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As slider-to-disk spacing becomes smaller than 10 nm, electrostatic and intermolecular forces become increasingly important. Even if the slider and disk are both grounded, a potential difference can exist between them due to the contact potential, which, as we show here, can generate an electrostatic force greater than the van der Waals force. We have developed a method for measuring the contact potential between the slider and disk by monitoring the induced slider motion with a Laser-Doppler-Vibrometer (LDV) and lock-in amplifier while applying ac and dc bias voltages. We find that the first harmonic of the ac driving frequency is minimized when the dc bias voltage cancels the contact potential. We have preformed air bearing modeling of the slider-disk interfaces that also incorporates electrostatic forces. From simulation, we find that the flying height is reduced and the pitch angle increased as the potential difference between the slider and disk is increased.