THE bonding potential between atoms determines all the key properties of matter. It is usually deduced from a nide variety of experimental parameters such as elastic moduli, binding energy and vibrational nonlinearities. These provide an indirect route to quantities such as virial coefficients which characterize the variation of potential energy,vith interatomic spacing(1). Here we report use of a modified atomic force microscope(2) to measure mechanically the interatomic forces between a tip and the sample surface as a function of separation. We use a magnetically controlled feedback mechanism to resist the ’jump to contact’ that commonly occurs in mechanical force measurements at small separations, enabling us to map out reversible curves to separations closer than the point of inflection in the potential-energy curve. This method provides a direct means for continuous measurement of forces between atoms as they approach towards contact.