<p>A vibrating wire strain meter has a steel proving ring across the bore of which extends a taut wire. The outer surface of the proving ring has sloping or tapered grooves extending axially along the entire length of the ring. A pair of diametrically opposed wedges sloping at the same angle as the grooves are used to wedge the proving ring in a borehole. The tapered grooves provide positive guiding of the wedges such that the device can be positioned accurately and repeatably in relation to the borehole wall. Preferably, the wire extends across a diameter which is perpendicular to the diameter on which the grooves and wedges are located. A novel tool for inserting and withdrawing the strain meter is also described.</p>
