This apparatus is intended to show the duration of an electric spark. An electric spark is generated between two electrodes just before a mirror that is turned away from the viewer. The mirror is not directly visible for the eye. The flash reflects on a spinning disc divided into 16 radial segments alternately coloured light and dark. Before the flash, the eye sees the spinning disc as a uniformly grey surface. Because a spark lasts only one millionth of a second, we see a stationary afterimage of the bright flash. The pattern of the disc stands out sharply for a short while. An alternating voltage that produces several sparks in succession, will yield a stationary image at a certain rotational speed.
Discharger for stroboscopic observation of a spark
pre 1859