An instrument for detecting or measuring minute extension
   or movements of solid bodies. It consists essentially of a small rod,
   disk, or button of carbon, forming part of an electrical circuit, the
   resistance of which, being varied by the changes of pressure produced
   by the movements of the object to be measured, causes variations in the
   strength of the current, which variations are indicated by a sensitive
   galvanometer. It is also used for measuring minute changes of
   temperature.