Abstract: A measuring instrument comprising a pair of probe elements moveable relative to one another to vary the spacing between sensing areas of each probe, means for producing a signal representing the spacing of said sensing areas, means for monitoring the rate of change of said signal, and means for recording and/or displaying a value representative of said signal upon the rate of change of said signal reaching a predetermined value. Preferably there is a probe including a probe element and a sheath, the probe element being slideable within the sheath and protruding therefrom by a variable amount, means for producing a depth signal representing the amount of protrusion of the probe element from the sheath, means for monitoring the rate of change of said depth signal, and means for recording and/or displaying a value representative of said depth signal upon the rate of change of said depth signal reaching a predetermined value.