not for the arrangement according to the invention in fact to eliminate this drawback said arrangement uses the formation provided by the luminous beam 22 as a reference to allow an automatic rating of the luminous beam 21 it is to be noted that during normal working of the weft feeding device the luminous beam 22 is free from the presence of yarn turns during several time intervals the electronic circuit of fig3 uses these time intervals to measure the amount of light reflected by the beam 22 and to consequently dose the amount of light sent by the beam 21 in order to compensate any light which may have been absorbed by the dust settled on the protection glass 15 with reference to fig3 which illustrates the block diagram of an electronic circuit adapted to provide the aforespecified automatic compensation the signal 31 amplified and processed by the amplifier 50 generates a signal 33 which operates an inverter circuit 61 this latter generates a three phase voltage 45 with variable frequency and voltage in order to run the motor which operates the rotating reel 2 the signal 33 is moreover compared with a reference value 34 in comparator 51 when the signal 33 exceeds the reference value 34 the luminous beam 22 is positively not influenced by the presence of a reserve as said reserve will first have to intercept the beam 21 determining a reduction of the signal 33 below the reference value 34 an oscillator 52 generates a square wave 35 of suitable frequency an and circuit 53 allows the signal 35 to pass only if the output from the comparator 51 is at a high logic.