Max Kohl A.G. Chemnitz
10 × 7 × 10 cm
Invented sometime before 1843, Charles Wheatstone’s photometer was praised at the time, although it was later superseded. It was based on comparing the brightnesses of reflections of two sources in a small sphere rapidly moved in a pattern so that persistence of vision smears out and reduces the intensities, allowing more sensitive comparison.
Using a sun and planet gear arrangement the sphere was moved in a trochoidal path. Although obsolete by the time this one was purchased (1913?), such instruments were still being manufactured for teaching purposes.
Ours was described in an early inventory as an ‘epicycloid generator’ and when identified, had lost the planetary gear and all the external moving parts, so it was decided to restore it to working order.
Unfortunately the engravings in the Max Kohl catalogue of the time show something a little different, essentially the same as in the textbook of Ganot dating from many years earlier, so it was necessary to use some advice from expert sources and imagination. With a 60 tooth sun gear, a 4-lobed epitrochoid pattern as shown in the catalogue can be obtained with a 15 tooth planetary wheel and it was possible to source one of the correct module (0.7M) cheaply in plastic. The rest of the assembly was fabricated in brass with a steel axle. As in other extant Wheatstone photometers, a steel ball bearing reflector was mounted with sealing wax on a black painted cork disc pinned to the table rotated by the planetary wheel. It seems that users liked to try different numbers and arrangements of balls, and this allowed easy exchange.