The History of the Thermometer

A common thermometer

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Thermometers measure temperature by using materials that change in some way when they are heated or cooled. In a mercury or alcohol thermometer, the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit (used in the United States) or Celsius (used in Canada), or Kelvin (used mostly by scientists).

The Thermoscope

Galileo thermometer
Galileo thermometer.

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Before there was the thermometer, there was the earlier and closely related thermoscope, best described as a thermometer without a scale. A thermoscope only showed the differences in temperatures; for example, it could show something was getting hotter. However, the thermoscope did not measure all the data that a thermometer could, such as an exact temperature in degrees.

Early History

Galileo Galilei (1564-1642), wood engraving, published in 1864
Galileo Galilei (1564-1642), wood engraving, published in 1864.

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Several people invented a version of the thermoscope at the same time. In 1593, Galileo Galilei invented a rudimentary water thermoscope, which for the first time allowed temperature variations to be measured. Today, Galileo's invention is called the Galileo Thermometer, even though by definition it was really a thermoscope. It was a container filled with bulbs of varying mass, each with a temperature marking. The buoyancy of water changes with temperature. Some of the bulbs sink while others float, and the lowest bulb indicated what temperature it was.

In 1612, the Italian inventor Santorio Santorio became the first inventor to put a numerical scale on his thermoscope. It was perhaps the first crude clinical thermometer, as it was designed to be placed in a patient's mouth for temperature taking.

Neither Galileo's nor Santorio's instruments were very accurate.

In 1654, the first enclosed liquid-in-a-glass thermometer was invented by the Grand Duke of Tuscany, Ferdinand II. The Duke used alcohol as his liquid. However, it was still inaccurate and did not use a standardized scale.

Fahrenheit Scale: Daniel Gabriel Fahrenheit

An old style mercury thermometer, which isn't safe if it breaks, and could be hard to read anyway.
An old style mercury thermometer, which isn't safe if it breaks, and could be hard to read anyway.

What can be considered the first modern thermometer, the mercury thermometer with a standardized scale, was invented by Daniel Gabriel Fahrenheit in 1714.

Daniel Gabriel Fahrenheit was the German physicist who invented the alcohol thermometer in 1709 and the mercury thermometer in 1714. In 1724, he introduced the standard temperature scale that bears his name—Fahrenheit scale—that was used to record changes in temperature in an accurate fashion.

The Fahrenheit scale divided the freezing and boiling points of water into 180 degrees; 32 degrees was the freezing point of water and 212 degrees was its boiling point. Zero degrees was based on the temperature of an equal mixture of water, ice, and salt. Fahrenheit based his temperature scale on the temperature of the human body. Originally, the human body temperature was 100 degrees on the Fahrenheit scale, but it has since been adjusted to 98.6 degrees.

Centigrade Scale: Anders Celsius

Anders Celsius portrait in full color.

Public Domain

The Celsius temperature scale is also referred to as the "centigrade" scale. Centigrade means "consisting of or divided into 100 degrees." In 1742, the Celsius scale was invented by Swedish Astronomer Anders Celsius. The Celsius scale has 100 degrees between the freezing point (0 degrees) and boiling point (100 degrees) of pure water at sea level air pressure. The term "Celsius" was adopted in 1948 by an international conference on weights and measures.

Kelvin Scale: Lord Kelvin

Frost-covered statue of Lord Kelvin
Frost-covered statue of Lord Kelvin.

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Lord Kelvin took the whole process one step further with his invention of the Kelvin Scale in 1848. The Kelvin Scale measures the ultimate extremes of hot and cold. Kelvin developed the idea of absolute temperature—called the "Second Law of Thermodynamics—and developed the dynamical theory of heat.

In the 19th century, scientists were researching what the lowest temperature possible was. The Kelvin scale uses the same units as the Celsius scale, but it starts at Absolute Zero, the temperature at which everything, including air, freezes solid. Absolute zero is 0 degrees Kelvin, which is equal to minus 273 degrees Celsius.

When a thermometer was used to measure the temperature of a liquid or of air, the thermometer was kept in the liquid or air while a temperature reading was being taken. Obviously, when you take the temperature of the human body you can't do the same thing. The mercury thermometer was adapted so it could be taken out of the body to read the temperature. The clinical or medical thermometer was modified with a sharp bend in its tube that was narrower than the rest of the tube. This narrow bend kept the temperature reading in place after you removed the thermometer from the patient by creating a break in the mercury column. That is why you shake a mercury medical thermometer before and after you use it to reconnect the mercury and get the thermometer to return to room temperature.

Mouth Thermometers

Woman with thermometer in her mouth

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In 1612, the Italian inventor Santorio Santorio invented the mouth thermometer and perhaps the first crude clinical thermometer. However, it was both bulky, inaccurate, and took too long to get a reading.

The first doctors to routinely take the temperature of their patients were Hermann Boerhaave (1668–1738); Gerard L.B. Van Swieten (1700–1772), founder of the Viennese School of Medicine; and Anton De Haen (1704–1776). These doctors found temperature correlated to the progress of an illness. However, few of their contemporaries agreed, and the thermometer was not widely used.

First Practical Medical Thermometer

Medic screening temperature of coronavirus patient with digital thermometer
Modern digital thermometers all descend from the first medical thermometer invented by Sir Thomas Allbutt.

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English physician Sir Thomas Allbutt (1836–1925) invented the first practical medical thermometer used for taking the temperature of a person in 1867. It was portable, 6 inches in length, and able to record a patient's temperature in 5 minutes.

Ear Thermometer

Mother taking young boy's temperature with ear thermometer

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Theodore Hannes, a pioneering biothermodynamics scientist and flight surgeon with the Luftwaffe during World War II, invented the ear thermometer. David Phillips invented the infrared ear thermometer in 1984, the same year that Dr. Jacob Fraden, CEO of Advanced Monitors Corporation, invented the popular Thermoscan Human Ear Thermometer.

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Bellis, Mary. "The History of the Thermometer." ThoughtCo, Apr. 5, 2023, Bellis, Mary. (2023, April 5). The History of the Thermometer. Retrieved from Bellis, Mary. "The History of the Thermometer." ThoughtCo. (accessed June 3, 2023).