thermometer (ther-mom'et-er)

An instrument or apparatus for measuring degrees or variations of sensible heat. Its construction is based on the property po e ed by certain substances of expanding or contracting regularly as the temperature is increased or diminished. The ordinary t. consists of a bulb communicating with a tube of very small bore, the bulb and adjacent part of the tube containing mercury, while the distal part of the tube is empty. With a rise of temperature the mercury expands so as to fill more and more of the tube. The temperature is indicated by a scale attached to the tube. The scale is graduated by marking off equal intervals as degrees, the unvarying distance between the marks that indicate the freezing point and boiling point of water being divided into 180 of these degrees in the Fahrenheit t., 100 degrees in the centigrade t. or Celsius's t., and 80 in Reaumur's t. The freezing point of water is the zero in the centigrade t. and Reaumur's t., but Fahrenheit put boiling his zero 32° of below point, and the points water that in the three are thus ioo°, 80 ° and 2120. The _Fahrenheit scale (symbol, Fahr. or F.) is that in general use, especially for meteorological and clinical observations; but in some countries the centigrade (symbol C), which is that now chiefly employed in scientific research, takes its place; they are connected by the formula F. = 9/sC. + 32°. For very low temperatures alcohol is used instead of mercury, and very high temperatures are measured by the pyrometer, air t. A t. in which the temperature is measured by the expansion of air. Celsius's t. See under Celsius, centigrade t. See Celsius's t. clinical t. A t. for testing the temperature of the body. Fahrenheit's t. See under Fahrenheit. Reaumur's t. See under Reaumur. recording t. A t. which makes automatically a tracing indicative of the variation of temperature to which it has been subjected, registering t., self -registering t. An matic maximum or minimum t. auto- The maximum or minimum is shown by some indicator remaining in position after the mercury falls or rises, surface t. A t. for taking the temperature of the surface of the body. [Gr., therme, heat, + metron, a measure.]