Complex sentence: The directly-heated cathode (or indirectly by means of a filament) produces an electron charge by thermionic emission when AC is applied; this electron stream is attracted to the positively-charged plate (anode), inducing a current.
Split into simpler sentences: The directly-heated cathode (or indirectly by means of a filament) produces an electron charge by thermionic emission. This electron stream is attracted to the positively-charged plate (anode), inducing a current.