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The most pressing problem in electric contact testing is to define the modes by which contacts fail. Until we understand why and by what mechanism contact spots stop passing electricity between the mating members, it will be impossible to devise relevant life-testing procedures. This paper describes an experimental teehnique whereby interfaces can be observed during exposure to moist, polluted air and the degradation of contact spots studied in situ. The reactions that occur are analyzed in terms of the physical effects of the ingress of reactants into contact joints, and the resistance changes observed. The main experimental tool was metallography, including macro- and micro-cinephotography. The distinction between ingress-limited corrosion and reaction-limited corrosion is introduced, and the death of contact spots is observed under the microscope. This technique makes possible the direct observation of the progressive corrosion of an array of contact spots, and reveals in detail how the conducting area of each spot is eroded by the chemical action.