Oscillograph, instrument that records, on paper or film, wave patterns that are equivalent to the oscillations of an electric voltage, or that converts mechanical oscillations into an equivalent electric current before recording them on paper or film in the form of wave patterns. The electromechanical type of oscillograph consists in principle of a galvanometer (see Electric Meters) to which is attached an apparatus that records the movements of the galvanometer coil.

The recording apparatus may consist of a mirror that reflects a beam of light from the moving coil upon a photographic film moving at a fixed rate, or it may be a lightweight pen or stylus connected to the coil that writes on a moving paper chart. The recordings are called oscillograms. Electromechanical oscillographs are limited to recording oscillations having not much more than 500 Hz. The electrocardiograph is an electromechanical oscillograph that records the electric currents produced by a beating heart.