An instrument for exhibiting optically or graphically these variations is called an oscillograph, or sometimes an ondograph.
For the scientific problems connected with oscillation see Mechanics and Oscillograph.
In one form of Blondel's oscillograph, the vibrating system is a small magnetic needle carrying a mirror, but the principle on which it operates is the same as that of the instrument above described.
In the Duddell oscillograph it is usual to place a pair of loops in the magnetic field, each with its own mirror, so that a pair of curves can be delineated at the same time, and if there is any difference in phase between them, it will be detected.
In this case the large vibrating mirror must be oscillated by a current from an alternator, on the shaft of which is a disk of nonconducting material with brass slips let into it and so arranged with contact brushes that in each period of the alternator a contact is made, charging say a condenser and discharging it through the oscillograph.
See Assoc. Franc. pour l'Avanc. des Sciences (1898), for a paper on oscillographs describing Blondel's original invention of the oscillograph in 1891.
The oscillograph can be made to exhibit optically the form of the current curve in non-cyclical phenomena, such as the discharge of a condenser.