Duddell's oscillograph 4 (fig.
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.
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.
A form of thermal oscillograph has been devised by J.
Blondel, and are exemplified by his oscillograph and that of W.
See Assoc. Franc. pour l'Avanc. des Sciences (1898), for a paper on oscillographs describing Blondel's original invention of the oscillograph in 1891.