Photometers are instruments for measuring the intensity of a light source or for comparing the intensities of two sources.
The Wheatstone photometer, used mostly in comparing the intensities of two gas burners, consists of a small brass-plated copper cylinder (diameter 5 cm, height 2 cm) with a metal shaft through its axis which is made to rotate by turning an external handle and two gear wheels inside.
The shaft turns a blackened metal arm on the upper base of the cylinder which has on its free end a very small gearwheel which fits the inside part of a toothed circumference attached to the side.
On the upper face of the gearwheel there are four vertical needles on which a disk of blackened cork is applied0. The latter has on it, off centre, a small ball of silvered glass or metal of high reflectivity.