A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. In medical instrumentation, medical thermometers are used for measuring human body temperature. Thermometer is a very important tool in medicine because many early symptoms of a disease can be easily detected by measuring body temperature. But, how it was invented ?
In 1603, the Italian scientist Galileo showed that a closed glass tube inserted in a container of a water could be arranged so that the height of the water sucked into the tube by a partial vacuum varied with the temperature. In 1625, Santorio Santonio, a Slavic physician, constructed a similar device, which he used to measure the temperature in the human body. The problem with the instrument (Figure a) was that the height of the water was also affected by the atmospheric pressure. This problem was solved a quarter century later when Ferdinand II, Grand Duke of Tuscany, sealed the water in a closed vessel to eliminate the effect of atmospheric pressure. The essentially modern thermometer shown in (Figure b) was introduced by the Dutch instrument maker Gabriel D. Fahrenheit, who in eighteenth century replaced the water with mercury and improved the instrument's accuracy. This thermometer is still widely used, although more recently crystal thermometer have been adopted for special applications. 
Today, as a result of developments in biomedical engineering field, there are various kinds of modern thermometers used to measure body temperature. This modern thermometers are no longer using mercury or liquid crystal. The other techniques of thermal measurement are adopted, for example ear thermometer that use infrared emitted from the inner ear to measure body temperature. -aw-
Source:1. Principles of Biomedical Instrumentation and Measurement - Ricard Ashton
2. http://en.wikipedia.org/wiki/Thermometer
3. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/eartherm.html
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