The Collaborative International Dictionary of English v.0.48:

Syntonizer \Syn"to*niz`er\, n. (Physics)
   One that syntonizes; specif., a device consisting essentially
   of a variable inductance coil and condenser with a pair of
   adjustable spark balls, for attuning the time periods of
   antennae in wireless telegraphy (called also syntonizing
   coil).
   [Webster 1913 Suppl.]