, rubber tires, and liquids such as water, alcohol, mercury, and halogen. However, the degree of expansion of each substance is different every time the temperature rises. This material constant is called the expansion coefficient.
Alcohol is more popular than water for the simple reason that it has a higher expansion coefficient. Even a small temperature change will cause its volume in the tube to change drastically. However, alcohol is so susceptible that these changes will cause the alcohol performance in the tube to be almost turbulent. The liquid interface changes continuously with small changes in temperature. This unpredictable change is worrying, because when the thermometer is taken out of a pot of boiling water, the reading will immediately become the temperature of the new environment, unable to measure the boiling water temperature we want.
To avoid this unreliability, Dutch inventor Daniel Warrenheit replaced mercury with mercury. The expansion coefficient of mercury is higher than that of water, which means that its volume changes more significantly with temperature. However, its change value is almost six times less than that of alcohol. That is to say, every time the temperature rises, the volume of alcohol rises 6 times larger than the volume of mercury.
This means that in a sealed glass tube, mercury will rise much slower than alcohol, but it also means that when the thermometer is removed from a pot filled with boiling water, the rate of mercury decline will be equally slow. Its reading will be effectively undisturbed, making the thermometer very reliable.
The thermometers prior to this invention were unique, and their readings did not meet any standardized scale range. However, Warren Hite provided a range of readings, applied to all mercury thermometers manufactured, and eventually spread to all thermometers. This transformation is not very cumbersome, because almost all mercury thermometers are made by Warren Hite. Later, this scale changed slightly, now this kind of thermometer is called Fahrenheit thermometer.
Examples of better alcohol
The hypersensitivity of alcohol can be compensated by its advantages. Compared to mercury, alcohol is much cheaper, and it is not so rare and non-toxic. If the mercury thermometer is damaged, the laboratory may need to be sealed for several hours because mercury inhalation can cause serious health problems. Alcohol has no such threat.
More importantly, the freezing point of mercury is -40 ℃, and the freezing point of alcohol is -115 ℃. This means that the mercury thermometer cannot measure the temperature of -40 ℃, then it cannot be often used in the scientific laboratory or the superconducting technology manufacturing industry.
But alcohol is colorless, and this feature forces manufacturers to add artificial dyes to alcohol to make them clearly visible. In addition, although alcohol can measure extremely low temperatures, it cannot measure temperatures above 78.37 ℃ because this is the boiling point of alcohol. Compared with the boiling point of mercury 356.7 ℃, it is almost pitiful.
Although the rare, expensive and toxic properties of mercury cannot be changed, we can still break through its temperature limits. To further increase its boiling point, mercury is usually sealed with an inert gas, such as nitrogen. The inert gas increases the pressure on liquid mercury, which further increases its boiling point.
We can also lower its freezing point by combining it with thallium. These new mercury-thallium thermometers can now measure temperatures of -62 ° C. Although the mercury thermometer has minor flaws, it is considered to be one of the most accurate thermometers available.
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