Thermometer

A thermometer is an instrument that measures temperature, usually by watching how a substance such as air, alcohol, mercury or a metal changes as it warms or cools. No single person invented it, because the device grew out of earlier thermoscopes that showed heat changes without a numbered scale. Today thermometers range from the sealed glass tube used at home to the platinum sensors that national laboratories use to realize the kelvin.

History of the thermometer

The oldest known design is credited to Philo of Byzantium, an engineer who lived about 2,000 years ago. His device was a hollow sphere of air and water joined to an open pitcher by a tube, so that heated air pushed water along the tube. The Roman physician Galen later created a device with a crude nine-point scale, although its marks stood for levels of hot and cold rather than measured amounts, according to the National Institute of Standards and Technology (NIST).

Modern thermometry began around 1600. A thermoscope, an inverted glass tube standing in a bowl of water or wine, was probably developed around 1593 by Galileo or by his friend Santorio Santorio, a professor of medicine at Padua. Santorio used a version of it to check patients for fever, and his 1612 commentary on Galen contains the first written mention of a clinical device of this kind.

Open thermoscopes reacted to air pressure as well as heat, so scientists in Florence sealed liquid inside thin glass tubes around the middle of the 1600s. Members of the Academia del Cimento, working under Grand Duke Ferdinand II, used colored alcohol. Sealed liquid thermometers had replaced the air thermoscope by the end of that century.

Fixed scales came next. In the early 1700s the Danish astronomer Ole Rømer and Isaac Newton each proposed reference points, which turned thermoscopes into true thermometers. Daniel Gabriel Fahrenheit is credited with the first widely used mercury thermometers, and the Swedish astronomer Anders Celsius followed roughly a decade after the French scales of the 1730s with a scale of 100 degrees between the freezing and boiling points of water. One account counts about 35 competing scales before Fahrenheit and Celsius became the two main ones, and William Thomson, later Lord Kelvin, proposed an absolute scale in 1848.

Inventor and origins of the thermometer

Credit for the first thermoscope is disputed. Galileo claimed it, but the evidence is described as inconclusive, and Santorio, Cornelis Drebbel in the Netherlands and Robert Fludd in England have all been proposed as alternatives. Authors cited by the World History Encyclopedia, Fermi and Bernardini, conclude that the thermometer seems to have been invented independently by several people in different places.

Sources also differ on the dates of Fahrenheit’s work. One reference says he made sealed alcohol thermometers by 1709 and sealed mercury thermometers in 1714, while NIST dates his published scale to 1724. Which date is right depends on whether the invention is counted as the instrument, the liquid inside it or the scale printed beside it.

How a thermometer works

A liquid thermometer relies on thermal expansion. When the bulb warms, the liquid inside expands and climbs a narrow tube, and a printed scale turns the height of the column into a reading. Mercury became a favorite filling because it is an excellent liquid for showing small changes in temperature, even though it is toxic.

Temperature measures the average energy of motion of atoms and molecules. NIST explains that thermodynamic temperature is expressed in kelvins above absolute zero, the theoretical point at which nothing can get colder. Scientists cannot easily measure that internal energy directly, so they watch heat flow, and two objects share the same temperature once heat no longer passes between them. This is why a thermometer must settle into thermal equilibrium with what it measures before the reading can be trusted. The ideas behind this are covered in thermodynamics.

Types of thermometers

The earliest type was the air thermoscope, which had no scale. It gave way to sealed alcohol thermometers in Florence and then to mercury-in-glass instruments, whose tubes and bulbs are made from glass. Santorio’s 1625 book described one uncalibrated and three calibrated clinical air thermometers, so the idea of a body-temperature device is older than the mercury fever thermometer many people know. A reference summary gives a normal body temperature range of 98.2 to 98.6 degrees Fahrenheit.

Laboratories use other designs. A standard platinum resistance thermometer relies on the fact that the electrical resistance of platinum rises in a reliably predictable way as it gets hotter. A radiation thermometer determines the temperature of an object from the optical or infrared radiation it emits, and NIST uses this type at the high end of its reference scale. Acoustic thermometers measure the speed of sound in a gas, and electronic thermometers measure tiny voltage fluctuations in resistors. NIST calls these primary thermometers when temperature can be calculated directly from well-known physical relationships.

Impact of the thermometer

Measurement turned fever and weather into numbers that could be compared. By the early twentieth century doctors used charts of expected temperature changes during illnesses to follow a patient’s progress. Fahrenheit’s scale was the main standard in English-speaking countries until the 1960s, and the Celsius scale later replaced it in most countries, while the kelvin became the unit of choice in science.

Temperature scales also changed how units are defined. The triple point of water, the single temperature at which ice, liquid water and vapor coexist, defined the kelvin from 1954. The kelvin has since been redefined using the Boltzmann constant, which links the kinetic energy of matter to its temperature through the relation E = kBT. A change of one kelvin equals a change of one degree Celsius, and 0 K is minus 273.15 degrees Celsius.

Uses of thermometers today

NIST notes that most everyday temperature measurements, from weather forecasts to food preparation and manufacturing, still use the Fahrenheit and Celsius scales. The kelvin appears in daily life mainly as the color temperature of lamps. An old incandescent bulb has a color temperature of about 3,000 K, while lamps labeled daylight run from 5,000 K to 5,600 K because the surface of the Sun is about 5,800 K.

Standards laboratories depend on the International Temperature Scale of 1990, which covers 0.65 K to 1,357.77 K, and on a separate Provisional Low Temperature Scale of 2000 for 0.9 millikelvin to 1 K. Together they provide 21 standard reference points. These include the triple point of mercury at 234.3156 K, the freezing point of aluminum at 933.473 K and the freezing point of gold at 1,337.33 K, and instruments are calibrated against them.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue