Thermometers and temperature sensors are an established and necessary tool for knowing how hot or how cold a place or an object is. But where did the idea come from and how have devices for measuring temperature evolved? Read on to find out…
Who invented the thermometer?
Like with many inventions throughout history, there does not seem to be one person who first created the thermometer, but several people all attempting the same thing at roughly the same time.
While the Ancient Greeks recognised that there were materials and devices that would respond to heat using air or water, they were not used to measure specific temperatures.
By the 16th and 17th centuries, scientists were using glass tubes with a container of water at one end to measure changes in temperature; however, these did not have a scale and so were not really thermometers — they were instead called thermoscopes.
Different versions included using small glass balls of different sizes and weights in the tube, with air bubbles inside that would shrink or grow depending on the temperature of the liquid; the balls would then rise or fall inside the tube to show the temperature.
Galileo Galilei is credited as being the first person to build a thermoscope using alcohol in the tube instead of water. Galileo's contemporary, Santorio Santorio, used a thermoscope for medical diagnostics, and even included a scale, effectively creating the first thermometer.
However, early thermoscopes and thermometers being open at one end meant that they were affected by air pressure in addition to temperature, which led to a lack of accuracy.
In the early 1650s, a few decades after Galileo and Santorio, the Grand Duke of Tuscany, Ferdinand II, had a thermometer built containing alcohol which was sealed at both ends. This relied on the liquid expanding rather than the air, and used pure alcohol for its sensitivity to temperature change. Ferdinand's glass-blowers were so skilled at their work that their thermometers were remarkably consistent in their readings. However, scales across the manufacturers differed, as no single measurement scale for temperature existed.
Fahrenheit vs Celsius vs Centigrade vs Kelvin vs Rankine
It has long been realised that the extremes of temperature can be easily seen with ice and boiling water, but it wasn't until Robert Hooke in 1664 that water's freezing point was used as the starting point for thermometers and thermoscopes. He experimented with alcohol and mercury in his thermometers, and around the same time Isaac Newton tried linseed oil. Newton also used three fixed points of temperature for his thermometers: water beginning to freeze, blood in the body, and boiling water.
Even so, these measurements were not widespread throughout the scientific world, and the need for a single scale became necessary for scientists to share results and compare readings.
One of the early contenders included René-Antonie Ferchault de Réaumur, who not only used his own temperature scale (such as those of a warm bath, silkworms, an orangery and black ice, in addition to the three used by Newton), but continued to refine it throughout his career — so eventually his scale even differed to itself!
Then, in 1712, German Gabriel Fahrenheit used mercury as the ideal liquid for measuring temperature, as it expands more clearly than alcohol. Fahrenheit worked out a system of temperature scale using sea water as his basis, using an interval of 12, with the freezing point of salt water at 0°F and the temperature inside a person's mouth as 96°F. The freezing point of water alone (without the salt of sea water) became 32°F, and boiling water became 212°F (180 degrees above the freezing point). A slightly adjusted version of this scale is still used today in a few countries, most prominently the USA.
However, the most commonly used scale for measuring temperature is now Celsius, which was developed by Anders Celsius in Sweden in 1742. He proposed using 100 degrees between the boiling and freezing points of water (non-salt water). This was called the Centigrade scale (cent = hundred), and was quickly adopted among the scientific community, and then more widely.
However, Celsius initially had the scale the opposite way around: 100 as the freezing point and 0 as the boiling point. It was Jean-Pierre Christin who proposed, in 1743, to reverse it.
Centigrade and Celsius degrees are the same — the centigrade scale was renamed by the International Committee for Weights and Measures to Celsius in 1948, both to honour Anders Celsius and to remove confusion with another measurement, also called centigrade, which related to the size of an angle.
A hundred years after Celsius and Christin, the Irish Lord Kelvin, William Thomson, examined theoretical temperatures, more extreme than even the Centigrade had previously noted. Kelvin used the same scale, but started with 0°K at the theoretical lowest temperature in the universe, when all molecular motion stops: absolute zero. Absolute zero is equivalent to -273.15°C and -459.67°F, but is so cold that it only exists theoretically and is believed to be unreachable.
Over fifteen years after Kelvin introduced the idea of using absolute zero as the base point on a scale, the Fahrenheit version was conceived by William Rankine, where each Rankine degree (°R) is the same as a Fahrenheit degree.
Thermometers' expanding use
Throughout the following two centuries, thermometers used these scales to become more standardised and more widely produced for different uses. Glass tubes containing mercury became the most common form of thermometer, used in medicine, meteorology, chemistry and other industries.
Modifications to the standard thermometer were being made for different purposes, including the creation of 'registering' thermometers, which keep their reading even when removed from the original temperature. Initially, a small spring was included to keep track of the previous temperature — it remained where it was when the mercury moved again, and could be reset by being shaken.
In more recent decades, the toxicity of mercury has also led to a ban on its use in many countries, which has led to the use of other liquid metals in its place.
Modern thermometers
Today, a wide range of objects and devices use temperature measurements as part of their normal function — but they don't use a liquid-filled glass tube.
Electric thermometers are called thermistors, which use resistance to electrical current to accurately measure temperature. Thermistors were first discovered in 1833 by English scientist Michael Faraday (inventor of the Faraday cage). However, their application was initially limited, so it took until the 1930s for thermistors to be commercially produced.
The most common form is the Negative Temperature Coefficient Thermistor, where higher temperatures lower resistance. The other type is Positive Temperature Coefficient thermistors, which are mainly used as fuses in circuits to prevent excessive current. In this way, an electrical temperature reading can be taken, and incorporated into electrical operations wherever a temperature reading might be needed, such as in cars, factories, food storage, toasters, hair dryers and many more.
Infrared thermometers are also widely used, as they do not require physical contact. They use thermal radiation to calculate temperature, and were commonly used during the Covid-19 pandemic to take temperatures without any contact.
HeatingSave's temperature sensors
Thermistors are used in HeatingSave's room temperature sensors, which are used to measure internal and external temperatures and feed the information to the HeatingSave controller.
HeatingSave initially uses this data to create a 'heat loss profile', which is used alongside ongoing internal and external readings, boiler temperature, and readings from any additional sensors (such as from the flow/return pipes or hot water cylinders) to ensure the room is kept at the optimal temperature.
The HeatingSave heating diary, room booking system and occupancy sensors inform the controller unit of the required times for heating or cooling, and the advanced AI algorithm ensures the optimal time to switch on the boiler, redirect heated water, or activate air conditioning units. This ensures the room (or heating zone) is only heated or cooled when necessary, which reduces energy use — and cuts costs on energy bills.