The construction and calibration of a rotational displacement transducer, using light sensitive resistors and a variable slit, are described. The instrument is capable of measuring both static and dynamic angular displacements and has a continuous range of sensitivity with a resolution as low as 0.01°. The linearity of the response over a 50° linear region is within 1% of the full scale deflection of 135°. The transducer exhibits no substantial phase lag and has response well suited for low frequency oscillation studies in the range 0–50 cps. The device is light in weight and has free undamped motion due to an external air‐bearing suspension system. A restriction which must be noted in applying the transducer is that because of high frequency response dropoff the instrument is suitable only for recording accurately low frequency angular displacements of quasiharmonic waveform.