A probe for measuring the dimensions of objects on a coordinate positioning machine such as a machine tool has a workpiece-contacting stylus 20. This is suspended via a sensor mechanism 30, including strain gauges 34 which provide an output when the stylus contacts a workpiece. A processor 16 processes the strain gauge outputs to produce a trigger signal. It does so in accordance with an algorithm or equation or look-up table which ensures equal sensitivity in all possible directions of approach to the workpiece in the three dimensions X, Y, Z.