The Collaborative International Dictionary of English v.0.48:

barretter \bar"ret*ter\, n. [OF. bareter to exchange. Cf.
   Barter.]
   1. (Electronics) a resistor inserted into a circuit to
      compensate for changes (such as those arising from
      temperature fluctuations); a thermal cymoscope.

   Syn: ballast resistor.
        [Webster 1913 Suppl. + WordNet 1.5]

   Note: The original form consisted of an extremely fine
         platinum wire loop attached to terminals and inclosed
         in a small glass or silver bulb. It operates by
         increased resistance when subjected to the influence of
         an electric current. In a later variety, called the

   liquid barretter, wire is replace by a column of liquid in
      a very fine capillary tube. A recent version has a
      resistor composed of an iron wire in a glass bulb
      containing hydrogen, with the temperature variation of
      resistance such that the current passing through it
      remains constant over a wide range of applied voltages.
      [Webster 1913 Suppl. + PJC]

WordNet (r) 3.0 (2006):

barretter
    n 1: a resistor inserted into a circuit to compensate for
         changes (as those arising from temperature fluctuations)
         [syn: ballast resistor, ballast, barretter]