Add punctuation: The rhythm is created by a rotating plate with holes through which a number of small bulbs illuminates activates photoelectric cells These cells are part of the oscillator circuit of the tubes and thus determines the speed of the rhythm
The rhythm is created by a rotating plate with holes through which a number of small bulbs illuminates (= activates) photo-electric cells. These cells are part of the oscillator circuit of the tubes - and thus determines the speed of the rhythm.