\Ther*mom"e*ter\, n. [Thermo- + -meter: cf. F.
thermom[`e]tre. See {Thermal}.] (Physics)
An instrument for measuring temperature, founded on the
principle that changes of temperature in bodies are
accompained by proportional changes in their volumes or
dimensions.
Note: The thermometer usually consists of a glass tube of
      capillary bore, terminating in a bulb, and containing
      mercury or alcohol, which expanding or contracting
      according to the temperature to which it is exposed,
      indicates the degree of heat or cold by the amount of
      space occupied, as shown by the position of the top of
      the liquid column on a graduated scale. See
      {Centigrade}, {Fahrenheit}, and {R['e]aumur}. To reduce
      degrees Fahrenheit to degrees Centigrade, substract
      32[deg] and multiply by 5/9; to reduce degrees
      Centigrade to degrees Fahrenheit, multiply by 9/5 and
      add 32[deg].
{Air thermometer}, {Balance thermometer}, etc. See under
   {Air}, {Balance}, etc.
{Metallic thermometer}, a form of thermometer indicating
   changes of temperature by the expansion or contraction of
   rods or strips of metal.
{Register thermometer}, or {Self-registering thermometer}, a
   thermometer that registers the maximum and minimum of
   temperature occurring in the interval of time between two
   consecutive settings of the instrument. A common form
   contains a bit of steel wire to be pushed before the
   column and left at the point of maximum temperature, or a
   slide of enamel, which is drawn back by the liquid, and
   left within it at the point of minimum temperature.