The History of Thermometers

Quick answer

No single person invented the modern thermometer in one step. Ancient pneumatic experiments showed that heated air could move a liquid. Around the end of the 16th century and the beginning of the 17th, several European experimenters developed air thermoscopes that made warming and cooling visible. Galileo is strongly associated with this stage, but Museo Galileo notes that the invention has also been attributed to Santorio Santorio, Robert Fludd, and Cornelis Drebbel.

A thermoscope indicates that temperature has changed; a thermometer adds a scale and a repeatable measurement method. Santorio described thermometric apparatus for medical measurement, Florentine instrument makers developed sealed spirit-in-glass thermometers, and Fahrenheit and Celsius helped establish reproducible scales. Kelvin later provided an absolute thermodynamic scale. Modern instruments may use electrical resistance, thermoelectric voltage, semiconductor junctions, or emitted infrared radiation instead of liquid expansion.

Thermoscope vs thermometer

The distinction is important because many popular histories call every early temperature-sensitive device a thermometer. A thermoscope shows that a body or the surrounding air has become warmer or cooler. It may move a liquid column, but without a calibrated scale it cannot report a standardized numerical temperature.

A thermometer combines a temperature-sensitive element with a scale, readout, or conversion function. The instrument also needs a defined procedure if measurements are to be compared across locations and over time. Modern thermometry adds calibration, traceability, measurement uncertainty, and controls for environmental effects.

Who invented the first thermometer?

Ancient pneumatic experiments

Devices described in antiquity by writers such as Philo of Byzantium and Hero of Alexandria used the expansion and contraction of air to move liquid. These demonstrations were not calibrated thermometers, but they established the physical behavior that later thermoscopes used.

Galileo and a shared historical attribution

Galileo is commonly connected with a bulb-and-tube air thermoscope. Museo Galileo reports that Vincenzo Viviani's later biography placed Galileo's development of such an instrument in Padua in 1597. That is historical testimony, not a surviving dated instrument or proof that no one else developed a similar device independently.

Museo Galileo's broader account explicitly says that the thermoscope has been attributed to Galileo, Santorio Santorio, Robert Fludd, and Cornelis Drebbel. The evidence therefore supports a cautious conclusion: Galileo played an important role in the early thermoscope tradition, but the modern thermometer was a cumulative invention.

Santorio and quantitative medical use

Santorio Santorio was a physician who promoted physical measurement in medicine. Museo Galileo states that he published descriptions of thermometric apparatus that were probably inspired by Galileo's thermoscope. Adding graduations and using a repeatable procedure allowed changes in a patient's temperature to be compared, although the early apparatus was bulky and still affected by atmospheric conditions.

It is therefore more accurate to describe Santorio as a pioneer of quantitative clinical thermometry than to assign him or Galileo sole ownership of the complete modern thermometer.

How sealed liquid thermometers improved measurement

Why an open air thermoscope was limited

In a typical open air thermoscope, warming the bulb expands the trapped air and moves liquid in the tube. Cooling reverses the movement. Because the system communicates with the atmosphere, changes in barometric pressure can also move the liquid. The reading is therefore not a response to temperature alone.

Florentine spirit-in-glass instruments

By the middle of the 17th century, Florentine instrument makers associated with the Medici court and the Accademia del Cimento were producing sealed thermometers filled with alcohol or another spirit. Sealing the liquid and removing the open air-pressure path made the indication more useful and repeatable.

These instruments were not yet governed by one universal scale. Museo Galileo documents several Accademia del Cimento scales, including 30-, 50-, 60-, 100-, and 180-division versions. Their reference procedures were not fully homogeneous, so measurements from different instruments could still disagree.

Why fixed points mattered

A useful scale requires reproducible reference conditions. Melting ice and boiling water became important reference points because other instrument makers could reproduce them more readily than an arbitrary mark on one thermometer. Pressure, water purity, thermometer construction, and the exact procedure still matter, so modern metrology defines reference conditions and uncertainty much more carefully.

How Fahrenheit and Celsius scales developed

Fahrenheit: reproducible instruments and a lasting scale

Daniel Gabriel Fahrenheit was an instrument maker and experimental physicist. Royal Society records document his thermometric research and papers in the 1720s. His contribution was not merely choosing numbers for a scale; he also worked on making liquid-in-glass instruments that could agree more closely with one another.

The modern Fahrenheit scale is related exactly to Celsius by °F = (°C × 9/5) + 32. Under the common water fixed-point convention, freezing is 32 °F and boiling is 212 °F at one standard atmosphere. Actual phase-change temperatures depend on pressure and sample conditions.

Celsius: a 100-division interval

Anders Celsius proposed his scale in 1742 after comparing existing thermometers and investigating reproducible reference points. Uppsala University records that his original orientation placed 100 at the freezing point of water and 0 at the boiling point. The direction was reversed in subsequent use, producing the familiar modern orientation of 0 °C for freezing and 100 °C for boiling under the stated reference conditions.

Popular accounts sometimes credit the inversion to a single named person. The historical record is less tidy, so the safer statement is that the modern orientation was adopted after Celsius's original proposal.

Réaumur and Rankine

Other scales remain relevant in historical documents or specialized calculations. Réaumur used an 80-division interval between the water fixed points. Rankine is an absolute scale with Fahrenheit-sized intervals. Neither replaces the SI kelvin in scientific measurement.

The water values in this table are conventional reference values. Boiling point varies with pressure, and high-accuracy work must state the realization method rather than relying on a household water test.

Kelvin and the current SI definition

From an absolute scale to an SI base unit

William Thomson, later Lord Kelvin, proposed a thermodynamic temperature scale in 1848. Its zero is absolute zero: 0 K = -273.15 °C = -459.67 °F. Its interval has the same magnitude as the Celsius interval. A difference of 1 K therefore equals a difference of 1 °C, while the zero points differ by 273.15.

The relationship is exact: t/°C = T/K - 273.15. A temperature of 0 °C is therefore 273.15 K. It is correct to write 300 K, not 300 °K or 300 degrees Kelvin.

The kelvin changed definition in 2019

The old article states that the triple point of water defines the kelvin as exactly 273.16 K. That was the earlier SI definition, but it is no longer current. Since 20 May 2019, the kelvin has been defined by fixing the numerical value of the Boltzmann constant, k, at 1.380 649 × 10^-23 J K^-1.

This change connects the unit to a fundamental constant and allows primary thermometry to be realized through more than one physical method. It did not make existing calibrated thermometers suddenly change their readings.

What happened to the triple point of water?

The triple point of water remains an important calibration reference near 273.16 K, but its thermodynamic temperature is now measured rather than used to define the kelvin. NIST explains that the International Temperature Scale of 1990, ITS-90, continues to use reproducible fixed points and specified instruments to disseminate practical temperature measurements.

Clinical and electronic thermometers

Smaller clinical instruments

Clinical thermometry became practical when instruments were made compact enough for routine use and their response time was reduced. Mercury-in-glass clinical thermometers were widely used because a constriction could retain the maximum reading after removal. They also carried breakage and mercury-exposure risks, so many health systems and laboratories later moved to safer alternatives.

Electronic thermometers appeared before the 1980s

The old article says the first digital thermometers appeared in the 1980s. That is too late as a general milestone. The Smithsonian catalogs a LaBarge electronic clinical thermometer made in 1972, with related work documented in 1971. Electronic temperature measurement existed earlier in laboratories and industry; later integrated electronics made compact digital readouts cheaper and more common.

A digital display does not identify the sensing principle. A digital thermometer may use a thermistor, RTD, thermocouple, semiconductor sensor, or infrared detector. Accuracy depends on the complete instrument, probe, calibration, placement, and use method.

Modern temperature sensor types

Resistance thermometers and RTDs

A resistance thermometer infers temperature from the predictable change in electrical resistance of a sensing element. Platinum resistance thermometers are important in precision and industrial work. Lead resistance, self-heating, mechanical strain, immersion depth, and readout quality can all affect the result.

Thermistors and semiconductor sensors

A thermistor offers a large resistance change over a defined range, which can provide high sensitivity. Its response is nonlinear, and self-heating or poor thermal coupling can introduce error. Semiconductor temperature ICs can add signal conditioning, calibration data, alarms, and digital communication, but their useful range and package-dependent thermal response are device-specific.

Thermocouples

A thermocouple produces a small voltage related to the temperature difference between its measuring junction and reference junction. Different alloy combinations cover different ranges. The measurement system must account for the reference junction, wiring, electrical noise, inhomogeneity, drift, and calibration. A thermocouple is valuable for broad-range or rugged measurements, but it is not automatically more accurate than an RTD or thermistor.

Infrared and radiation thermometers

An infrared thermometer estimates surface temperature from emitted thermal radiation. It can measure moving, hazardous, very hot, or inaccessible targets without contact. The result depends on emissivity, wavelength band, field of view, spot size, focus, reflections, intervening windows or atmosphere, surface condition, and instrument calibration.

A forehead scanner is therefore not universally more accurate than a contact thermometer, and an industrial infrared instrument does not directly measure internal temperature unless the application model supports that inference.

Accuracy, calibration, and ITS-90

A sensor does not measure temperature directly

Each thermometer observes a physical property that changes with temperature: liquid volume, resistance, voltage, semiconductor behavior, or radiation. A mathematical relationship and calibration convert that property into a temperature indication. The uncertainty belongs to the complete measurement chain, not just the sensing element.

Traceability and practical temperature scales

National metrology institutes maintain standards and calibration services that link working instruments to the SI. NIST calibrates resistance thermometers, thermistors, thermocouples, digital readouts with probes, and radiation thermometry standards across appropriate ranges. Many high-accuracy measurements are disseminated through ITS-90 fixed points, specified interpolation instruments, and defined equations.

Common sources of measurement error

• Poor thermal contact: the sensor has not reached the target temperature.

• Heat conduction: the probe stem or wires carry heat away from the measurement point.

• Self-heating: excitation current raises a resistance sensor's temperature.

• Response time: the reading is taken before the sensor settles.

• Environmental influence: airflow, radiation, pressure, humidity, or electrical noise changes the result.

• Infrared setup: emissivity, reflections, spot size, or an intervening window is not controlled.

• Calibration drift: the instrument has changed since its last calibration.

How to choose a thermometer

1. Define the expected temperature range, required uncertainty, resolution, and response time.

2. Decide whether the sensor can contact the target or must measure remotely.

3. Identify the target material, surface, dimensions, motion, pressure, and surrounding environment.

4. Check probe construction, chemical compatibility, electrical isolation, safety, and installation method.

5. For resistance sensors, account for wiring and self-heating; for thermocouples, account for reference-junction and extension-wire errors.

6. For infrared instruments, verify emissivity, spectral band, spot size, focus distance, reflections, and any window material.

7. Use the manufacturer's specified accuracy over the required range, not only the display resolution.

8. Define calibration interval, traceability, acceptance limits, and an in-process verification method.

For medical diagnosis, food safety, regulated production, and other critical applications, use an instrument approved for that purpose and follow the applicable procedure. A consumer wearable or general-purpose sensor should not be treated as a diagnostic instrument unless its labeling and evidence support that use.

Thermometer history timeline

Frequently asked questions

Did Galileo invent the thermometer?

Galileo is strongly associated with an early air thermoscope, but the invention is also attributed to other experimenters. The modern thermometer resulted from later additions including a scale, a sealed sensing medium, reproducible fixed points, and calibration methods.

What is the difference between a thermoscope and a thermometer?

A thermoscope shows relative temperature change. A thermometer reports a numerical temperature using a calibrated scale or conversion function.

Did Fahrenheit invent the mercury thermometer?

Fahrenheit made major advances in liquid-in-glass thermometry, instrument reproducibility, and the scale that bears his name. It is safer to describe those documented contributions than to frame the entire mercury thermometer as a one-person invention.

Was the Celsius scale originally reversed?

Yes. Uppsala University states that Celsius's original 1742 orientation used 100 at the freezing point and 0 at the boiling point. The familiar orientation was adopted afterward.

Is the kelvin still defined by the triple point of water?

No. Since 2019, the kelvin has been defined by the fixed numerical value of the Boltzmann constant. The triple point of water remains a useful calibration reference but no longer defines the unit.

Why is Kelvin written without a degree sign?

The SI unit name is kelvin and its symbol is K. Write 300 K, not 300 °K. Celsius and Fahrenheit use degree symbols: 27 °C and 80.6 °F.

Are infrared thermometers more accurate than contact thermometers?

Not inherently. Infrared instruments are valuable for fast non-contact surface measurements, but emissivity, optics, spot size, reflections, atmosphere, and calibration can dominate the error. The best method depends on the target and uncertainty requirement.

Does a digital display guarantee accuracy?

No. Display resolution is not the same as measurement accuracy. Sensor quality, calibration, probe placement, thermal coupling, environmental effects, electronics, and the specified operating range all matter.

Official sources

• Museo Galileo: Thermoscope

• Museo Galileo: From the thermoscope to the thermometer

• Museo Galileo: Santorio Santorio

• Museo Galileo: Thermometric scales of the Accademia del Cimento

• The Royal Society: Daniel Gabriel Fahrenheit archive record

• Uppsala University: Anders Celsius

• BIPM: SI base unit kelvin

• BIPM: Historical perspective on the kelvin

• NIST: Kelvin and ITS-90

• NIST: Thermometry standards, sensor types, and calibration

• Smithsonian: 1972 LaBarge electronic clinical thermometer