
Temperature Heroes: Standing on the Shoulders of Giants
The history of thermometry is a story of discoveries, errors, and groundbreaking insights. It begins with the first attempts of antiquity to understand heat and cold, leads through the ingenious simplicity of Galileo Galilei’s thermoscope, to the high-precision Standard Platinum Resistance Thermometers (SPRTs) of our time.
Every advancement in the history of thermometry builds on the knowledge and experiments of the scientists who came before us. Daniel Gabriel Fahrenheit, Anders Celsius, William Thomson (Lord Kelvin), and many others not only developed temperature scales but revolutionized our understanding of heat and energy. Without their ideas and measurement methods, reliable temperature measurement would be unthinkable today.
We stand on the shoulders of these “Temperature Heroes”. Their work not only shapes today’s science but also our modern life. I will take you on a journey through the history of thermometry – from the first simple attempts to the high-precision temperature measurement of the present.
Introduction – History of Thermometry
Temperature is one of the most important physical quantities in our daily lives. Without precise temperature measurement, many things would be more complicated or even dangerous. But this wasn’t always the case. For centuries, people had no way to measure temperature accurately. It was only with the development of the first thermometers in the 16th and 17th centuries that a new era of science and technology began. How have thermometers evolved over time? And what milestones have brought temperature measurement to today’s level of precision?
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Why is Temperature Measurement Important?
Temperature influences our daily lives more than we often consciously perceive. From reaching for a jacket in the morning to brewing the perfect coffee to controlling the heating in winter – without precise temperature measurement, many things would be impractical or even dangerous.
Even in the kitchen, temperature plays a crucial role. When cooking or baking, it determines taste and consistency. A steak is only perfect when the right core temperature is reached, and chocolate melts at body temperature – which is why it dissolves so pleasantly on the tongue. The morning coffee only tastes really good when it’s hot enough, but not so hot that it burns your tongue.
Temperature is important not only in preparation but also in food storage. A refrigerator must be cold enough to keep food fresh, but not so cold that fruits and vegetables freeze.
Temperature measurement is also indispensable in health care. When we have a fever, a thermometer immediately gives us an assessment of whether it’s a harmless cold or possibly a more serious illness.
A quick glance at the thermometer often determines how we dress or what activities we pursue. If it’s frosty outside, we dress warmly; in high temperatures, we opt for light clothing. Temperature is also crucial for our safety in road traffic: black ice can form when temperatures are below freezing.
Ultimately, temperature measurement is an invisible helper that makes our lives safer, more comfortable, and healthier. Whether at breakfast, at work, or on the street – it determines many decisions without us consciously noticing.
Brief Preview of the Development of Thermometry
The history of temperature measurement dates back a long way. Even in ancient times, scholars tried to understand heat and cold, but it wasn’t until the 16th and 17th centuries that the first measurable scales were created. Around 1593, Galileo Galilei developed the first thermoscope, which made temperature changes visible but didn’t yet provide precise values. In the 18th century, researchers like Fahrenheit, Celsius, and Réaumur introduced accurate temperature scales that laid the foundation for modern measurement methods. With the Industrial Revolution came new technologies such as the mercury thermometer, electrical resistance thermometers, and later digital sensors.
Today, the International Temperature Scale of 1990 (ITS-90) enables highly precise and globally uniform temperature measurement. It is based on defined fixed points, including the triple point of water (0.01 °C).
In high-precision measurement, Standard Platinum Resistance Thermometers (SPRTs) are used, allowing measurements with accuracy in the microkelvin range.
This development shows how simple observations became a scientific standard.
Early Attempts at Temperature Measurement
Before thermometers existed, people had to estimate temperature differences in simple and often subjective ways. In ancient times, there were no scales or accurate measurement methods, but various cultures developed methods to roughly evaluate heat and cold.
The Simplest Method: Feeling with the Hand
The most obvious way of assessing temperature was through touch. People would hold their hands in the sun, in water, or in the wind to feel heat or cold. However, this method was prone to errors – our skin quickly adapts to temperatures, so we often only perceive relative differences.
Air Expansion as an Early Temperature Indicator
Even in ancient times, scholars observed that air expands when heated and contracts when cooled. Although no specific individuals are recorded by name as having made this observation, early scientists used this principle to develop simple devices based on air expansion.
Medical Significance: The Temperature of the Body
In ancient medicine, temperature played an important role. The Greek physician Hippocrates (ca. 460–370 BC) recommended assessing a patient’s temperature by touching their forehead or hands. This was an early form of diagnosis that is still used in medicine today – even though we now have fever thermometers.
While the methods of antiquity were rudimentary, they laid the foundation for later developments. The observation of air expansion later led to the development of the thermoscope, and medical temperature assessment showed how important thermometry is for everyday life.
First Theoretical Considerations in Antiquity (e.g., by Philosophers such as Empedocles or Aristotle)
Long before thermometers existed, ancient philosophers dealt with the concepts of heat and cold. Since they did not yet have physical measurement methods, they interpreted temperature based on natural phenomena and philosophical principles. Two of the most significant thinkers in this field were Empedocles (5th century BC) and Aristotle (4th century BC), whose ideas influenced scientific thinking for centuries.
Empedocles: The Four Elements Theory and Temperature as a Property of Matter
Empedocles was one of the first philosophers who tried to explain nature through fundamental elements. He developed the Four Elements Theory, according to which everything consists of the four basic substances: fire, water, air, and earth. Heat was associated with fire and air, while cold was linked to water and earth. According to this theory, temperature was not an independent physical quantity, but a property of the elements themselves.

This approach was used as a basis for natural sciences for centuries. It wasn’t until much later that it was recognized that temperature depends not on the four elements, but on the movement of molecules – a concept that was only developed in modern times through the kinetic theory of gases.
Aristotle: Heat as the Counterpart to Cold
Aristotle expanded on the ideas of Empedocles and proposed a model in which heat and cold acted as opposing principles. He believed that each material naturally possessed a certain “natural heat” or “natural cold” that could be altered by external influences. According to Aristotle, heat was associated with rising (e.g., hot air or flames), while cold led to condensation and cooling.
Aristotle assigned specific properties to the four elements:
• Fire: hot and dry
• Water: cold and wet
• Earth: cold and dry
• Air: hot and wet

These associations formed the basis for his understanding of heat and cold as fundamental properties of matter.
These ideas continued to be used for centuries in medicine, alchemy, and natural philosophy. Especially in the humoral medicine of Hippocrates and Galen, temperature played a significant role – it was believed that the balance of “hot” and “cold” humors in the body determined health.
From Philosophy to Measurement Science
Although ancient theories did not yet allow for precise measurements, they laid the foundation for the scientific understanding of temperature. The idea that heat and cold are natural, measurable quantities eventually led to the development of the first temperature measuring devices in the 16th and 17th centuries.
Today, we know that temperature is a consequence of the movement of atoms and molecules – a concept that has little to do with ancient ideas. Nevertheless, the realization remains that philosophers were trying to systematically explain temperature over 2000 years ago.
The Invention of the First Thermometers
16th Century – Galileo Galilei and the Thermoscope (ca. 1593)
In the late 16th century, the systematic study of temperature measurement began. One of the first significant developments was the thermoscope, attributed to Galileo Galilei (ca. 1593). In fact, the exact authorship is disputed, as other scientists like Giambattista della Porta described similar devices. However, it is certain that Galilei further developed the concept and was the first to use it for physical observations.

The thermoscope was a simple device that could visualize temperature changes. It consisted of a glass bulb filled with air, connected to a water vessel through a narrow tube. As the air in the bulb warmed, it expanded and pushed the water in the tube downwards. When the air cooled, it contracted and the water rose again. While this allowed for qualitative observation of temperature changes, it lacked a uniform scale to determine precise measurements.

A major problem with the thermoscope was that it responded not only to temperature but also to changes in air pressure. This dependency made accurate measurements difficult and later led to the development of thermometers using liquids like alcohol or mercury, which functioned independently of ambient pressure.
Despite these limitations, the thermoscope was a significant milestone. It laid the foundation for later developments in thermometry and inspired scientists like Santorio Santorio, who was the first to attach a scale to numerically capture temperature differences. Thus, the thermoscope was the first attempt to systematically visualize temperature changes.
The 17th Century – The First Scaled Thermometers by Santorio Santorio and Ferdinand II de’ Medici
In the 17th century, significant advancements were made in temperature measurement. While Galileo Galilei’s thermoscope could already visualize temperature changes, it lacked a scale to obtain measurable values. Two scientists played an important role in the history of thermometry: Santorio Santorio and Ferdinand II de’ Medici.
Santorio Santorio: The First Scaled Thermometer (ca. 1612)
The Italian physician and scientist Santorio Santorio (1561–1636) was one of the first to develop a thermometer with a scale. Santorio was known for his work in medical metrology and combined the principle of the thermoscope with a numerical scale to enable objective temperature comparisons.
His thermometer consisted of a glass tube filled with alcohol that was equipped with a scale. However, it was not yet completely independent of air pressure, so fluctuations in the environment could influence the measurement results. Nevertheless, it was a decisive advancement as it allowed, for the first time, to quantitatively capture and compare temperature changes. Santorio used his thermometer particularly in medicine to measure body temperatures – an early precursor to the modern fever thermometer.
Ferdinand II de’ Medici: The First Closed Liquid Thermometer (ca. 1654)
Another major advance came from Ferdinand II de’ Medici (1610–1670), Grand Duke of Tuscany and an enthusiastic natural scientist. Under his patronage, researchers at the Accademia del Cimento developed a thermometer that used alcohol or wine as the measuring liquid.
The special feature of this thermometer was that, compared to earlier devices, it had a sealed capillary, making it less influenced by air pressure fluctuations. It thus represented an important step towards the development of stable temperature scales.

The Medici thermometers laid the foundation for the later work of Daniel Gabriel Fahrenheit, who invented the mercury thermometer in the 18th century.
The works of Santorio Santorio and Ferdinand II de’ Medici marked a first turning point in the history of temperature measurement.
The first scaled thermometer by Santorio and the improved liquid thermometer that was less affected by air pressure paved the way for later temperature scales and the development of more precise measuring instruments.
First Attempts at a Temperature Scale
With the development of the first thermometers in the 17th century, the need arose to make temperature measurements comparable. Without a uniform scale, temperature readings were purely relative and dependent on individual measuring instruments. The first attempts to define a temperature scale came from various scientists who used different reference points.
Ole Rømer and the First Documented Temperature Scale (1701)
The Danish astronomer and physicist Ole Rømer (1644–1710) was one of the first to develop a systematic temperature scale. His scale set the freezing point of water at 7.5° and the boiling point at 60°. This made temperature measurements reproducible for the first time.
However, Rømer’s scale had some disadvantages: The choice of his fixed points was arbitrary, and the division was not particularly practical. Nevertheless, it was an important step towards standardizing temperature measurement.
Isaac Newton’s Temperature Scale (1701)
Almost simultaneously, Isaac Newton (1643–1727) proposed a temperature scale that was more oriented towards practical experiences.

Instead of using absolute fixed points like the freezing or boiling point of water, Newton oriented himself on everyday temperature phenomena and assigned them values on a scale. Among his approximately 20 scale points were, among others, “cold air in winter” as a low reference point and “glowing coals in the kitchen fire” as an upper fixed point.
Later, Newton set the temperature of melting snow (0°) as a reference point and measured other temperatures relative to it based on the expansion of mercury.
Newton’s scale was primarily intended for scientific purposes and was later replaced by more precise scales. Nevertheless, it was important on the path to modern thermometry.
Development of Uniform Temperature Scales
The Foundations for More Precise Scales
The early temperature scales were not yet universally standardized. Different researchers used different fixed points, and many scales were based on subjective empirical values. The scales, such as those by Ole Rømer (1701) or Isaac Newton (1701), were also not widespread. With the further development of thermometry in the 18th century, it became clear that a uniform temperature scale was necessary.
It wasn’t until the 18th century that scientists like Daniel Gabriel Fahrenheit, Anders Celsius, and René Antoine Ferchault de Réaumur succeeded in developing generally accepted scales that eventually became the basis for modern temperature measurement.
Daniel Gabriel Fahrenheit (1724) – Mercury Thermometer and Fahrenheit Scale
In 1724, the German physicist Daniel Gabriel Fahrenheit (1686–1736) introduced one of the first standardized temperature scales, which is still used today in countries such as the USA. In addition to the scale, he also developed the first reliable mercury thermometer, which allowed for more accurate measurements than earlier alcohol thermometers.
The Mercury Thermometer – More Precise Measurements
Fahrenheit initially experimented with alcohol thermometers but found that alcohol freezes at low temperatures and expands unevenly at higher temperatures. Therefore, he began using mercury as a measuring liquid.
The Advantages of Mercury:
- Remains liquid over a wide temperature range (-39 °C to 357 °C).
- Expands linearly, allowing for more accurate measurements.
- Does not evaporate easily, which extends the lifespan of the thermometer.
With these properties, the mercury thermometer became the standard method for temperature measurements in science and technology.
The Fahrenheit Scale – Three Fixed Points for Temperature Measurements
Fahrenheit established three fixed points for his temperature scale:
- 0 °F: The lowest temperature he could produce with a mixture of ice, water, and ammonium chloride
- 32 °F: Freezing point of water
- 96 °F: Body temperature of a “healthy person”
- 212 °F: Boiling point of water
These fixed points allowed for a reproducible scale that functioned independently of individual thermometers.
The Fahrenheit scale quickly gained acceptance in England and the British colonies but was replaced by the Celsius scale in most countries during the 19th and 20th centuries. Today, it is used almost exclusively in the United States.
The Réaumur Scale (1730)
The French scientist René Antoine Ferchault de Réaumur (1683–1757) developed a temperature scale for alcohol thermometers in 1730, which was used for a long time in France and parts of Europe.

Characteristics of the Réaumur Scale
- 0 °Ré: Freezing point of water
- 80 °Ré: Boiling point of water
Réaumur chose a division into 80 degrees, as he assumed that alcohol expands linearly with temperature. However, this assumption proved to be inaccurate, as liquids expand differently at various temperatures.
The Réaumur scale was mainly used in France, Italy, and Russia, but gradually lost importance with the introduction of the Celsius scale.
Anders Celsius (1742) – Celsius Scale
The Swedish astronomer and physicist Anders Celsius (1701–1744) developed a new temperature scale in 1742 that later became established as an international standard. Unlike the Fahrenheit scale, Celsius used a decimal division, which allowed for intuitive handling.
The Celsius Scale
In his work Observationer om twänne beständiga grader på en thermometer, Celsius proposed a temperature scale with two fixed points at normal pressure:
- 0 °C: The boiling point of water.
- 100 °C: The freezing point of water.
This inverted scaling was initially unusual. After Celsius’ death in 1744, his students, especially Carl von Linné (1707–1778), advocated for a reversal of the scale. This set the freezing point at 0 °C and the boiling point at 100 °C – a more intuitive order that became established worldwide.
Advantages of the Celsius Scale
The Celsius scale had two major advantages over earlier temperature scales:
- Easy handling: The decimal division into 100 steps facilitated measurements and calculations.
- Precise fixed points: The scale was based on the physical properties of water (at normal pressure), which were reproducible everywhere.
Celsius Scale and Its Current Significance
Today, the Celsius scale, known as degrees Celsius (°C), is one of the most widely used temperature scales and is used as the standard for temperature measurements in almost all countries. Only in the USA and a few other countries is the Fahrenheit scale still used.
The Celsius scale also forms the basis for the Kelvin scale (K), which is used in science. The relationship is:
0 °C = 273.15 K (Kelvin begins at absolute zero).
The introduction of the Celsius scale was another major step in thermometry. Due to its simple division, clear fixed points, and intuitive handling, it quickly became the international standard. Although Anders Celsius himself did not live to see the current scaling, his work is among the most important developments in the history of temperature measurement.
The Kelvin Scale (1848)
In 1848, the Scottish physicist William Thomson, Lord Kelvin (1824–1907) introduced the first absolute temperature scale. The Kelvin scale (K) is based on the absolute zero, the lowest possible temperature value at which all thermal motion ceases. The Kelvin scale (1848) is the first scientifically based absolute temperature scale.
Characteristics of the Kelvin Scale:
- 0 K: Absolute zero (-273.15 °C).
- 273.15 K: Freezing point of water (0 °C).
- 373.15 K: Boiling point of water (100 °C).
The Kelvin scale is particularly used in science, physics, and thermodynamics, as it is independent of specific fixed points such as water and is based on the energy movement of particles.
The Kelvin scale is now the official temperature scale of the International System of Units (SI). A major advantage of Kelvin is that it allows temperature values without negative numbers.
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Image Rights
“Four Elements of Alchemy”, public domain, available on Wikimedia Commons
“The Four Elements” in the Königslutter Imperial Cathedral – August von Essenwein (1831-1892); Adolf Quensen (1851-1911), Public domain, photographed by Rabanus Flavus, Wikimedia Commons, February 15, 2012
Portrait of Galileo Galilei, painted by Domenico Tintoretto (1602–1607), photo from the National Maritime Museum, Greenwich, London, available on Wikimedia Commons
Photo of Galileo Galilei’s thermoscope in the Musée des Arts et Métiers, taken by Chatsam, licensed under CC BY-SA 3.0 – available on Wikimedia Commons
Medici Thermometer – Accademia del Cimento. (1667) – Saggi di Naturali Esperienze
Portrait of Sir Isaac Newton, English School, ca. 1715–1720, Wikimedia Commons
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