The Use of Arcs and other Fluctuating Sources in Photoelectric Photometry

Abstract

IN his paper on the registering microphotometer, P. P. Koch (Annalen der Physik, iv. 39, p. 705, 1912) describes an arrangement consisting of two similar photoelectric cells connected to an electrometer, both illuminated by the same source, but through different optical paths. The arrangement described has the advantage of being independent of intensity fluctuations in the source (provided that these are uniform over the area of the source), since the electrometer reading depends only upon the ratio of the intensities falling on the two cells. An example is given in which a change from 10 to 100 in the light intensity of the source produced only a 1 per cent, change in the electrometer deflexion. However, it remained impossible to use arcs, or other light sources which not only fluctuate in intensity but also flicker or change their position; for since the two beams come from different parts of the source, and traverse different optical paths, flickering changes the ratio of their intensities, as well as the absolute values.