The first development on the road to thermometers was Galileo's invention consisting of a series of glass spheres filled with varying densities of alcohol that would rise or fall in a closed tube of water depending on the temperature. Around the same time, in the 16th/17th century, an invention known as the thermoscope was developed by Robert Fludd, though the invention has also been attributed to Galileo, Santorio, and Cornelius Drebbel. This device is known to us today as an air thermometer. The open end of a closed tube was submerged in water, alterations in the air temperature would cause the water in the tube to rise and fall, which also makes it susceptible to air pressure and, thus, a barometer. The word thermometer is French in origin and is noted to have first been used in a 1624 French Mathematics text. The first modern thermometer, made with completely closed ends and containing alcohol, was made about 20 years later by the Grand Duke of Tuscany de'Medici.
A body temperature reading at that time in the thermometer's history would take 20 minutes. In 1866, Sir Thomas Allbutt created a clinical thermometer that could take a temperature reading in 5 minutes.
The thermometers used to measure body temperature are called medical thermometers. They measure a person's body temperature by inserting the tip into the mouth (oral), the armpit (axillary), or the rectum (rectal). Traditionally mercury thermometers were used in the same way as weather thermometers. Heating the liquid mercury in the tip would cause it to expand into the tube of the thermometer, thus fluctuating according to temperature and stopping at a point on the scale. Also, disposable plastic heat sensor dot thermometers and disposable probe covers have been developed for clinical use to prevent contamination between patients.
The scale of the thermometer was debated until Daniel Fahrenheit developed the Fahrenheit scale nearly 100 years after the air thermometer was invented. The Fahrenheit scale was developed using mercury as the liquid in the glass tubing, which was to become the standard until the 1990s when digital thermometers began replacing traditional thermometers in medicine due to safety concerns.
Another type of thermometer used to measure body temperature is called a basal thermometer. These devices measure core body temperature upon waking. This is the most efficient way to eliminate environmental factors on body temperature such as exercise or diet. Basal temperature alterations are used to determine fertility and ovulation in women trying to conceive. Most medical mercury thermometers are sensitive to 0.2 degree, basal body temperature fluctuations require a sensitivity of 0.1 degree. Most digital thermometers are sensitive enough to also measure basal temperature.