The first recorded thermometer was produced by the Italian, Santorio Santorio (1561-1636) who was one of a group of Venetian scientists working at the end of the Sixteenth Century. As with many inventions the thermometer came about through the work of many scientists and was improved upon by many others.
Galileo Galilei is often claimed to be the inventor of the thermometer. However the instrument he invented could not strictly be called a thermometer: to be a thermometer an instrument must measure temperature differences; Galileo's instrument did not do this, but merely indicated temperature differences. His instrument should rightly be called a thermoscope.
The predecessor to the thermometer, the thermoscope is a thermometer without a scale; it indicates differences in temperature only ie it can show if the temperature is higher, lower or the same, but unlike a thermometer it cannot measure the difference nor can the result be recorded for future reference. The thermoscope was widely used by a group of scientists in Venice that included Galileo. It was only a small step from the thermoscope to the thermometer.
The Italian, Santorio Santorio (1561-1636) is generally credited with having applied a scale to an air thermoscope at least as early as 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 as the effects of varying air pressure on the thermometer were not understood at that time.
The sealed liquid-in-glass thermometer, more familiar to us today, was first produced in 1654 by the Grand Duke of Tuscany, Ferdinand II (1610-1670). His thermometer had an alcohol filling. Although this was a significant development his thermometer was inaccurate and there was no standardised scale in use.
Gabriel Fahrenheit (1686-1736) was the first person to make a thermometer using mercury. The more predictable expansion of mercury combined with improved glassworking techniques led to a much more accurate thermometer.
Fahrenheit used the newly discovered fixed points to devise the first standard temperature scale for his thermometer. Fahrenheit divided the freezing and boiling points of water into 180 degrees. 32 was chosen as the the figure for the lower fixed point as this produced a scale that would not fall below zero even when measuring the lowest possible temperatures that he could produce in his laboratory - a mixture of ice, salt and water. It is sometimes suggested that Fahrenheit divided his scale into 100 degrees using blood temperature (incorrectly measured) and the freezing point of water as fixed points - this is not true. The Fahrenheit scale is still in use today.
In 1731 the Frenchman, René Antoine Ferchauld de Réamur (1683-1757) proposed a thermometer scale on which the freezing point of water was 0° and the boiling point was 80°. The Réamur scale is not in use today.
In 1742 a Swedish scientist named Anders Celsius (1701-1744) devised a thermometer scale dividing the freezing and boiling points of water into 100 degrees. Celsius chose 0 degrees for the boiling point of water, and 100 degrees for the freezing point. A year later, the Frenchman Jean Pierre Cristin (1683-1755) inverted the Celsius scale to produce the Centigrade scale used today (freezing point 0°, boiling point 100°). By international agreement in 1948 Cristin's adapted scale became known as Celsius and is still in use today.
In 1848 Sir William Thomson, Baron Kelvin of Largs, Lord Kelvin of Scotland (1824 - 1907) proposed the absolute temperature scale with zero degrees being the theoretical lowest temperature possible where molecular motion ceases. Kelvin defined 1 Kelvin degree as being equal to one Celsius. The Degree Kelvin is the current Standard Unit of temperature measurement.