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Safety matches and Avo Smethurst exposure meter, c 1930s.
This box of safety matches contains a strip of red phosphorus. When an oxidised match head strikes it, the phosphorus helps ignite the match. White phosphorus had been used in matches since 1669, but by 1845 was considered too toxic. The exposure meter (right) 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. It 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.