Abstract: 1. A photoconductive target for a pickup tube comprising a layer of porous lead telluride on a transparent conductive layer, said porous lead telluride having a resistivity of approximately 10.sup.-11 ohm cm when maintained at a temperature substantially equal to liquid nitrogen temperature.

Abstract: 1. In a frequency measuring system for measuring and indicating the frequency position of a signal within an extended band of frequencies, wherein said extended band of signals is divisible into a plurality of first sub-bands of equal extent and a plurality of second sub-bands of equal extent, the frequency extents of said first and second sub-bands being respectively different from each other, means for converting said signal to first signals only one of which occurs within each of said first sub-bands, means for further converting said signal to second signals only one of which occurs within each of said second sub-bands, means capable of measuring plural frequency locations within said plurality of first sub-bands for indicating the frequency location within said first sub-band of said first signal, and means capable of measuring plural frequency locations within said plurality of second sub-bands for indicating the frequency location within said second sub-band of said second signal.

Abstract: A twisting distortion correcting circuit to remove the twisting distortion in a picture on a television receiver in such a way that beam current of the television receiver is detected, and the high frequency components and the d.c. components in the detected beam current are removed so as to obtain a voltage wave form analogous to the mean horizontal deflection current wave form, and then the voltage wave form obtained is fed to the horizontal oscillation circuit whose oscillating frequency is controlled by the output voltage of the AFC circuit, whereby the twisting distortion of the picture is corrected by producing the twisting distortion opposite to that of picture caused by a substantial variation of the beam current.