Who Invented the Thermometer

Temperature is a parameter which helps measure the average heat or thermal energy of a particle in a substance. It does not depend on the number of particles in an object, as it is an average measurement. It is the physical property that helps you measure hot and cold. How is the temperature measured? It is measured by a thermometer. The most common scale to measure temperature on a thermometer is Celsius (°C ). The thermometer is a very important instrument for measurement, but have you ever wondered, what is the history behind the thermometer? When was the thermometer invented? The following few paragraphs will help answer all your questions.

History of the Thermometer

People have always known the difference between different temperatures. Even the stone age man knew, that fire is hot and snow is cold. The only thing people did not understand was how to measure temperature. People did not have any specific scale on which they could measure temperature. Instead they relied on subjective scales, like a glow on the metal indicates hot. But how to measure how hot or how cold was the temperature? This remained a mystery for many centuries. Also, there is a large gap of written record related to measurement of temperature in history. Therefore, one cannot believe that the ancient Egyptians, Chinese, Romans, Greeks or the Assyrians had no specific way to measure temperature. During this period the history of the thermometer remains a mystery.

Yet people who lived as far as 300 BC, knew the basic principle behind temperature measurement. They knew that expansion of air by heat and contraction by cold, could be used to measure temperature. This principle was explored and proven by Philo of Byzantium, in 300 BC. However, it needed a genius to truly explore and inculcate the principle to make a working thermometer.

Galileo Galilei and His Crude Thermoscope
It is said that Galileo invented the thermometer in the year 1596. But, in reality what Galileo invented was not actually a thermometer, instead a thermoscope. What is the difference between the two instruments? Well, although they both are related to temperature, the 'themoscope' did not measure the temperature differences. It was an instrument that just indicated temperature differences. The thermometer on the other hand measures temperature.

The thermoscale did not have any scales, therefore one could know there are temperature differences, but could not measure the difference between temperatures. Also, one could not record any data for future reference. However, it was a small step for others to take lead and continue with the invention of the thermometer. Benedetto Castelli, the consultant to Pope in 1626 and a professor of mathematics at the university of Rome, wrote in 1638 about the thermoscope, he saw in Galileo's hands around 1603:

"He took a small glass flask, about as large as a small hen's egg, with a neck about two spans long (perhaps 16 inches) and as fine as a wheat straw, and warmed the flask well in his hands, then turned its mouth upside down into the vessel placed underneath, in which there was a little water. When he took away the heat of his hands from the flask, the water at once began to rise in the neck, and mounted to more than a span above the level of the water in the vessel. The same Sig. Galileo had then made use of this effect to build an instrument for examining the degrees of heat and cold."

Santorio Santorio and his Thermometer
Often, it is concluded that Santorio Santorio invented the thermometer in the year 1654. Santorio was an Italian who applied a scale to an air thermoscope. Thus, he is often given the credit for invention of the thermometer. However, he invented the air thermometer that has very poor accuracy levels. This was because people did not understand the details behind the effects of varying air pressure during Santorio's time.

Grand Duke of Tuscany and His Liquid-in-Glass Thermometer
The liquid-in-glass thermometer is more commonly known to people today. This liquid-in-glass thermometer was invented by Ferdinand II, the Grand Duke of Tuscany (1610 - 1670) in 1654. He used alcohol filling in his thermometer. Yet, it too was inaccurate as no standardized measurement scale was in use.

Gabriel Fahrenheit and His Mercury Thermometer
Gabriel Fahrenheit, is however, the real inventor of the thermometer that we use today. He was the first person to make a thermometer using mercury in 1714 as the filling. Mercury helped in measurement of more accurate readings than any thermometer before.

Fahrenheit and His Temperature Scale
Fahrenheit was able to make more accurate measurements of temperature, as he used fixed points, as the temperature scale for his thermometer. He divided freezing and boiling points of water into 180°. He chose 32 as the lower fixed point of the scale. This lower point would not fall below zero, even if one measured the lowest temperatures. Even, today, Fahrenheit's scale is widely accepted and used to measure temperature.

Reamur and His Temperature Scale
Rene Antoine Ferchauld de Reamur, a Frenchman, in 1731 proposed a new thermometer scale. His scale had freezing point of water that was 0° and boiling point of water was 80°. The Reamur scale was quickly scrapped and is not in use today.

Celsius and His Scale
The next person after Fahrenheit to come up with a correct scale to measure temperature was Andres Celsius in 1742. He came up with a temperature scale that divided the freezing and boiling points of water into 100 degrees. The 0 degree was chosen as the boiling point of water and 100 degrees as the freezing point by Celsius. Then about after a year, Jean Pierre Cristin, a Frenchman inverted the Celsius scale. He came up with his Centigrade scale that had freezing point as 0° and boiling point as 100°. The international agreement in 1948 accepted Cristin's scale adaptation and it is known today as Celsius. This became one of the most widely used temperature measurement scales in the world, even today.

Lord Kelvin and His Absolute Temperature Scale
Sir William Thomson, Baron Kelvin of Largs, who later became known as Lord Kelvin of Scotland proposed the existence of an absolute zero in 1848. In his proposal, the absolute temperature scale with zero degrees, was the lowest temperature possible theoretically where the molecular motion ceases. 1 Kelvin degree was defined as begin equal to 1 Celsius degree by Lord Kelvin. The current Standard Unit of Temperature measurement in Degree Kelvin.

The history of the thermometer is a long story that has seen many ups and downs. Many concepts needed to be explained, many theories to be proven to the successful invention of a thermometer.