Considering the moisture that exist in the atmosphere, the water is thought to be complexly connected with the degradation process of polymeric materials due to the partial discharge. Therefore, in this paper, investigation was done from the standpoint that water has effects on the molecules degrading the polymer (i. e. the active molecules for the degradation). The degradation tests were carried out by using the parallel plate electrode system. High frequency potential was applied to the sample (Polyethylene), the particular atmospheric conditions having been established by appropriate mixture of N
2, O
2 and H
20. The following results were obtained in respect to the measurment of the weight change of the samples, the inception voltage and the discharge magnitude by Lissajous's figure. In the N
2+H
2O atmosphere, the active oxygens were generated due to the discharge similar to the N
2+O
2 atmosphere, but the amount of generation per discharge magnitude was approximately 1/3 lower than in the N
2+O
2 atmosphere. In the case of the N
2+H
2O+O
2 atmosphere, the water molecules had three types of degradation effects to the polymer corresponding to the enclosed density of the oxygen, namely the acceleration, inhibition and both of the effects.
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