We describe a method for measuring forces in the atomic force microscope (AFM), in which a small amplitude oscillation(similar to 1 Angstrom(p-p)) is applied to a stiff(similar to 40 N/m) cantilever below its first resonant frequency, and the force gradient is measured directly as a function of separation. We have used this instrument to measure electrostatic forces by applying an ac voltage between the tip and the sample, and observed a variation in contact potential difference with separation. We also show how the benefits of this instrument may be exploited to make meaningful capacitance measurements, especially at small tip-surface separations, and demonstrate the potential of this technique for quantitative dopant profiling in semiconductors.