A Differential Circuit for Blocking-Layer Photo-Cells

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Abstract
The use of a differential photo‐cell circuit employing two cells of the blocking‐layer dry disk type is of advantage where a difference between two light intensities is to be measured or to serve as control, especially where fluctuations of the source prove troublesome, or where the difference to be measured is small. Experimental as well as theoretical consideration of the circuit in which terminals of opposite polarity from the two cells are connected together, with a meter or relay in parallel with the cells, shows many desirable features in respect to sensitivity, linearity of response, and stability. The advantage arises fundamentally from the fact that the effective resistance across each cell under these conditions is very low. The circuit is useful either as a null method or for obtaining, on a current‐measuring instrument, scale readings proportional to light‐intensity difference. Linearity is obtained over approximately the same range as for a single cell, the rate of change with light intensity being approximately the same. A comparison with the arrangement hitherto chiefly employed, which balances open‐circuit voltages, indicates advantage in all respects of the circuit here described, which should consequently prove of wide utility.

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