instrument for measuring temperature. Galileo and Sanctorius devised thermometers consisting essentially of a bulb with a tubular projection, the open end of which was immersed in a liquid. Heating or cooling the bulb affected the height of the column of liquid in the tube, on which a scale was marked. Over a century later appeared the three thermometers now most widely used—the Fahrenheit, the centigrade (Celsius), and the Réaumur (used to some extent in parts of Europe). The first, invented by Fahrenheit c.1714 in Danzig, initiated the use of mercury as a heat-measuring medium; the thermometer of Réaumur, invented c.1730, used alcohol; the Celsius, invented by Anders Celsius at Uppsala (probably 1742) is now most used in laboratory work. The clinical thermometer is a small tubular instrument of rather thick glass. It consists essentially of a small vacuum tube of uniform bore closed at one end and connected at the other with a mercury chamber (either a bulb or a short tube of larger bore). A Celsius or a Fahrenheit scale (or both) is etched on the front of the thermometer; opposite this the glass is milky or semiopaque, to facilitate reading the temperature. When heat is applied, the mercury expands and rises from the chamber past a narrowed point and up the small tube. This narrowed point prevents the mercury from sinking back until shaking forces it down. A thermocouple can be used as a thermometer for measuring temperatures outside the range of liquid-in-glass thermometers. It is based on the thermoelectric effect occurring when the two junctions of a closed loop made of two different metals are at different temperatures (see thermoelectricity).

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