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thermometer, 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 REaumur (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 REaumur, 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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