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Kodak has been making photographic film for more than a century, but the
company admits it still can’t always get the emulsion mix just right (GB 2 335
988). The acidity is crucial, but it’s tricky to measure the pH of the
hot suspension of silver salts in gelatin that is used to coat the film. So
Kodak suspends a field effect transistor whose output current is sensitive to
ion concentrations in the hot photo emulsion. Kodak says that the voltage
difference between the hot transistor and an electrode in the neutral solution
at the same temperature gives a read-out of the emulsion’s pH.