that is the sliver trumpet 9 is connected with a microcomputer 18 by means of an integrating device 16 such as an r c member and a memory 17 the computer 18 is coupled to an inputting device 19 for manual inputting of for example the actual sliver weight further the computer 18 is connected with the regulating device 14 by means of a desired value setter 20 in operation the desired sliver number is manually inputted in the computer 18 which is a microcomputer controlling the operation of the carding machine at a given desired value in conjunction with a fiber specific characterizing value there may be determined by means of the inputted calibrating values or a formula the clearance width of the measuring trumpet 9 at which the zero balancing for the regulating system 14 is to be effected for the desired sliver number the zero balancing for determining the desired sliver value is automatically performed under the control of the microcomputer 18 a time wise limited test phase is started during which the actual sliver thickness is measured integrated and stored the sliver produced during the test phase is manually removed and weighed from the result of the weighing and a length determination the actual sliver number may be.