It is reasoned that the large photovoltages observed by Cheroff and Keller on ZnS crystals are theoretically plausible and can result from a wide class of different mechanisms. Conversely, the conditions under which a photoconductor with periodic inhomogeneities does not show a cumulative photovoltage are shown to be very restrictive and improbable. General theorems concerning the magnitude of the photovoltage are proved, and remarks are made on its directionality.
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