An oscillograph traces or records an electrical signal against time, making its waveform observable. The signal-to-deflection conversion and the time-axis mechanism require separate calibration.
A historical string design passed electric current through a fine wire in a magnetic field. An illuminated image of the wire and a rotating mirror made its oscillation visible on a screen. [ref]
General Radio type 338 string oscillograph by the Smithsonian Institution. Smithsonian, General Radio type 338 string oscillograph: wire, lamp and rotating-mirror arrangement
Resonance and inertia limit how faithfully such a recorder follows rapid signals; a distorted trace need not reproduce the input waveform. New to topics? Read the docs here!