Abstract: The invention relates to probe for use in measuring crack depth in a conducting workpiece, apparatus including the probe for measuring crack depth and methods of measuring crack depth. The probe has a pair of spaced parallel electrodes mounted on a body and electrically insulated from each other. The probe includes a lead for each electrode. The leads extend in a twisted pair through the body and between electrodes to a point lying in or adjacent the plane containing the contact surfaces of electrodes. The leads then separate and extend directly to a respective one of electrodes and are connected thereto adjacent the contact surfaces thereof. The probe is used to detect crack depths by measuring and comparing potential difference between contact surfaces when the electrodes are engaged on cracked and uncracked portions of an electrically conducting workpiece, when an alternating current is passed through the workpiece.