A surgical instrument assembly has (i) a surgical instrument including a handpiece and a probe or horn attached to the handpiece, (ii) a source of ultrasonic vibratory energy operatively connected to the probe or horn, (iii) a source of electrical current operatively connected to deliver electrical current to organic tissues of a patient at a surgical site contacted by a distal end of the probe or horn, and (iv) a sensor of electrical potential disposable in contact with the patient at a desired distance from the surgical site. The sensor is operatively connected to the source of ultrasonic vibratory energy to automatically attenuate an output thereof in response to a detected potential of a predetermined magnitude.