In an air-water-only system under batch operation, a bubble interface was created in a laboratory column by introducing a step change in gas Row, and the rise velocity of this interface, u(in), was measured using a novel method based on electrical conductivity with a custom-designed fast-response conductivity meter. The u(in) was plotted against J(g2)/J(g1), where J(g1) and J(g2) are the superficial gas velocities before and after the step change, respectively. The velocity of the undisturbed system was estimated by interpolation at J(g2)/J(g1) = 1. This velocity was referred to as the hindered velocity, u(h). The local average gas velocity, u(g,loc) (=J(g,loc)/epsilon(g,loc)) [corrected from the average gas velocity (=J(g)/epsilon(g)) to account for the local pressure], was different from u(h). It is argued that u(h) is the real bubble swarm velocity and u(g,loc) is the bubble swaem velocity for the ideal case of uniform bubble size. Unlike u(g,loc), the u(h) was constant along the column, a result supported by theoretical momentum analysis. The u(h) is proposed as the key characteristic swarm velocity of the system.