1) Electrical potential field distributions have been obtained for SEN and CTS fracture testpieces and used to derive the optimum conditions for current input and probe positions. 2) The most suitable position for the potential measurement probes is on the top face of the specimen as close to the open end of the notch as possible, since attachment near the crack tip leads to much larger errors from any slight variations in positioning the probes. 3) For SEN specimens, the greatest sensitivity is obtained by introducing the current at points close to the notch. However, the uniform current configuration (input of current at specimen ends) is prefered since it is not liable to errors from slight variations in current input position. 4) Introduction of the current by area contact on the top surface in CTS specimens is preferred because very low sensitivity is achieved for the initial stages of crack growth when the current is applied from the side flanks. 5) The mode of current lead attachment on CTS testpieces is dependent on the testpiece materials. In steels, reproducible area contact mayy be achieved by the use of brazed or soldered copper terminals. Howwever, in aluminium and its alloys, the samde degree of reproducibility cannot be achieved for any practical application by area contact. Point application of current through a single screw is therefore advocated.