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 how hot an object is. Temperature is measured by an instrument called thermometer, and the most common scale to measure it is Celsius (°C). The thermometer is a very important instrument for measurement, but have you ever wondered about its history? When was the thermometer invented, and by whom? The following few paragraphs will answer all your questions. History of 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. They did not have any specific scale on which they could measure how hot an object is to touch. 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 back as 300 BCE 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 1593. In reality, what Galileo invented was actually a thermoscope. Although both these instruments are related to temperature, the themoscope did not measure the temperature differences. Instead, it was an instrument that just indicated these differences. The thermometer, on the other hand, measures absolute temperature. The thermoscope did not have any scales, therefore one could know about a change in temperature, but could not measure the exact difference. 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 that 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 Santorii and his Thermometer: Often, it is concluded that Santorio Santorii invented the thermometer in the year 1654. He was an Italian who applied a scale to an air thermoscope. Thus, he is often given the credit for the invention of modern thermometer. However, what he invented was an 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 the most commonly known to people today. This liquid-in-glass instrument was invented by Ferdinando II de' Medici, the Grand Duke of Tuscany (1610-1670) in 1654. He used alcohol filling in his instrument. Yet, this too was inaccurate, as no standardized measurement scale was in use.
• Fahrenheit and His Mercury Thermometer: Daniel Gabriel Fahrenheit is the inventor of the modern temperature-reading instrument that we use today. In 1714, he was the first person to make a thermometer using mercury as the filling. Mercury helped in measurement of more accurate readings than any thermometers that were invented before. 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, proposed a new temperature scale in 1713. His scale had freezing point of water at 0° and the boiling point at 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 Anders Celsius, in 1742. He came up with a temperature scale that divided the freezing and boiling points of water into 100 degrees. 0° was chosen as the boiling point of water and 100° as the freezing point by Celsius. Then, about a year later, 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, which is known today as Celsius. This is one of the most widely used temperature measurement scales in the world today.
• Lord Kelvin and His Absolute Temperature Scale: Sir William Thomson, 1st Baron Kelvin, who later became known as Lord Kelvin, proposed the existence of an absolute zero in 1848. In his proposal, the absolute temperature scale with zero degrees was the lowest temperature possible, where theoretically the molecular motion ceases. 0K was defined as begin equal to -272°C by Lord Kelvin. Kelvin is the current scientific scale of temperature measurement.

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