If the instrument is placed in motion by turning the handle, the small ball is subject to two uniform, circular motions, one around the axis of the cylinder and the other around the axis of the gearwheel; since the gearwheel has 18 teeth and the circumference has 72, the ball, while completing one revolution around the axis of the cylinder completes four revolutions around the axis of the gearwheel and describes a rosette trajectory with four nodes.
To compare the intensity of two light sources, the photometer is placed between them and, holding it with one hand the crank is turned quickly with the other; at that point, the two shiny dots produced by the reflection on the ball of the light coming from the two sources create two luminous trajectories in the form of a rosette due to the persistence of vision on the retina. If one appears more brilliant than the other, the instrument is gradually moved away from the source producing it until the two appear equally bright.
The distance of the two sources from the instrument is then measured and the two intensities compare as the squares of the respective distances.