The implications of elastic deformation on techniques for the direct measurement of the surface forces of colloid science are discussed using a new model of contact elasticity (soft elastic contact). Under experimental conditions deformation of the measuring apparatus may occur which is neither negligible nor able to be described by the classical contact theory of Hertz.

In experiments using soft rubber spheres misuse of Hertz's theory may lead to under-estimation of surface potentials and over-estimates of the Debye screening length. The errors are largest at low electrolyte concentrations and large distances.

A direct estimate of the effect of elastic deformation on experiments on crossed mica cylinders indicates that the experimental apparatus suffers negligible deformation until well within the range of the anomalous short range force reported by the experimenters.