Lines of hydrogen bubbles generated by electrolysis from a fine wire may be usedto visualise and quantify velocity fields in low velocity flow. Because thebubbles are released into the wake of the generating wire their velocity differsfrom that of the free stream. As the velocity deficit can amount to twentypercent of the free stream velocity at a distance of one hundred diameters fromthe wire, the wake effect cannot be ignored in quantative work. This paperexamines the influence of wire orientation, wire Reynolds number, bubble pulseduration and rate of bubble production on the velocity defect experienced bythe bubbles.