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The precursors to the technology of the body temperature thermometer were the ordinary thermometers used for measuring the temperature of the air.

In 1592 Galileo Galilei measured temperature using a device that he called a thermoscope which was based on the fact that changes in temperature can cause changes in pressure and this can be observed as changes in water level in a glass bulb and tube system immersed in water.

The first usage of thermometer' was by Jean Leurechon in 1624 and the first thermometer sealed from the air and not affected by atmospheric pressure was invented by Ferdinand II, Grand Duke of Tuscany, in 1641. He used colored alcohol but later Tuscan researchers used mercury.

Gabriel Fahrenheit invented a new temperature scale in 1724, which we now name after him. He set zero at the freezing point of water mixed with ice and sea salt and set body temperature as 96 degrees, having worked it out from inserting the bulb of his thermometer in the mouths and armpits of his subjects.

Anders Celsius then introduced the scale named after him in 1742. We now have 0 C as the freezing point of water and 100 C as the boiling point of water although Celsius originally suggested that the scale be defined the other way around (Jean Pierre Christin suggesting the form we know today in 1743).

But despite these tools being in place it was over a century before regular diagnosis involving systematic use of the body thermometer was used. One important milestone being the work of Karl August Wunderlich who published temperature data on 25000 patients in 1868 under the title The Temperature in Diseases.

One key advance was the introduction of a constriction to the tube of the thermometer in 1852 meant that the mercury no longer fell back down. Also, early thermometers were about a foot long. This was put right by Clifford Allbutt in 1867 who provided a shortened version.

Learn more about this author, Darian Peters.
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