characteristic of the signal from the selector 31 is indicative of the desired or optimum final moisture content of tobacco particles 13 when the characteristic of signal which is furnished by the detector 28 deviates from the characteristic e g voltage of signal supplied by the selector 31 the moisture content of tobacco particles 13 is either too high or too low and the junction 29 then transmits a signal to a second junction 30 which transmits signals to the amplifier 32 the junction 30 is further connected with a temperature detector 35 in the supply pipe 20 the detector 35 is preferably a temperature sensitive semiconductor of the ptc or ntc type the motor 27 transmits signals to a junction 130 which is further connected to a preferably adjustable potentiometer 33 a characteristic e g voltage of signal from the motor 27 to the junction 130 matches the corresponding characteristic of signal from the potentiometer 33 when the valve 24 assumes an intermediate or neutral position namely a position in which the valve 24 is normally held and which is selected in such a way that the temperature of air in the pipe 20 increases abruptly if the valve 24 is pivoted clockwise and that the temperature of air in the pipe 20 decreases abruptly if the valve 24 is pivoted counterclockwise as viewed in the drawing the retention of valve 24 in such neutral position is desirable because the control unit 25 is then ready to rapidly increase or decrease the moisture content of tobacco particles as soon as the detector 28 in the trough 16 produces a first signal whose characteristic deviates from the characteristic of signal from the selector.