Bipolar Electrosurgical Instruments

An electrosurgical system comprising: a bipolar surgical instrument including a body, a shaft at whose distal end are located first and second opposed elongate jaws having respective first and second electrodes, the first jaw being movable with respect to the second jaw between an open and a closed position; and a controller to control a radio frequency (RF) signal generator. The controller is operable in a heating stage 2 to measure the impedance between the electrodes whilst supplying an RF electrode signal, that at the start of the heating stage 2 is at a starting voltage and at the end of the heating stage 2 is at a final voltage, during which a minimum measured impedance is recorded, and where the end of the heating stage 2 is determined by the measured impedance being greater than the recorded minimum impedance by a predetermined amount. The controller may be operable in an initialisation stage 1 prior to heating stage 2. The controller may be operable in a sealing stage 3 following heating stage 2. The controller is described in terms of operating to determine a boiling point for tissue between the jaws through measuring the impedance between the jaws.