The Collaborative International Dictionary of English v.0.48:

Coherer \Co*her"er\, n. (Elec.)
   Any device in which an imperfectly conducting contact between
   pieces of metal or other conductors loosely resting against
   each other is materially improved in conductivity by the
   influence of Hertzian waves; -- so called by Sir O. J. Lodge
   in 1894 on the assumption that the impact of the electic
   waves caused the loosely connected parts to cohere, or weld
   together, a condition easily destroyed by tapping. A common
   form of coherer as used in wireless telegraphy consists of a
   tube containing filings (usually a pinch of nickel and silver
   filings in equal parts) between terminal wires or plugs
   (called

   conductor plugs).
      [Webster 1913 Suppl.]