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Avo Smethurst Exposure Meter, c 1930s.
This meter would have been used by photographers to measure the intensity of light. This particular model was made by the Automatic Winder and Electrical Equipment Co Ltd and made available to customers in the 1930s. The meter works by the photoelectric effect, and its mechanism is straightforward: photons of light hit a strip of sensitive material, such as selenium, and produce an equivalent number of electrons. These electrons are then measured by a current meter, and the quality of light can be accurately asesed.