A high-precision photoelectric polarimeter

In this instrument, which has been designed primarily for standardization work on quartz control plates, the half-shade principle familiar in visual polarimeters is realized, in a slightly different form, by means of a revolving disk carrying segments of optically-active quartz. These cause the plane of polarization of the light incident on the analyser to alternate between two positions at a low audio-frequency; the setting position of the analyser is that in which the intensity of the emergent light, as indicated by means of a photomultiplier and a.c. amplifier, is unmodulated. The precision of setting is about ten times better than that of a visual instrument. For measuring specimens of small optical activity, use is made of the Faraday effect to balance out the rotation produced by the specimen, thus obviating the need for angular scale readings.