Most classic thermometers measure temperature using liquids that expand and contract inside a glass column when they are heated or cooled. Galileo (1564-1643) constructed a thermometer as early as 1592, but it had no scale and could only show whether the temperature was rising or falling. The Italian physician Sanctorius (1561-1636) was the first person to put a scale on a thermometer, allowing him to measure patients' absolute temperatures.
But early thermometers were very inaccurate. This was because it was not known how liquids expand and because glass-makers could not produce very regular thin glass tubes. Gabriel Fahrenheit (1686-1736) was the first person to make a thermometer filled with mercury. The more predictable expansion of mercury, combined with better glass-working techniques, led to a much more accurate thermometer. In 1742 the Swedish scientist Anders Celsius (1701-44) developed the scale we still use today, with 0 degrees as the freezing point of water and 100 degrees as its boiling point.
Hermann Boerhaave was perhaps the first physician to use a thermometer at his patients' bedside, but the instrument did not become part of everyday medical practice until the 1800s. In 1868 the German physician Carl Wunderlich published the results of measurements on more than 25,000 patients. He had recorded the temperatures of patients at Leipzig University Hospital as numbers and curves and established the range of 36.3 to 37.5 °C as normal human body temperature. Wunderlich also observed that specific diseases have their own characteristic fever curves. His work prompted hospitals to record patients' temperatures frequently, and to display those temperature curves on a chart by the patient's bed. With these charts, the physician could see the development of the patient's temperature at a glance.
W E Knowles Middleton, A History of the Thermometer and Its Use in Meteorology (Baltimore: Johns Hopkins, 2003)
B Noyes, 'The history of the thermometer and the sphygmomanometer', Bull Med Libr Assoc, 24/3 (1936), pp 155-165
J M S Pearce, 'A brief history of the clinical thermometer', QJM: An International Journal of Medicine, 95/4 (2002), pp 251-252