The Collaborative International Dictionary of English v.0.48:

Rheotome \Rhe"o*tome\ (r[=e]"[-o]*t[=o]m), n. [Gr. "rei^n to
   flow + te`mnein to cut.] (Elec.)
   An instrument which periodically or otherwise interrupts an
   electric current. --Wheatstone.
   [1913 Webster]