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Such a defect signal is approximately 50 volts minus 23' volts which equals 27 volts and its peak level at the grid of tri ode 181 is +150 volts and at the cathode of triode 181 it is +252 volts. Thus the control grid of the normally cut-off triode 175 of the trigger generator is raised above the potential level of 250 volts of its cathode and the output pulse 184 is produced. The relative potentials of the cathodes of the trigger generator triodes and the cathodes of the coupling diodes are thus so arranged that defect signals raising the potential of one of the output plates to about +250 volts will produce an output pulse of constant amplitude (184), while the noise content of the output signals do not elfect the output trigger gen erator. The second channel of the amplifying system shown in FIG. 10 consists of coupling diodes 159 and 168, paral leled ‘by resistors 161 and 162 with capacitor 163 coupling the junction of the anodes of diodes 159 ‘and 160 to the control grid of the Miller integrator 185 through resistor 186. The following components associated with the Mil ler integrator are the grid-leak resistor 187, plate load resistor 188, the plate to grid teedeback capacitors 189 and 190 which are coupled to the control grid by switch 191, the integrator output coupling diode 164, the plate resetting diode 192 and the screen grid de-coupling ca pacitor 193. The principle of the Miller integrator is well known in the art and its main function as illustrated in FIG. 10 is the production of a linearly increasing plate potential. The rate of increase of the plate potential FLY-L dT _C' R Where v is the potential applied to the resistor 186 at the junction of resistors 186 ‘and 187 and capacitor 163, vC is the capacitance of the capacitors 189,0r 19!} and R is the resistance of resistor 186. The control grid and the cath ode of the Miller pentode 185 are at substantially the same potentials while the resulting plate current is such that the plate potential of tube 185 will be only a few volts, say ten volts, above ground level (cathode poten tial). By applying to the screen grid of this Miller inte grator a high enough potential by pider 194, such “bot t-omed” condition of the plate can always be attained. Now when a negative potential is applied to R (186), the current of the tube is reduced and its plate potential will linearly increase toward the positive line potential +HT set at say +350 volts. When the plate potential reaches l+223 volts, that is the level to which the grid of the cathode follower output tube 181 is set, coupling diode 164 conducts and the further rising plate potential of the integrator 185 appears at the output of the ampli~ fying system producing an output pulse similar to sig nal 184, the duration of which is equal to the time period during which the integrator plate potential remains above 250 volts. 20 30 45 50 55 60 65 70 75 This second integrator channel of the amplifying sys tem senses defect signals which are below the trigger level of the output pulse generator and which can be of the same order of magnitude or smaller than the basic noise level (173, 174) of the sheet material under inspection and which are due to defects which are ex tended in the direction of travel of the sheet material. For example, the integrator constant CR may be set to have such value that the input signal v speci?ed in the table below raise the integrator plate potential to the hereinbefore de?ned trigger level of 250 volts for defects speci?ed in the following table. Dimension of the defects at right angles to the direc tion of travel of the sheet material equal 1/32 of an inch. Time intefgval Dimension of in millisecond defect in the during which Intensity of defect, direction of Input signal plate of inte 100 percent full travel of the amplitude 11 grater rises to black in percentage sheet material in volts +250 volts, in inches sheet velocity equals 100 inches per second 1A0 25 0.625 ls 12. 5 1. 25 1,4 6. 25 2. 5 V: 3.125 5. 0 1 1. 56 10. 0 For defects extending in the direction of travel of the sheet material, the integrator response is the same as that of the amplitude differentiating channel, channel 1, for defects extended at right angles to the direction of travel of the sheet material. In both channels the out put signal peak is proportional to the product of the intensity and the area of the defects.