The mechanical properties of very small volumes of material can vary greatly from bulk properties. These modified properties are of interest in many areas including the operation of atomic force microscopy (AFM), the study of adhesion and fracture, and the evaluation of electrical contact response. Despite the importance of these properties, AFM has not yet been successfully utilized for their investigation. Most existing AFMs still rely on the control and monitoring of displacements, with forces being inferred from spring constants. This would be fine if other interactions, such as those between the tip and the surface, were minor perturbations. However, this is frequently not the case, particularly for contact mode AFM. Hence very little is known about the forces applied in the contact and their affect on both the tip and the sample. In this article we describe an AFM probe where forces rather than displacements are applied to the tip. This allows absolute determination of contact compliance and hence provides a measure of the tip-surface interaction. As an example of its use we show quantitatively the effect of the adsorbed water and meniscus forces present in ambient probe microscopy.