Thermometer is a device used to measure temperature, by using materials that change in some way when either heated or cooled. In a mercury or alcohol thermometer, the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius.
In 1596, Galileo Galilei is often claimed to be the inventor of the thermometer. However the instrument he invented was a thermoscope, the predecessor to the thermometer. The thermoscope is a thermometer without a scale. It indicates differences in temperature if the temperature is higher or lower.
Savants had found out that it might be possible to use air and water to invent a "ruler" or scale that would mark the grades from cold to hot and back again. They experimented with thermoscope involving a column of air in a tube with one end in a container of coloured water. In 1610, Galileo tried it with wine instead, and so is credited with the first alcohol thermometer.
The Italian, Santorio Santorio (1561-1636) is generally appreciated for applying a scale to an air thermoscope around 1612 and thus is thought to be the inventor of the thermometer as a temperature measuring device. Santorio's instrument was an air thermometer. Its accuracy was poor due to the effects of varying air pressure on the thermometer.
The first sealed thermometer was designed in 1641 for the grand duke of Tuscany. It used alcohol, and it had degree marks. But the man who used the freezing point of water as the "zero" or starting point was a Londoner, Robert Hooke, in 1664. An astronomer called Roemer in Copenhagen chose ice and the boiling point of water as his two reference points, and started keeping weather records.
Daniel Gabriel Fahrenheit (1686-1736) was the German physicist who invented the alcohol thermometer in 1709, and the mercury thermometer in 1714. In 1724, he introduced the temperature scale that bears his name - Fahrenheit Scale.
In 1724, a German instrument maker called Gabriel Fahrenheit settled on mercury as the most suitable liquid for measuring temperature.
He calibrated his first thermometer using a mixture of ice and water with sea salt as his zero. But salt water has a much lower freezing point than ordinary water, so for his purposes he chose his freezing point as 30, and the temperature inside the mouth of a healthy human as 96.
With those three points, he established the boiling point of water at 212 and later adjusted his freezing point of water to 32. This way, he could count 180 degrees between boiling and freezing, at sea level.
Two decades later, Linnaeus - the Swede who invented the taxonomic system naturalists now use for naming species - and a Swedish astronomer called Anders Celsius separately worked out a scale of just one hundred degrees between freezing and boiling points. Because there were 100 steps between the two states, it was called a "centigrade" scale.
Lord Kelvin took the whole process one step further and led to the invention of the Kelvin Scale in 1848. The Kelvin Scale measures the ultimate extremes of hot and cold. Kelvin developed the idea of absolute temperature, often called the "Second Law of Thermodynamics", and developed the dynamical theory of heat.
He used the centigrade scale, but started from absolute zero, the point at which all molecular motion stops, the lowest conceivable temperature in the universe. This turned out to be -273.16C.