We present a novel experimental design for high sensitivity measurements of the electrical resistance of samples at high pressures (0-6GPa) and high temperatures (0-1000K) utilizing the ’Paris-Edinburgh’ type large volume press. Uniquely, in the method the electrical resistance measurement is carried out directly on the sample, thus greatly increasing its sensitivity. This sensitivity can be used to clearly identify phase transitions in material samples. The method is demonstrated on Pb, Sn and Bi across a wide domain of temperature-pressure phase space and employing it to identify the loci of phase transitions. Based on these results, the phase diagrams of these elements are reconstructed to high accuracy and found to be in excellent agreement with previous studies. Further measurements, carried out on pure melted bismuth revealed an anomalous sharp decrease in resistivity with increasing temperature at constant pressures.  These findings imply on a possible novel transition in the melt. The proposed transition was observed across a broad pressure range both upon heating and cooling cycles of the sample demonstrating its reversibility. From the measurements it was possible to determine a 'phase-line' of this transition on the bismuth pressure-temperature phase diagram terminating at the melting curve.