A device is described which converts a train of electrical impulses into a succession of hyperbolic wave forms of amplitude proportional to the interval between successive impulses. These wave forms when applied to an oscilloscope furnish a display capable of showing rapid changes in the frequency of an electrical signal. Unlike conventional counting-rate meters response to sudden changes in frequency is immediate so that averaging over several cycles is not required. The instrument as constructed operates in the lower audio-frequency range and is being used in the study of the electrical discharge of single fibers of the optic nerve.