with the source 332 at a frequency which is determined by the interrupter 331 whereby the resulting sparks burn holes into the web 302 and the permeability of the web increases to the extent determined by the intensity of signal from the control circuit 326 to the servomotor 338 i e the permeability is a function of the ratio of the transmission 333 when the testing unit 315 determines that the permeability of the web 302 is unsatisfactory either excessive or insufficient the intensity of signal from the diode 316 to the peak voltmeter 321 changes so that the output of the signal comparing stage 323 transmits a different signal which causes the servomotor 338 to change the speed ratio of the transmission 333 and hence the frequency at which the electrodes 328 329 are connected with the source 332 of high potential since the diode 314 is operated in pulsating fashion the amplitude of pulses transmitted by the generator 319 can be higher than during continuous operation of the diode 314 the peak voltmeter 321 detects the amplitude of signals which are transmitted by the diode 316 and furnishes a continuous signal whose intensity or another characteristic is indicative of monitored permeability of the web 302 between the optical systems 317 and 318 as a rule the permeability of the web 302 changes only gradually i e the rate at which the speed ratio of the transmission 333 is changed by the motor 338 is also gradual whenever the speed ratio of the transmission 333 is changed in response to signals from the testing unit.