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The electrostatic potential in air resulting from an ion space charge of one predominant sign is difficult to measure with existing potential-measuring instruments. An ideal instrument should not deplete the ion space charge and should be capable of measuring relatively low potentials. An instrument using the null-and-substitution method, and combining an electronic pulse widener and a vacuum-tube voltmeter, is described. This instrument has an application in measuring ion density in air by measuring the electrostatic potential attributable to ion space charge; Poisson's equation can then be solved for ion density.