Source : Webster's Revised Unabridged Dictionary (1913)

Thermometer \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.