The paper describes a combination of beat-frequency oscillator and cathode-ray oscillograph so arranged that the oscillator output is rapidly varied in frequency over a limited range without appreciable change in amplitude, and, after passing through the apparatus whose response curve is to be measured, is applied to the vertical deflector plates of the oscillograph, the horizontal deflection being proportional to the frequency.A frequency scale is obtained on the oscillograph by means of piezo-electric crystals, so that the calibration of the instrument is unaffected by variations in supply voltages, etc.The variation of frequency is obtained by electrical means instead of by the usual rotating condenser, and the absence of mechanically moving parts renders the apparatus compact and portable.