A simple and cheap photosensitive transformer bridge for the analysis of powder mixtures.

Two cadmium sulphide cells are mounted at the lower ends of vertical copper tubes holding lenses to concentrate light on the cells, and illuminated powder samples are placed above the upper ends of the tubes. The cells are included in the arms of a transformer bridge which is energized by 50 Hz alternating current. The bridge output may be examined either by a valve voltmeter, or used to drive an amplifier to provide a larger output. A response is obtained which is linear to within some 10% over the whole composition range for coarse granular powder mixtures of a wide range of colours.