An instrument 10 for delivering a high voltage pulse to tissue is disclosed. The instrument 10 can include an outer support member 12 with a liquid reservoir 14 that has a liquid-contacting interior surface 16, an opening 18 at a distal end 20 of the outer support member 12, and a ground electrode 22 extending in a longitudinal direction and having a lower surface 23 proximate the opening 18. The instrument 10 can also include a working electrode 26 extending longitudinally from the liquid-contacting interior surface 16 with a needle-shaped distal portion 28 proximate the distal end 20; and an inlet port 31 and an outlet port 34 in liquid communication with the liquid reservoir 14. The working electrode 26 can be electrically isolated from the ground electrode 22 by an insulating portion 30 of the outer support member 12, and a direct path can exist through the liquid reservoir 14 between the ground electrode 22 and the working electrode 26.