A surgical tool comprising: an end effector comprising a first jaw member, a second jaw member, the first and second jaw members comprising corresponding first and second electrodes (709a and 709b), and a blade, the end effector configured to receive at least one signal (728, 708, 808, 1008, 1116); an actuator for reciprocating the blade along a longitudinal axis; and a control circuit (726, 1006, 1206) for controlling the actuator, wherein the control circuit: provides the at least one signal to the end effector; receives a feedback signal (710, 810) indicative of a tissue impedance of tissue in contact with the end effector; determines a rate of change of the tissue impedance based on the feedback signal; compares the rate of change of the tissue impedance to a predetermined rate of change of the tissue impedence; and varies the at least one signal to maintain the rate of change of the tissue impedance at the predetermined rate of change of the tissue impedance; characterised in that the surgical tool further comprises: a motor coupled to the actuator; a motor control logic coupled to the motor, wherein the motor control logic is configured to vary an output signal to the motor to increase or decrease the longitudinal speed of the blade, wherein the control circuit is configured to provide a control signal to the motor control logic and maintain the rate of change of tissue impedance by varying the speed of the motor. an end effector comprising a first jaw member, a second jaw member, the first and second jaw members comprising corresponding first and second electrodes (709a and 709b), and a blade, the end effector configured to receive at least one signal (728, 708, 808, 1008, 1116); an actuator for reciprocating the blade along a longitudinal axis; and a control circuit (726, 1006, 1206) for controlling the actuator, wherein the control circuit: provides the at least one signal to the end effector; receives a feedback signal (710, 810) indicative of a tissue impedance of tissue in contact with the end effector; determines a rate of change of the tissue impedance based on the feedback signal; compares the rate of change of the tissue impedance to a predetermined rate of change of the tissue impedence; and varies the at least one signal to maintain the rate of change of the tissue impedance at the predetermined rate of change of the tissue impedance; provides the at least one signal to the end effector; receives a feedback signal (710, 810) indicative of a tissue impedance of tissue in contact with the end effector; determines a rate of change of the tissue impedance based on the feedback signal; compares the rate of change of the tissue impedance to a predetermined rate of change of the tissue impedence; and varies the at least one signal to maintain the rate of change of the tissue impedance at the predetermined rate of change of the tissue impedance; characterised in that the surgical tool further comprises: a motor coupled to the actuator; a motor control logic coupled to the motor, wherein the motor control logic is configured to vary an output signal to the motor to increase or decrease the longitudinal speed of the blade, wherein the control circuit is configured to provide a control signal to the motor control logic and maintain the rate of change of tissue impedance by varying the speed of the motor. a motor coupled to the actuator; a motor control logic coupled to the motor, wherein the motor control logic is configured to vary an output signal to the motor to increase or decrease the longitudinal speed of the blade, wherein the control circuit is configured to provide a control signal to the motor control logic and maintain the rate of change of tissue impedance by varying the speed of the motor.