The Collaborative International Dictionary of English v.0.48:

Decoherer \De`co*her"er\ (d[=e]`k[-o]*h[=e]r"[~e]r), n. [Pref.
   de- + coherer.] (Elec.)
   A device for restoring a coherer to its normal condition
   after it has been affected by an electric wave, a process
   usually accomplished by some method of tapping or shaking, or
   by rotation of the coherer.
   [Webster 1913 Suppl.]