An instrument for introducing a vascular aid into a vessel during surgery comprises two L-shaped elements, first releasable means for locking the two L-shaped elements so as to form a T-shaped element having a stem and two oppositely directed arms, and second releasable means for connecting said vascular aid along said arms. This penile clamp is designed to provide an external pressure applied to the underside of the penis at the base thereof to close the urethral canal. A second circuit is coupled to an output of the first circuit and is configured to input a current into the cell in response to the sample values of the cosine waveform.