Abstract: A work-contacting probe system is selectively operable as a single-stage or as a two-stage system. The system consists of a first central probe head of large diameter and of one or more small or auxiliary probe heads of dimensions which are reduced as compared with the first probe head. The small probe head can be selectively accommodated, in substitution of a rigid probe pin on the probe-chucking receptacle of the central probe head. Upon substitution of the auxiliary probe-head system for a rigid probe pin, associated electronic circuitry automatically responds to the fact of substitution, (1) by substantially increasing spring-preload force on the probe-chucking receptacle of the central probe head, thereby converting the central probe head for collision-detection service, while (2) connecting work-contacting signals from the auxiliary probe-head system for exclusive service of measurement functions.