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.
x1 emits light characteristic of/is of the color that is described by/(as) arising from/associated with the approximately twenty-one centimeter wavelength, hydrogen hyperfine proton/electron spin-flip transition (from parallel to antiparallel configuration, id est: from the higher energy state to the lower energy state), electromagnetic radiation