The History of Thermometers

Executive Summary: The Evolution of Temperature Measurement (2026 Update)

Core Answer: The thermometer was not invented by a single person but evolved through the contributions of Galileo Galilei (1593), Gabriel Fahrenheit (1714), and Anders Celsius (1742). As of January 2026, modern thermometry has shifted from mercury-based glass tubes to AI-integrated digital sensors, infrared technology, and wearable continuous monitoring systems.

• First Device: Galileo's Gas Thermoscope (1593).

• First Modern Scale: Fahrenheit (1724) by Gabriel Fahrenheit.

• Current Standard: The Kelvin (K) and Celsius (°C) scales are the global standards under the International System of Units (SI).

Ⅰ. Who invented the first thermometer? (Galileo's "Gas")

The first device to measure temperature changes, known as a thermoscope, was invented by the Italian scientist Galileo Galilei in 1593. While lecturing at the University of Padua in Venice in 1592, Galileo observed that water volume changed with heat, leading to a breakthrough in thermal measurement.

Key Historical Context:

• The Discovery: Galileo employed the principle of heat expansion and cold contraction. Inspired by his students' observations that water levels rise when heated, he developed the "bubble-shaped glass tube thermometer."

• Mechanism: This early device consisted of a glass bulb connected to a tube containing colored liquid, inverted into a cup of water. As the air in the bulb expanded or contracted due to temperature, the liquid column would move accordingly.

• Limitation: It was a "gas thermometer" (or thermoscope) because it relied on air volume. However, it was imperfect because it was also affected by changes in atmospheric pressure, not just temperature.

While ancient Greek scientists like Philon and Hero of Alexandria had explored air expansion, Galileo is credited with creating the first scaled instrument for this purpose, driven by the medical community's need to diagnose fever.

Ⅱ. The Evolution of Thermometers: From Gas to Liquid

The transition from inaccurate gas thermoscopes to sealed liquid thermometers marked a major leap in scientific precision. This evolution occurred rapidly throughout the 17th century.

Timeline of Key Improvements:

• 1612 - First Medical Application: Professor Santorio Santorio, a colleague of Galileo, refined the design by adding a scale to the glass tube. This allowed for the first comparison of human body temperatures, making it the world's first clinical thermometer, though still an air thermoscope.

• 1632 - The Shift to Liquid: French physician Jean Rey inverted the design to use water expansion instead of air. This eliminated barometric pressure errors but introduced new errors due to evaporation (as the nozzle remained open).

• 1641 - The First Sealed Thermometer: Grand Duke Ferdinand II of Tuscany introduced the first sealed thermometer using red-stained alcohol. By sealing the glass tube with wax, he created a device independent of air pressure, a design that became famous across Europe by 1654.

• 1658 - Mercury's Precursor: The French astronomer Ismaël Boulliau (often cited historically as Ismail Borio) is credited with early work on mercury thermometers, paving the way for high-precision instruments.

By the late 17th century, scientists like Robert Boyle and Christiaan Huygens realized that for thermometers to be universally useful, they needed standardized "fixed points"—such as the freezing and boiling points of water—to calibrate scales. This realization set the stage for the modern temperature scales we use in 2026.

Ⅲ. Understanding Temperature Scales: Fahrenheit, Celsius, & Kelvin

A temperature scale provides a standardized way to quantify "hotness." Without agreed-upon scales, scientific collaboration is impossible. Below are the five historical and modern scales that defined thermometry.

1. The Fahrenheit Scale (°F)

Invented by the physicist Gabriel Daniel Fahrenheit (formerly misidentified in some texts as "Warren Heights") between 1709 and 1724. Fahrenheit revolutionized thermometry by inventing the precise mercury thermometer.

• Calibration: He set 0°F as the temperature of a brine mixture (ice, water, and ammonium chloride) and originally set human body temperature at 96°F.

• Fixed Points: Later adjustments set the freezing point of water at 32°F and the boiling point at 212°F.

• Usage in 2026: Still the official scale in the United States, Liberia, and the Cayman Islands.

2. The Réaumur Scale (°Re)

French scientist René Antoine Ferchault de Réaumur (historically miscited as "Leo Muir") created this alcohol-based scale in 1730. He set the freezing point of water at 0°Re and the boiling point at 80°Re. While once popular in Europe, it is largely obsolete today.

3. The Celsius Scale (°C)

Proposed by Swedish astronomer Anders Celsius in 1742. Interestingly, Celsius originally defined 0°C as boiling and 100°C as freezing.

• The "Inversion": After Celsius's death, his colleague Carl von Linnaeus (or possibly Mårten Strömer) inverted the scale to the modern standard: 0°C for freezing and 100°C for boiling.

• Usage: This is the standard scale for almost all countries and scientific applications worldwide.

4. The Kelvin Scale (K) - The Absolute Standard

Proposed by Lord Kelvin (William Thomson) in 1848, the thermodynamic temperature scale measures energy directly, starting at Absolute Zero (0 K), the point where molecular motion theoretically stops.

• Scientific Standard: It avoids negative numbers and uses the same increment size as Celsius (1 K = 1°C).

• Triple Point of Water: Defined precisely as 273.16 K (0.01°C).

• Modern Context: In 2026, Kelvin is the primary unit for the International System of Units (SI).

5. The Rankine Scale (°R)

Established by engineer William John Macquorn Rankine, this is the "Absolute Fahrenheit" scale. It starts at absolute zero but uses Fahrenheit-sized degrees. It has historically been used in US aerospace and thermodynamics engineering, though it is often referred to as "Lan's scale" in some Asian translations.

Ⅳ. Modern Thermometry: 2026 Standards & Technology

As we move through 2026, the humble glass thermometer has largely been replaced by sophisticated digital and infrared sensors. The evolution continues to be driven by the need for speed, safety (eliminating mercury), and connectivity.

Key Milestones in Modern Thermometry:

• The Clinical Thermometer: Invented by Sir Thomas Clifford Allbutt in 1867. His portable design reduced reading time from 20 minutes to just 5 minutes, revolutionizing fever diagnosis.

• The Electronic Revolution: The first digital thermometers appeared in the 1980s, utilizing thermistors to provide readings in seconds.

• Infrared (IR) Technology: Non-contact forehead and ear thermometers became ubiquitous during the 2020s global health events, allowing for instant, hygienic screening.

2026 Industry Landscape:

Today's thermometry integrates with the Internet of Things (IoT) and AI.

• Wearables: Smartwatches and rings (e.g., Oura, Apple Watch) now perform continuous skin temperature monitoring to predict illness or fertility cycles.

• Smart Industrial Sensors: In manufacturing, fiber-optic thermometry measures temperatures in extreme environments (like semiconductor fabrication) where electronic sensors would fail.

• AI Diagnostics: Modern medical thermometers often pair with smartphone apps to interpret data, differentiating between viral fevers and normal fluctuations.

From Galileo's air-filled glass tube to the semiconductor-based sensors of 2026, the history of the thermometer is a testament to humanity's quest for precision.