c1 is an electromagnetic relay which allows electric current c2 = df1 to flow under conditions c3 in / on / through df2 flowing in direction to / towards df3 from direction/source df4.
This only covers relays that operate using a solenoid to magnetically attract one or more moving parts. This includes reed relays and mercury relays, but not solid-state relays. Cf. maksi, dicflecru, dicfle, dicka'u.