The CdS x‐ray detector is compared with the photo‐multiplier x‐ray detector. Reduced to the same solid angle the sensitivity of the CdS x‐ray detector is of the same order of magnitude as the sensitivity of the photo‐multiplier x‐ray detector. As the internal amplification of the primarily released electron currents in CdS is of the order 103to 104compared with the amplification in the multiplier tube of ½×106the conversion process of x‐ray quanta in slow electrons in CdS must be more efficient than the corresponding processes in the combination crystal phosphor screen photo‐multiplier. Calibration curves show a linear relation between photo‐current and intensity of x‐rays at low irradiation levels. At high irradiation saturation effects occur. Oscilloscope curves of the response at 60 x‐ray pulses per second are given. At strong irradiation the photo‐current over a long time increases in a completely reversible way. X‐ray intensities as low as 150 quanta CuK&agr;per second are registered with a circuit employing a WL 759 trigger tube.