A gas is ionized in the immeiate vicinity of the surface of plane electrode `I` (Fig) separated from electrodes 2 by a distanae `l`. An alternating voltage varying with time `t` as `V = V_(0) sin omega t` is applie dto the electrodes. On decreasing the frequency `omega` it was observed that the galvonometer `G` indicates a current only at `omega lt omega_(0)`, where `omega_(0)` is a certain cut-off frequency. FInd the mobility of ions reaching electrode 2 under these conditions.