Abstract :
This paper is devoted to a detailed study of the selenium rectifier type of photo-electric cell, only brief reference being made to the development of the dry rectifier cell from the original ¿Lange¿ cell. It is mainly an investigation into the difficulties met with and the precautions necessary when employing this type of photo-electric cell for photometric measurements. Hence experiments were carried out to show:¿ (1) The effect of illumination and current generated on the internal resistance. (2) The effect of the external resistance on the linearity of the current response. (3) The effect of temperature on the current generated, internal resistance, and voltage developed, for a given illumination. (4) The effect of intermittent illumination on the cell sensitivity, etc. In an endeavour to overcome the effect of the external resistance and temperature the possibilities of the Campbell- Freeth compensation circuit were investigated. The results obtained indicate that the generated current is strictly proportional to the illumination and that it is unaffected by temperature. Further, the effect of applying an external voltage was investigated for different values of the illumination. The advantages of the slightly increased sensitivity gained are greatly outweighed by the disadvantages of having auxiliary apparatus. It should be noted that only a limited number of cells were available for test, and as cells vary considerably between one another the numerical results must only be taken as a guide to the values to be expected.
