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Dissipation of leakage power in the surface film of water on an insulator at high humidity is thought to cause the observed improvement in surface resistance by elevating the temperature and evaporating part of the water. Surface resistance of a printed circuit test pattern varied from 0.3 to over 3000 megohms in response to the magnitude of electrical potential applied (5 to 400 volts) and the cleanliness of the sample. In addition to such gradual changes, oscilloscopic observation revealed transient changes, possibly arising from sudden shifting of current paths from one area to another in the surface film of water. Contamination produced transient reductions in resistance as great as 50 per cent. With clean samples, less pronounced and less frequent variations in resistance were observed.