oscillograph, instrument for indicating and recording time-varying electrical quantities, such as current and voltage. The two basic forms of the instrument in common use are the electromagnetic oscillograph and the cathode-ray oscillograph; the latter is also known as a cathode-ray oscilloscope (q.v.), which, strictly speaking, is purely an indicating instrument, while the oscillograph can make permanent records.
The operation of an electromagnetic oscillograph, like the operation of a d’Arsonval galvanometer, depends on the interaction of the field of a permanent magnet and a coil of wire through which an electric current is flowing.
Some oscillographs are provided with a pen arm, attached to the coil, that traces an ink record on a moving paper chart. The most common device of this nature is the electrocardiograph, which employs a coil of fine wire with many turns and is used for studying heart action.
The light-beam oscillograph has much less weight to move than does the pen-writing instrument and so responds satisfactorily to higher frequencies, about 500 Hz, or cycles per second, compared with 100 Hz for the pen assembly. It uses a coil to which a small mirror is attached. A beam of light is reflected from the mirror onto a photographic film moving at a constant speed. The cathode-ray oscillograph makes use of a sharply focused electron beam to display the relationship between variable quantities on a luminescent screen similar to that used in a television. It may be either a three- or a four-dimensional recording device, the latter being very versatile. Recording is by transfer of energy to a light-sensitive record, using either an optical-lens system or a fibre-optic faceplate. It can be employed at frequencies in the megahertz range.